Traffic Increase

This is probably one of the most obvious and common cases where in getting what you wanted, you end up regretting ever asking for it.

Let’s say you made a post in Hacker News about your brand-new app, and it suddenly got the attention of way too many people. Or maybe you published a new mobile app into Google’s Play store and it was featured, and now your APIs are receiving 400% more traffic than you expected for the first five months. Or maybe your online massive multiplayer game is now popular and even though your game servers are able to handle the load, your tiny log server is crashing every 2 hours from the amount of internal traffic it’s receiving.

With any of these, you now have a problem. It might directly affect your entire application, or it might be a challenge for only part of it, but you will have to fix it if you want your platform to perform as expected.

An increase in incoming traffic could affect your system in different ways; we can describe these as direct or indirect.

Indirect Effects

An increase in traffic can affect your application indirectly by overloading one of your internal processes. In a microservices-based architecture, the communication between services needs to be carefully planned. The fact that you’re capable of handling the increased traffic on your user-facing service doesn’t mean the rest of your architecture will be able to handle it.

Figure 1-1 shows exactly that case, where a resource-bound log server is crashing randomly and limiting the number of logs properly saved in your database. Good luck troubleshooting any other issues or bugs in your platform when that happens.

Another possible indirect effect occurs when an increase in traffic starts affecting the performance of a service you share with other platforms. This is what is normally known as service degradation , when the service is still active and working, but is responding slower than usual.

In this scenario, your lack of planning and scaling capabilities will start affecting those who use your service. This is why you always want to make sure that whenever you depend on third-party services, they can actually assure you that their service will not be degraded by anything.

Increased Processing Power Required

A need for increased processing power could be related to the previous case, and sometimes can even be caused by it; but it can also develop independently, which is why it’s worth discussing as a whole different category.

Specifically, this is purely a resource-related problem; you’re trying to process more information than your current resource utilization technique allows you to (“trying to bite off more than you can chew”). Notice that I didn’t blame the server directly; instead, I’m sharing the blame between you and your server.

In this scenario, you’re trying to do something with one or more sources of data, and suddenly, they start providing considerably more data than you expected. And when this happens, things can go wrong in one of two ways: your service may be degraded, or it may crash completely.

High Availability

For an architecture to be highly available, it must ensure that whatever service it provides will always be available and will not lose performance, despite having internal problems (such as a loss of processing nodes). The availability of a system is usually also known as its uptime, and commercially for service providers you sign onto an SLA (Service Level Agreement) which is measured in “nines” of availability. For instance, Amazon ensures three nines of availability for their S3 service, which in practice means they ensure 99.9% of monthly availability. Put another way, if their service fails more than 43.2 minutes a month, then they’ll be over service credits. More critical services, like mobile carriers, ensure five nines of availability, which translates to 99.999% of uptime, which in turn, translates to 5.2 minutes a year of downtime allowed.

There are several techniques that can be used to achieve this, but the most common one is the master-slave pattern.

You will often see this pattern discussed while reading about database scaling configurations, since most of them tend to go this route. It basically consists of having at least one more node for each of your main ones, and some kind of monitoring on the main nodes, so that if they go down, you can promote one of these “extra” nodes into a main one. In Figure 1-2, you have a standard three-node service, where node 3’s input depends on node 2’s output, and node 2’s input depends on node 1’s output. You can also see the “slaves,” which are the smaller dotted boxes next to them.

The final component of the master-slave model is the monitoring service, which makes sure all master nodes work correctly.

The second half of Figure 1-2 shows what happens when one of the nodes fails (in this case M2). Its slave node is promoted to master (essentially taking the place of M2, connecting M1’s output into its input and its output into M3’s input).

In practice you normally don’t need to worry about switching the connections or manually monitoring nodes for that matter; usually load balancers are used between nodes to do exactly that, act as “fixed” points of connectivity and decide by themselves (and a set of conditions you configure on them) whether or not to promote a slave.

That works well if your nodes are simple processing nodes; in other words, if you can simply exchange a master for a slave without any loss of data or any kind of information on your platform. But what happens if your nodes are part of a storage system, like a database? This scenario is slightly different from the previous one, because here, you’re not trying to avoid a lack of processing power, or a processing step in your flow. You’re trying to prevent data loss, without affecting your performance at the same time.

In this case, what you’ll want between your master and slaves (you could potentially have more than one slave per master), is a passive data-replication process (write only) from your master into your slaves. This will make sure that if your master is lost at any point, your slaves will be able to take over with minimal negative effects, as shown in Figure 1-3. (Note that you can’t prevent the master going down in mid-write, which would negate the transaction, preventing the data transfer from completing.)

In some cases, like Redis (with Sentinel enabled), slaves are not just there waiting to be promoted, they’re used for read-only queries, thus helping shed some load off their masters, which in turn, take care of all the write operations.

MongoDB, for another example, provides something called replica sets ,¹ which allow you to set up a group of nodes, in which one of them is the primary and therefore the one your code is constantly talking to, but if something happens to it, the secondary nodes will “elect” one of them to be the new primary (as seen in Figure 1-4).

Finally, another real-world example of HA is the newer versions of Hadoop’s HDFS. Prior to version 2, the HDFS (or Hadoop Distributed File System) had a single point of failure in its only NameNode. In other words, if that machine were to fail or be brought down due to maintenance, the entire cluster would be rendered inaccessible until it was brought back up. In newer versions, there is an option to set up as many redundant secondary NameNodes as you want. With this feature, these nodes effectively act as passive slaves to their master, getting sufficient state (via external services called journal nodes) to provide a fast failover if required (shown in Figure 1-5).

As a side-note, all DataNodes need to send heartbeats to all NameNodes to ensure that if a failover is required, it will happen as fast as possible.

As you can see, the slave-master approach can have slightly different implementations, but if you dig deep enough, they all end-up being the same.

Let’s take a look now at fault tolerance, to understand how it differs from HA.

Fault Tolerance

You can think of fault tolerance as a less strict version of HA. The latter was all about keeping the offline time of your platform to a minimum and always trying to keep performance unaffected. With FT, we will again try to minimize downtime, but performance will not be a concern—in fact, you could say that degraded performance is to be expected.

That being said, the most important difference between these two is that if an error occurs during an action, a highly available system does not ensure the correct end state of that action, while a fault-tolerant one does.

For example, if a web request is being processed by your highly available platform, and one of the nodes crashes, the user making that request will probably get a 500 error back from the API, but the system will still be responsive for following requests. In the case of a fault-tolerant platform, the failure will somehow (more on this in a minute) be worked-around and the request will finish correctly, so the user can get a valid response. The second case will most likely take longer, because of the extra steps.

This distinction is crucial because it will be the key to understanding which approach you’ll want to implement for your particular use case.

Usually fault-tolerant systems try to catch the error at its source and find a solution before it becomes critical. An example of this is having mirrored hard drives in case one of them fails, instead of letting a single drive fail. That would require replacing the entire server, affecting whatever actions the server could have been performing at the time.

Hardware-level fault tolerance is beyond the scope of this book; here, I will cover some of the most common techniques used to ensure FT at a software level.

Redundancy

One way to design fault-tolerant architectures is by incorporating redundancy into your key components. Essentially, this means that you have one or more components performing the same task and some form of checking logic to determine when one of them is has failed and its output needs to be ignored.

This is a very common practice for mission-critical components, and it can be applied to many scenarios.

For example, in 2012, SpaceX sent its Dragon capsule to berth with the International Space Station. During the ascent, the Falcon9 rocket used suffered a failure on one of its nine Merlin engines; and thanks to the implemented redundancy, the onboard computer was able to reconfigure the other eight engines to ensure the success of the mission.

Because these systems are so complex to code and to test, the cost-benefit ratio is not always something the normal software project can handle. Instead, these types of systems are usually present in critical applications, where human lives might be at risk (such as air traffic controllers, rocket guidance systems, and nuclear power plants).

Let’s go over some techniques to provide software redundancy and fault tolerance.

Triple Modular Redundancy

Also known as triple mode redundancy, TMR is a form of redundancy in which three systems perform the same process and their results are checked by a majority voting system that in turn produces a single output (see Figure 1-6). If one of the three systems fails, then the other two will correct it by providing the accurate output to the voting system.

This is a particular implementation of the N-modular redundancy systems, where as you might’ve guessed, you can add as many parallel systems as you see the need for, in order to provide a higher degree of fault tolerance for a given component. A particularly interesting real-world use case for this type of solution (in this case a 5-modular redundancy system) is the FlexRay² system.

FlexRay is a network communication protocol used in cars; it was developed by the FlexRay Consortium to govern onboard car computing. The consortium disbanded in 2009, but the protocol became a standard. Cars such as the Audi A4 and BMW 7 series use FlexRay. This protocol uses both data redundancy, sending extra information for problem detection purposes as metadata in the same messages, and structural redundancy in the form of a redundant communication channel.

Forward Error Correction

Yet another way to add a form of redundancy to the system, Forward Error Correction (FEC) adds redundancy into the message itself. That way, the receiver can verify the actual data and correct a limited number of detected errors caused by noisy or unstable channels.

Depending on the algorithm used to encode the data, the degree of redundancy on the channel may vary and with it, the amount of actual data that can be transferred through it.

There are two main types of encoding algorithms: block codes and convolutional codes. The first kind deals with fixed-length blocks of data, and one of the most common algorithms is Reed-Solomon. A classic example of this is two-dimensional bar codes, which are encoded in such a way that the reader can withstand a certain number of missing bits from the code.

Another very interesting real-world example of this type of redundancy can be found on the messages sent by the Voyager space probe and similar probes. As you can imagine, the communication with these devices can’t really afford retransmissions due to a faulty bit, so this type of encoding is used to ensure that the receiving end takes care of solving as many errors caused by a problematic channel as it can.

By contrast, convolutional codes deal with streams of arbitrary length of data, and the most common algorithm used for this is the Viterbi algorithm. This algorithm is used for CDMA (Code Division Multiple Access) and GSM (Global System for Mobiles) cellular networks, dial-up models, and deep-space communications (sometimes it’s even used in combination with Reed-Solomon to ensure that whatever defect can’t be fixed using Viterbi is fixed using R-S).

Byzantine Fault-Tolerance

I intentionally left this one for last, because it could be considered the sum of all of the above. What we have here is the “Byzantine Generals Problem”, basically a distributed system where some components fail, but the majority of the monitoring modules can’t reach a consensus. In other words, you’re in trouble.

Figure 1-7 shows a basic and high-level example of what this problem means for a platform architecture. In it, you have five replicas of the component C, which send their output to four different status checkers (A, R, M and Y), they in turn, exchange “notes” and try to reach a consensus regarding the data they all received. But because there is a problem, maybe with the data channel or with the fifth component, different values are sent to different checkers, so in this case a majority consensus can’t be reached.

There are different approaches to tackle this type of problem; in fact, there are too many out there to cover in a single chapter, so I’ll just go over the most common ones, to try to give you an idea of where to start.

The simplest approach, and it is not so much a solution as a workaround, is to let your status checkers default to a specific value whenever consensus can’t be reached. That way, the system is not stalled, and the current operation can continue.

Another possible solution, especially when the fault is on the data channel and not on the component generating the message itself, is to sign the messages with some sort of CRC algorithm, so that faulty messages can be detected and ignored.

Finally, yet another approach to ensure the authenticity of the message sent is to use blockchain, just as Bitcoin does, with a Proof of Work approach, in which each node that needs to send a message must authenticate it by performing a heavy computation. I’m simply mentioning this approach, since it could be the subject of an entire book, but the idea behind this approach is that it solves the Byzantine Generals problem without any inconvenience.

Layered Architecture

Figure 2-1 shows a very high-level overview of how this pattern expects the layers to be organized. The level of abstraction, as shown in the image, refers to the specifics of the business that the logic for your application is built around. In other words, the deeper you go into your layers, the more detail you’ll have to deal with regarding your specific business data model and business rules.

One common misconception about this pattern is that people confuse it with MVC (Model-View-Controller), thinking the latter to be the 3-tier version of this one. In the next pattern, I’ll go over the main differences between the two.

As a side-effect of using a layered architecture, the code inside a layer is all related to the same functionality (or at least, follows a set of standards common to the rest). This in turn helps developers work independently from each other on different layers. This allows the teams to make internal changes to the layers without affecting anything else (as long as the actual interface remains the same).

Some people also split the business layer into two, one in which the business knowledge remains, and another, often called the application layer, which owns the interaction logic between the UI and the business layer and also provides some common services useful for that interaction.

MVC Is Not Layered

If you’ve been doing any kind of web development for the last few years, you’ve probably heard or read about Model-View-Controller (MVC) . This pattern is one that many web frameworks have adopted (Ruby on Rails, Django, and Sails.js, to name a few), because the structure of most web projects resembles this approach.

That is, in most web projects you have a UI (or View); you most likely will want to handle the requests of your UI somewhere (the Controller); and finally, you most likely have a storage engine, inside of which you can probably force your data to fit into a set of Models that represent your resources. You’ll also want to perform some transformations on this.

Now, don’t get me wrong; MVC is not web-specific. In fact, you can use this pattern on any application that handles some sort of domain knowledge, has a representation for it, and is capable of performing actions on it. Let me summarize what the definition says for this pattern:

• The model is the boss: here’s where the domain knowledge or business logic is stored, here is where the actual data is handled, and here is where all the business specific coding should take place.

• The view is a simple representation of the model; there can even be several views for the same model. Remember that “representation” doesn’t mean “web page”; it means anything that can be read and understood by another system. A JSON object can be a view; this pattern could be applied to RESTful APIs just as much as it could be implemented to the front-end architecture of your very complex SPA.

• Finally, the controller is the poor guy who drew the short stick and is in charge of interconnecting the model and the view. It basically takes input from the view and passes it along to the model, while executing whatever command is needed on that input. Then, once the controller gets something back from the model, it sends it back to the view, updating it. In some cases, the controller can even avoid the last part (as seen in Figure 2-2), letting the model directly update the view.

We now have enough information to answer the question of how this is different from a 3-layered architecture. You have seen that the layered architectural pattern does not allow jumping layers when sending messages between them. You could say that for the n-tier architecture, the communication is completely linear (it needs to go through all intermediate layers in all cases), while for the MVC pattern, you can work around that (as seen in Figure 2-2), since it is more of a triangular setup. This flexibility can potentially be a negative for this pattern if performance is a big concern, since every extra layer that you add to it will definitely add latency to the communication (no matter how fast you make it work).

Finally, I want to mention some of the variations on this pattern. Over the years, many adaptations have been created to improve on it, depending on whether the goal is to have less component coupling, better testing capabilities, or simply to follow a similar logic but adapted to particular needs. For instance, the MVP (Model, View, Presenter) pattern aims to remove that (normally) unwanted interaction between Model and View, making the Presenter the sole man-in-the-middle taking care of passing information between its associated View (there is only one view for each presenter) and it’s Model. Another very common variation is the MVVM pattern (Model, View, ViewModel), which aims for a two-way data binding between the View and ViewModel. This in turns allows for automatic updates on the view, based on changes in the model.

Client-Server

The client-server pattern is a very simple yet powerful one. It consists of having a powerful server that provides meaningful services to many clients.

This pattern should sound very familiar to anyone who’s done any web development, since it’s the basic pattern for the World Wide Web. Browsers act as the many clients, which in turn request resources (web pages) from the different servers they interact with. Figure 2-3 shows what this looks like.

This pattern favors the off-loading of application logic into the server, where more hardware resources can easily be allocated. It also keeps the clients “thin” and “dumb,” in the sense that they usually don’t have a lot of business logic knowledge; instead, they simply know what to request from the server, and it is in the latter where all the heavy business-related computation will take place.

One of the main benefits of this approach is that making changes and fixes to either side does not necessarily mean affecting the other. That is, fixing a server bug doesn’t mean you need to even touch the client code, and vice-versa. Inherent security is another plus that comes out of this setup, since any core security check can be done on the server side, making sure any clients that are tampered with can still remain secure. (Think of a multiplayer game client, for example, where hackers can modify a player’s position; if the server is still checking for that, then the modification has no effect.) The clear separation of concerns between client and server is what give you that ability.

Another benefit from thin and dumb clients is that they’re easier to distribute (you don’t need a 2GB client when a simple 10MB will do, and you can keep the rest of the 2GB code in the server).

Finally, consider that even though Figure 2-3 shows a single server dealing with all the clients (essentially representing a monolithic approach), your server “box” can actually be expanded into a set of microservices or any other distributed architecture you might find more useful, where there is a single point of contact between dumb clients and business logic.

In fact, if you think about it, doing that you could very well end-up with a layered pattern, in which every layer is usually a different tier (physically separated from each other). As long as your client and server layers are physically separated, your client-server architecture is essentially a 2-tier one, in which the two constraints described earlier apply.

Master-Slave

The master-slave pattern at first glance could be confused with the previous one, since it implies a single channel of communication between two parties. But conceptually they’re very different, because instead of having one centralized hub for the business logic and heavy processing of data, you have a one-way controlled communication between a controlling node (the master) and many decentralized nodes (the slaves). Figure 2-4 shows an example of this architecture.

Event-Bus or Event-Driven Architectures

This pattern is a very interesting one in the sense that it breaks the mold of what I’ve been telling you about so far. Event-driven architectures don’t have the one-to-many or many-to one type of relationship between components; instead, they are usually many-to-many relationships. Let’s first take a look at Figure 2-5, which shows a basic example of an event-driven architecture and then I’ll go into more detail about it.

This pattern is extremely powerful for providing a highly available platform, or even when trying to scale up—as long as your architecture follows the guidelines of the diagram in Figure 2-5. In other words, you need to avoid component-to-component communication and allow them to interact with each other only through the event bus.

If you do that, you can potentially replace crashed nodes with new instances in the time it takes them to boot up. The same happens if you need more processing power; you simply add new consumer or sources and connect them to the event bus, and that’s all.

A good idea when dealing with this type of architecture is to use a third-party data bus (as long as that’s an option), because that will allow you to focus on creating the event sources and event consumers, while at the same time using a tried-and-tested bus, one that can reliably transmit the data and scale when needed.

A classic example of this approach consists of using a message queue as the event bus. In this case you wouldn’t want to create your own bus; you would most likely want to use one of the many existing solutions such as RabbitMQ, Kafka, ZeroMQ or any other.

Microservices Architecture

This pattern is one of the best-known , since it’s been growing in popularity in the last few years. Everybody and their mother is jumping into the microservices bandwagon, whether they have use for them or not. Just like with anything in our industry, there is no silver bullet solution; so hoping that microservices will solve all your problems without taking into consideration its pros and cons is reckless, at best.

The core behind this pattern is your understanding of the different features your platform is supposed to have (that is, the different services it needs to provide). If you properly understand your platform, you can probably split the services into a set of smaller services that, when used together by the client app, yield the same result you would get from a single block of code with all features bundled together. Figure 2-6 shows an example.

By splitting your platform into individual services, you gain a new level of control over it that you never had with a monolithic approach. Figure 2-6 shows an example of a fake platform that takes care of payments. It also needs a proper authentication strategy, so it also has a dedicated Auth service. Every user needs to be authenticated against it to start using the platform, and then every request is authorized against the service. Users also have a way to register into the system, which is why there is a UserMgnmt service .

This pattern might sound like the best solution for most problems, but you need to take into account that, as with any other option, you might run into problems because you’re trying to meet your needs with the wrong architecture.

In the end, it’ll be a matter of picking the right tool for the job, as with everything else.

The Broker Pattern

You can think of the broker pattern as a specialization of the microservices architecture. One of the pain points for the latter was that given a high enough number of microservices, you begin to need a form of orchestration; otherwise, your clients start to lose the ability to communicate easily with your platform. You start to burden them with the knowledge of where everything is in your system, and that should not be the case. Ideally, clients should be able to discover your services organically and with minimal previous knowledge.

Here is where the broker pattern comes into play. Its main component is a node called broker, whose purpose is to centralize and redistribute requests among different services.

Another key characteristic of this pattern is that by default it is not the broker that “knows” about its servers; instead, it’s the servers that register with the broker once they come online, and provide all the information it needs to understand the services they provide.

Figure 2-7 shows an example of a broker providing services from three different servers. This pattern is very common among message queues, such as RabbitMQ,¹ Apache Kafka,² and Apache ActiveMQ.³

Vertical or Horizontal Scaling?

When it’s time to start scaling, the most basic things you can do are either improving the hardware your code is running on, or simply adding more computers to distribute the load among them.

That is the essence of vertical and horizontal scaling (as seen in Figure 3-1). You can think of this as changing your old car’s tires compared to buying a new Ferrari when trying to get more horsepower. With the first method you’re always trying to add resources to your existing hardware: memory, processing power, disk, anything that might help out depending on your needs.

This approach normally requires some downtime because of the physical changes that need to take place. Whatever benefits you might get from doing this are limited by the resources you must add to the hardware. There is, of course, a sort of improved vertical scaling option, which is available if you’re in a cloud-based environment. In these situations, your cloud provider will allow you to switch your application from one type of hardware to another. This can be done with a few clicks. Options are limited, but at least you’re not constrained by your original hardware’s specs. For example, Amazon will let you switch your t2.nano instance that only has half a Gigabyte worth of RAM for a t2.2xlarge one, which actually has 32 GB of RAM and 8 virtual CPUs. If memory or CPU power was your bottleneck, this would solve it).

The main problem with vertical scaling, though, is that sooner or later you’re going to hit the cap for your scaling capacity. No matter what service you’re paying for, eventually you’ll run out of specs to improve.

And here is where horizontal scaling enters. Instead of improving a single server, you’re now working with a pool of them, and adding to that pool is as simple as connecting the new server to it (or removing one from it). You can design your application to withstand such changes and avoid any real downtime when scaling. With that you gain “elasticity” in your capacity, and control over how much money you want to spend, during both normal operation and peak hours.

Taking Advantage of the Cloud

The Power of a Multi-Zone Deployment

Although it is not strictly related to scaling your architecture, taking advantage of a multi-zone deployment on the cloud is a must if you have the chance. This is usually something that most of the major cloud providers already allow, so you should consider, when planning your architecture, deploying and even scaling into multiple zones.

By doing this, you gain tolerance for big network outages that tend to affect an entire geographical zone. These types of network problems aren’t very common, but when they happen, big sections of the internet are isolated from the world, and this is something you want to avoid.

There are two types of resources to consider this technique for: services and data. If it’s services you’re deploying, then all you have to do is make sure you’re deploying behind a load balancer (such as Elastic Load Balancing for AWS). Figure 3-2 shows an example of such a case. By doing so, you’ve made sure that no matter what happens, your services will always be available.

But if you’re actually deploying data, or a data storage platform such as a database, this is also a good idea for you. To make it work, however, requires some extra work. In the case of data, what you want is to make it available no matter what. To accomplish this, you must make sure you’re properly replicating your data. Figure 3-3 shows one example of what such a deployment might look like.

Clustering with PM2

The cluster module is great, because it allows you to make the most of your CPU with minimum effort; but that’s about all it can do for you. If you want more control over how it’s done, or need to know a bit more about what exactly is happening with your cluster, there is very little you can do out of the box.

With that command, you’ll be starting up your application, naming it “my app” in PM2’s list of processes, and taking advantage of all your CPU cores (thanks to the -i max modifier). Figure 3-5 shows the output from the start command .

Figure 3-6 shows the possible output from the logs. As you can see, the same line is repeated four times, thanks to the four processes running in parallel (because of my four cores).

In Conclusion

Clustering is and should be your first step toward scaling whatever Node.js application you might be working on. Depending on your needs, you might want to go with the cluster module. You don’t need anything extra; simply add a few lines of code and you’re done. On the other hand, if you’re looking to avoid changing your code, and you need extra support for your production environment, then PM2 or similar solutions should definitely be your choice.

In Conclusion

Microservices is a topic that you’ll read about throughout this book because it is a very helpful pattern for scaling Node.js platforms (since Node is usually used to create APIs).

That being said, and as I’ve already covered in previous chapters, it is not a silver bullet and will not work for you every time. You need to remember when this pattern is helpful and how you need to prepare your code and your team to be able to get the most out of it.

You’re Just Logging into stdout and stderr

And what makes it even worse is that you’re not wrapping the output function/method of your language of choice into a construct under your control. In other words, and in the Node.js universe, you’re logging using console.log and console.error.

This is great for small projects and quick PoC (Proof of Concept) , but if you’re interested in getting anything out of your logs, then you need to do something about it. In particular, because both the stdout and stderr are local to each server instance, if you start scaling your application into multiple servers, you’ll have to deal with distributed logs that aren’t being saved anywhere (or maybe they are, depending on your setup).

This will work, even if you’re not saving the data anywhere, since PM2 will catch all your output and save it automatically, just in case. But that will only get you halfway, since we also need to send those log files into a centralized location.

Because there are so many options, and so many variations of similar solutions, I’m just going to cover a simple one, assuming you have an ELK⁴ (Elastic, Logstash, and Kibana) cluster configured and ready to receive logs somewhere on your architecture. This will act as the centralized logging and analytics system from Figure 4-1.

What you want to do in this situation is configure something that will ship the log files stored by PM2, into Logstash, which in turn will apply any transformation you might need to the data and then send it and index it into Elastic for your consumption using Kibana.

This might sound like a lot at first glance, especially if this is your first time dealing with something like this, but it is a scalable way of going about it. If you do it right, you gain the ability to support failures and downtimes on your Elastic cluster, you get back-pressure, on your ogging pipeline, making sure you’re not overwhelming your analytics platform, and so on.

To achieve this, you’ll install and configure Filebeat in all your servers (all those that need to send data out). Filebeat is essentially a log shipper that follows a standard protocol called Beat. This shipper (and its associated protocol) is the result of several years of iteration by the team of Elastic to get the best and most lightweight log shipper possible.

That configuration will pull the contents of the log files for your app (stored in a default location by PM2) and into a Logstash server. You need to replace the constants YOURHOMEFOLDER and LOGSTASH-HOST by the actual values to make it work.

With that, you’re ready to retrieve your log files (if you’re crazy enough to log using only console.log). But you still need to configure Logstash to make sure you can parse these logs, transform them (if needed) and then index them into Elastic. So stop smiling, and keep reading.

Note that the configuration from Listing 4-3 will only receive and index data; it will not transform (which is one of the key benefits of Logstash) anything. So if you wanted to do some extra tweaking of your logs before indexing them, I recommend looking at the full documentation for Logstash.⁶ Also, make sure to match the port under the configuration for the Beat plugin with the port specified on the Filebeat config file (Listing 4-1).

You’re now set. Congratulations, you’ve managed to avoid a major problem by using the right set of tools.

If, on the other hand, you weren’t using anything like PM2 that would catch the output of your process and save it into a file, you’re out of luck. You’ve lost your logs to the black hole that lives inside every server and there is no way for you to retrieve them. So don’t do it like this.

You’re Logging into a Single File

You could potentially take care of both from inside your own code, by adding extra logic to your logger and make sure you properly keep the size and history of your logs in check. You can also use an external tool, such as the classic logrotate⁷ command line utility, which is already part of most (if not all) Linux distributions.

This is definitely the preferred way of handling this logic instead of writing it directly into your own logger’s code. But you’re not there yet. You now have your own log file, and you’re properly making sure it doesn’t grow out of hand, but you still need to send its content into a centralized location. You can look at the previous point in order to understand how to configure Filebeat and Logstash.

With that last bit of configuration, you’re ready to move on with your development, because you again have a stable logging system within your platform.

Single Points of Failure? No Thank You!

It may seem obvious, but you should avoid having single points of failure (SPOFs) whenever possible. They are basically your weakest link and if they break, your entire application/platform/system will be rendered useless.

That being said, they also have the habit of sneaking into some architectures when you’re not looking for them. This is especially true when systems start growing organically instead of systematically, and you start adding new bits and pieces based on your most recent needs, without thinking about the future. For example, without going too far from what we’ve been talk about, imagine having to start sharing memory between processes. You’ve followed my earlier advice, but miss the very crucial part about setting up your external memory in cluster mode, so you end up with something that looks like Figure 4-3.

In this scenario, all services depend on Redis, but Redis is not set up to be able to handle any type of problems; in fact, there is only one instance of Redis running. If it fails, your entire system will be rendered useless. The fix for this one is easy enough; just change the way you have your Redis running. You’d probably set it up as a cluster, or with Sentinel,¹³ so your master nodes will have a slave assigned just in case.

Getting rid of single points of failure can be as easy as setting up a cluster or as hard as adding redundancy to your entire platform. It mostly depends on how far you can take the process until it no longer provides enough bang for your buck.

Let me give you another example, going back to Figure 4-3. After you set up your minimum Redis cluster (which by definition will already contain three master nodes and three slave nodes) you realize that all nodes could fail, because they’re servers you actually have inside your own datacenter. So you decide to move to the cloud and deploy your own Redis cluster in AWS. You then realize that all those servers could still be unreachable if there is a region-level outage, so you again redeploy on a multi-region setup. Great, but what if Amazon goes out of business (unlikely, I know, but bear with me for a second)? You now redeploy your multi-zone Redis cluster in Azure as well, and you have both multi-zone load balancers and extra logic in your code to determine when to pick each one.

You’re now paying a whole lot more money for a level of stability that you might never need.

Alerting

A final concept for monitoring in general, before we start delving deeper into each of the levels I just mentioned, is alerting . This can mean anything; it can be sending an email to the user or it can be notifying some system that can it perform predefined actions. Whatever it might mean to you, it’s the whole point of this practice: you monitor and then you react to the values received.

Deciding when to send those alerts is also very important; both the triggering condition and the periodicity of the messages need to be taken into account to provide a good user experience and add value. Alerting for the sake of alerting is the worst thing you can do; the only thing you’ll achieve by doing that is overwhelming your users to the point where received alerts will be ignored.

Monitoring Your Apps

Specifically when it comes to monitoring Node.js applications, there are too many possibilities to cover them all in this book, and eventually we would start seeing repeated features. And we’re not just looking for a monitoring tool, remember that we’re trying to set up an environment that can scale up and down, according to our needs. So we need a solution that can monitor and react to the values receive by triggering a set of scaling rules.

So instead of reviewing products such as PM2 Enterprise,¹ or even Prometheus,² I’m going to review how to set up AWS CloudWatch³ to monitor your application and your infrastructure at the same time (so you’ll be able to cover the entire pyramid from Figure 5-1) and on top of that, how to make it react and trigger new instances of your application. Figure 5-2 shows the base architecture we’ll use for this example.

The idea behind this example is that CloudWatch will be gathering metrics both from our own application (ad-hoc metrics) and from AWS services that already provide them automatically (such as EC2). These metrics will be monitored by AWS CloudWatch Alerts and displayed and charted by AWS CloudWatch Dashboard. The alerts will trigger actions, which can be anything ranging from a notification (via email, SMS, or even an SQS queue) to a call to the AWS AutoScaling service.

In order to look at this, we’ll use a sample application, so you can see how the ad-hoc metrics can be provided to AWS using their own libraries. This sample app is going to be mind-numbingly simple, but the point is to avoid distracting you by looking at unimportant code, so bear with me.

To start the refactoring process, we’ll begin by installing the SDK module from AWS, and with it, we’ll create a basic metric reporter, which will let us send custom metrics back to CloudWatch.

That file should be stored in ~/.aws/credentials (or your usual Windows equivalent), and you can get that information from AWS Console, while creating your users or by regenerating these keys (for security reasons the AWS system does not allow you to download existing keys).

Let’s break down the code from Listing 5-6, because there is a lot in there, even though it might not seem like it.

I’ve added all the code inside the request handler; this is for simplicity, but you can refactor it as you see fit. In it you can see I’m just calculating the time difference between the previous line actually getting the current date and the line after it. In other words, I’m just going to be reporting the performance of this endpoint and the number of requests it received (simply by sending the first of the two report calls).

Because this is clearly a very simple endpoint, I’m adding a few random numbers to the time difference calculation, just to add some variability to our results (you’ll want to omit that random bit in your own code, obviously).

I’m reporting the data using our wrapper module from Listing 5-4. The parameters seen here are what are important. In both cases, the first one is the namespace. This is key because it allows you to group your metrics in CloudWatch’s console. As you’ll see in Figure 5-3, both of our metrics will be saved in the same API/TRAFFIC namespace. The second one is the actual metric itself; the first report simply sends a 1, as a count, but the second report call sends the time difference, on milliseconds (see the unit parameter). Finally the third parameter is the dimension; you can have up to 10 dimensions for each namespace and they will allow you to further group your metrics.

After you’re ready with your code and you’re properly sending the data up to CloudWatch, you can begin creating your plots Figure 5-4 shows the first one, which is plotting the number of requests received by our API.

We do the same for the other metric (see Figure 5-5), but instead of adding up the numbers, we leave the Statistic column at Average, because in this case, we want the average of the delay for all requests received within a minute.

Adding AWS Metrics into Your Dashboard

Compared to adding your own custom metrics, this should be a walk in the park; but nevertheless it’s also something you should be aware of, since it will allow you to monitor your infrastructure.

The great thing about using CloudWatch is that it is very straightforward. Remember the earlier section on the AWS/ namespace? Well, for every one of its services you use, several namespaces will pop-up under this section, letting you pick the metrics you care most about. Maybe you want to look at CPU utilization, or memory consumption, disk read operations, network traffic or a lot of other indicators.

Again, this will happen automatically, so all you have to do is start using your services, and wait a few minutes until those resources finish booting up and connecting (usually no more than 5 minutes). After that, you can go to the CloudWatch console inside the metrics section, and you’ll find all the new namespaces waiting for you. Figure 5-6 shows how after starting a single EC2 instance and setting up an RDS MySQL instance, three different official namespaces have appeared.

As you can see, AWS provides a lot of metrics for each service, so you can make sure you are aware of everything you need for your infrastructure-related metrics. And based on that, you can create whatever scaling rules you need (more on that in a bit).

Reacting to Your Metrics

This is the last step in the process. So far, you’ve being setting up your monitoring system to look for both custom and default metrics received by your infrastructure and your own Node.js application. You’ve created at least one dashboard (maybe more), and you can log in into AWS Console any time you want and look at those numbers.

But we want that process to also be automated; it makes no sense to have you, or anyone else, systematically logging in and checking numbers, does it? So instead, let’s look at how you can use AWS Console and its services to understand how to automate that process.

That is the basic process you need to go through to automate the reaction to your metrics. Let’s now go into a bit more detail on each step.

Step 1: Creating Your Alerts

From within the CloudWatch console, you can create a new Alert, first by specifying the metrics to use, and then by specifying how to treat those numbers (as we did for the charts), as seen in Figure 5-7. You’ll have to select the namespace, which will have your own ones and the default ones from AWS. After that, you’ll get the list of metrics within that namespace. Simply select the one you want to alert on and the chart below it will update, so you can verify the data.

You can play around with the chart in Figure 5-7 as much as you like, even changing the operation to perform on the data (as we did for the Statistic column a while ago) and the time period for the sampling of these numbers. After you’re happy with the results, you can move on to the final step, which will take care of setting up the actions for this alert.

Figure 5-8 shows the second step of this process, where you can set the name for the alert and the values to react to (the threshold). Finally, you can select a notification list, or create one on the spot.

Note

After you’re done with the alert, it will appear on your alert list (under the CloudWatch console). Initially this alarm will appear on the INSUFFICIENT list, simply because this alert will take a few minutes to start reporting the data needed. Just wait 5 minutes and the alert should go into another section (either OK or ALARM if it’s being triggered).

The Definition

This example is still quite simple, but I’ve added a bit more complex internal logic to the function called myFunction by adding the ability to pass in the mathematical function to apply as the first parameter. With this new logic, the function we’re testing is bigger, and it does different things, so if we design our tests as in Listing 6-2, we can run into a problem: what happens if our test fails?

I haven’t really covered what it means to “run” our tests, but it should be pretty obvious by now: your code is executed and the assertions are verified; if they are true, then your test will succeed, but if they fail (your assertion stated that two values were to be equal and in practice, they aren’t), then your entire test fails. You can see the problem if you’re testing several things within the same test. Once you get the results back from the execution, you will have to dig deeper into the execution logs (if there are any) to understand where your problem lies.

Now whenever a test fails, you’ll get better details from your test runner, because you’ll know exactly which test failed, and thus, you’ll be able to determine immediately which block of code in your complex function failed.

The word unit is the most important one there, because if you go online, you’ll probably find a lot of people defining it as your functions (provided you’re using a procedural programming language) or your methods (if you’re using an OOP language). But as you can see from a simple generic and pseudocode-based example, a unit of code can actually be smaller than that. It’s true that in all these examples I didn’t really show the actual code of the function, and you could argue that for each test case of Listing 6-3, our function is actually calling other, smaller functions, and that is a very good point!

So putting both definitions together, you get a pretty accurate idea of what testing your code means and a good basis for the rest of this chapter.

The Tools

Now that we’ve covered what unit testing is, let’s review the tools provided by this methodology that will allow you to test your code.

These are not software tools; they aren’t libraries or frameworks you can use. We’re not there yet. What I’m trying to give you here are the concepts, the wheels you’ll use to build your car down the road.

Best Practices

I’ve already covered some of these briefly in the previous section, but it’s a good idea to review the full list of recommended practices when writing tests. Like anything in software development, it’s never a solo effort; even if you’re the only one writing code right now, you have to think about the future.

Testing without External Modules

Let me first talk about this option, it’s probably not the way to go, since the provided module is pretty basic, but if you’re looking for something that’s quick and dirty, this will do the job.

One of the major things you’ll notice this library is missing is the rest of the framework; with it, you only get the assertion support. The rest will have to come from you or someplace else, but let’s look into it anyway.

As I already mentioned, this module does not require any kind of installation steps, since it’s already provided with Node’s installation. All you have to do to use it is to require the module assert. After you do so, you’ll gain access to a set of assertion methods, which basically help you compare two values (or objects).

This method performs a deep comparison between two objects. That means Node will recursively compare (using the strictly equal operand) properties within the objects, and if that comparison fails it will throw an AssertionError.

As expected, because in JavaScript, the number 1 and the string literal ‘1’ aren’t strictly the same, the objects compared in Listing 6-8 aren’t equal.

The other method I wanted to highlight is this one, which will test your block of code for a thrown exception. The only mandatory parameter here, is (as the method signature indicates) the first one, but you can also add pretty interesting behaviors using the second one.

There are many other methods to use, but they’re simply variations on the three we’ve just covered, so I’ll let you browse the documentation. Let’s now look at adding tests in Node using one of the most common libraries, Mocha.

Mocha

When it comes to testing libraries for Node, the list is always growing. You have some that add assertions, others that are full testing frameworks for TDD, others provide the tools you need if you’re practicing BDD, and I could keep going. Here I’ll focus on the one most people in the community seem to be using these days, and see what testing with it looks like.

Mocha² is a testing framework (not just an assertion library, it actually provides a full set of tools for us) that allows both asynchronous and synchronous testing, so considering that asynchronous functions are quite common in Node.js, this is a great choice already.

Testing Asynchronous Code

Before going into the specifics of how to test our project, I’m going to talk about one more feature provided by Mocha, since it will come in handy: asynchronous tests.

In order to test asynchronous functions using Mocha, you simply add a parameter to the callback on the function. This will tell Mocha that the test is asynchronous, so it will know to wait until that parameter is called upon (it’s going to be a function indicating the end of the test). It is worth noting that this function can only be called once per test, so if you try to call it more than once (or do so by accident), your test will fail.

There are many other features for this library that I haven’t covered (and won’t) in this chapter, so I urge you to go to its main website and browse through its documentation. Let’s now look at what it would look like to add some tests to our API project.

Testing: a Practical Example

Let’s now apply everything I’ve discussed so far into a single example. Consider an API project, one that is designed for a bookstore. This is a very straightforward API, so it will have a lot of classic CRUD endpoints. Let’s assume this API is using the classic MVC pattern for its internal architecture.

All resources are grouped into controllers, with each one having its own set of actions to perform.

For the purpose of this exercise and to show how you can add tests yourself, I’m going to show how to create tests for the create method of the BookSales controller . This controller simply takes care of listing and creating new resources (new book sales if you will), nothing else, and these actions are simple interactions with the database; so again, there is nothing too complex, since the focus here is on the test cases to add, not the actual code to test.

Let’s look at the code in Listing 6-16 first, and then I’ll do a quick overview of what’s being presented.

const assert = require("assert");

const restifyErrors = require("restify-errors")

const sinon = require("sinon")

const mongoose = require("mongoose")

const lib = require("../lib");

describe("Controllers", function () {

        describe("BookSales", function() {

                describe("#create", function() {

                        let BookSales;

                        //setup all we need for the tests

                        beforeEach(function() {

                                BookSales = require("../controllers/booksales")(lib);

                                sinon.spy(BookSales, "writeHAL")

                        })

                        //and tear down whatever we changed

                        afterEach(function(){

                                BookSales.writeHAL.restore();

                        })

                        //tests

                        it("should return an InvalidArgument exception if no body is provided in the request", function (done) {

                                BookSales.create({}, {}, function(err) {

                                        assert.ok(err instanceof restifyErrors.InvalidArgumentError)

                                        done();

                                })

                        })

                        it("should call the save method for the booksale model", function() {

                                //we'll spy on this method to understand when and how we call it

                                sinon.spy(mongoose.Model.prototype, "save")

                                BookSales.create({body: {totalAmount: 1}}, {})

                                assert.ok(mongoose.Model.prototype.save.calledOnce)

                                mongoose.Model.prototype.save.restore();

                        })

                        it("should call the writeHAL method", function() {

                                //we stub the method, so it can actually succeed even without a valid connection

                                sinon.stub(mongoose.Model.prototype, "save").callsFake( cb => cb() )

                                //we create a simple fake "json" property that will be called by writeHAL

                                BookSales.create({body: {totalAmount: 1}}, {json: () => {} })

                                assert.ok(BookSales.writeHAL.calledOnce)

                                mongoose.Model.prototype.save.restore();

                        })

                })

        })

}        )

Listing 6-16

Unit tests for the BookSales controller’s create method.

We begin by creating the groups for our tests. As I mentioned before, these groups can be anything we want; in my case I felt that grouping as Controllers ➤ [Controller name] ➤ [Method name] would come in handy.

After that, for the specific method we’re testing here, we’ll test the following:

• Make sure it returns the correct type of error message whenever the body for a new book sale is not present.

• Make sure it calls the save method on the model being created.

• Check that after a successful data save on the database, the controller is actually calling the writeHAL method , to create the correctly formatted response.

All three tests have different mechanics. The first one shows you how to use the done callback optionally available within each test. If you’re dealing with an asynchronous function, that’s how you tell it when to stop waiting for a response.

The second test is actually creating a spy on a method, so we can tell whether it was called. Note that in order to create the spy, we’re using yet another module, called SinonJS; this particular library works together with Mocha (or any other unit testing framework) and provides the same tools we saw earlier in this chapter: mocks, spies and stubs.

Finally, the third test case is creating a stub, because we need to control exactly how the insertion into the database works. (In this case, we end with a returned value of success as if the database was actually there.) This particular test also does not directly create or restore the spy on the writeHAL method for the controller; instead, that happens in the beforeEach and afterEach function callbacks, which are part of the testing flow executed by Mocha. They’re provided to exclude from the test case’s code anything that needs to happen for every single test.

Now that we’ve covered the code, let’s quickly look at its output to understand what you should be aiming to see. First, you execute it with the following line, assuming you’ve added the code from Listing 6-16 in a folder called tests:

$mocha tests/

The output should be something like Figure 6-1.

Here you can see the other point of having the groups; namely that the results are much easier to understand if the tests are properly grouped. Also notice the error message; even though the tests are all green, we’re showing an error message, and that’s completely normal, since the very first test is actually testing for the error type.

Tip

Even if error messages are normal, you still might want to catch them somehow (depending on how you’re printing out the error, possibly using a try-catch block, or redirecting the output based on an environment variable) in order to show only messages related to the output of the test, and avoid any clutter related to actual error messages. This could be useful if you have too much clutter from your own code.

Testing Approaches

There are different ways to implement integration testing into your system, each with its pros and cons. The most common ones are the big bang, top-down, and bottom-up approaches.

Let’s take a closer look at each one to understand when to use them and when to avoid them.

The Big Bang Approach

This approach consists of testing the entire system integration in one go. Forget about module-to-module communication; this approach goes for the entire platform at once.

This is great for small systems, with not a lot of interaction between components, that way if something goes wrong and there is an issue, you can quickly find the root cause for it. If your platform is complex and consists of multi-step interactions between modules, on the other hand, then this approach is not really recommended.

Because you’re testing everything at once, finding the root cause of a problem becomes a challenge. you can’t simply look at the output of your test and know which module failed; all you’ll know is that something went wrong, AND then it’s dive-into-logfiles time to try to see where the error is.

Figure 6-2 will help you visualize when a big bang approach is actually useful and when it can become a pain in the neck.

With this approach, finding root causes for issues turned up in the second example of Figure 6-2 would require you to go through the logs of four different modules in the hope of finding out what went wrong.

Another big problem with this approach is that it requires the entire system to be present to start applying it. You can’t partially test a user flow if you’re going big bang; you have to test the entire thing, so you need to wait for your team (or teams) to have a working version of all required modules before you can start implementing the integration tests between them.

If you’re dealing with complex systems, there are other ways to go about integration testing that might better suit your needs.

Biggest Challenges

According to LinkedIn’s engineers, some of the biggest challenges they met while changing their entire architecture and tech stack were monitoring and profiling.

Traffic Increase

This is probably one of the most obvious and common cases where in getting what you wanted, you end up regretting ever asking for it.

Let’s say you made a post in Hacker News about your brand-new app, and it suddenly got the attention of way too many people. Or maybe you published a new mobile app into Google’s Play store and it was featured, and now your APIs are receiving 400% more traffic than you expected for the first five months. Or maybe your online massive multiplayer game is now popular and even though your game servers are able to handle the load, your tiny log server is crashing every 2 hours from the amount of internal traffic it’s receiving.

With any of these, you now have a problem. It might directly affect your entire application, or it might be a challenge for only part of it, but you will have to fix it if you want your platform to perform as expected.

An increase in incoming traffic could affect your system in different ways; we can describe these as direct or indirect.

Indirect Effects

An increase in traffic can affect your application indirectly by overloading one of your internal processes. In a microservices-based architecture, the communication between services needs to be carefully planned. The fact that you’re capable of handling the increased traffic on your user-facing service doesn’t mean the rest of your architecture will be able to handle it.

Figure 1-1 shows exactly that case, where a resource-bound log server is crashing randomly and limiting the number of logs properly saved in your database. Good luck troubleshooting any other issues or bugs in your platform when that happens.

Another possible indirect effect occurs when an increase in traffic starts affecting the performance of a service you share with other platforms. This is what is normally known as service degradation , when the service is still active and working, but is responding slower than usual.

In this scenario, your lack of planning and scaling capabilities will start affecting those who use your service. This is why you always want to make sure that whenever you depend on third-party services, they can actually assure you that their service will not be degraded by anything.

Increased Processing Power Required

A need for increased processing power could be related to the previous case, and sometimes can even be caused by it; but it can also develop independently, which is why it’s worth discussing as a whole different category.

Specifically, this is purely a resource-related problem; you’re trying to process more information than your current resource utilization technique allows you to (“trying to bite off more than you can chew”). Notice that I didn’t blame the server directly; instead, I’m sharing the blame between you and your server.

In this scenario, you’re trying to do something with one or more sources of data, and suddenly, they start providing considerably more data than you expected. And when this happens, things can go wrong in one of two ways: your service may be degraded, or it may crash completely.

High Availability

For an architecture to be highly available, it must ensure that whatever service it provides will always be available and will not lose performance, despite having internal problems (such as a loss of processing nodes). The availability of a system is usually also known as its uptime, and commercially for service providers you sign onto an SLA (Service Level Agreement) which is measured in “nines” of availability. For instance, Amazon ensures three nines of availability for their S3 service, which in practice means they ensure 99.9% of monthly availability. Put another way, if their service fails more than 43.2 minutes a month, then they’ll be over service credits. More critical services, like mobile carriers, ensure five nines of availability, which translates to 99.999% of uptime, which in turn, translates to 5.2 minutes a year of downtime allowed.

There are several techniques that can be used to achieve this, but the most common one is the master-slave pattern.

You will often see this pattern discussed while reading about database scaling configurations, since most of them tend to go this route. It basically consists of having at least one more node for each of your main ones, and some kind of monitoring on the main nodes, so that if they go down, you can promote one of these “extra” nodes into a main one. In Figure 1-2, you have a standard three-node service, where node 3’s input depends on node 2’s output, and node 2’s input depends on node 1’s output. You can also see the “slaves,” which are the smaller dotted boxes next to them.

The final component of the master-slave model is the monitoring service, which makes sure all master nodes work correctly.

The second half of Figure 1-2 shows what happens when one of the nodes fails (in this case M2). Its slave node is promoted to master (essentially taking the place of M2, connecting M1’s output into its input and its output into M3’s input).

In practice you normally don’t need to worry about switching the connections or manually monitoring nodes for that matter; usually load balancers are used between nodes to do exactly that, act as “fixed” points of connectivity and decide by themselves (and a set of conditions you configure on them) whether or not to promote a slave.

That works well if your nodes are simple processing nodes; in other words, if you can simply exchange a master for a slave without any loss of data or any kind of information on your platform. But what happens if your nodes are part of a storage system, like a database? This scenario is slightly different from the previous one, because here, you’re not trying to avoid a lack of processing power, or a processing step in your flow. You’re trying to prevent data loss, without affecting your performance at the same time.

In this case, what you’ll want between your master and slaves (you could potentially have more than one slave per master), is a passive data-replication process (write only) from your master into your slaves. This will make sure that if your master is lost at any point, your slaves will be able to take over with minimal negative effects, as shown in Figure 1-3. (Note that you can’t prevent the master going down in mid-write, which would negate the transaction, preventing the data transfer from completing.)

In some cases, like Redis (with Sentinel enabled), slaves are not just there waiting to be promoted, they’re used for read-only queries, thus helping shed some load off their masters, which in turn, take care of all the write operations.

MongoDB, for another example, provides something called replica sets ,¹ which allow you to set up a group of nodes, in which one of them is the primary and therefore the one your code is constantly talking to, but if something happens to it, the secondary nodes will “elect” one of them to be the new primary (as seen in Figure 1-4).

Finally, another real-world example of HA is the newer versions of Hadoop’s HDFS. Prior to version 2, the HDFS (or Hadoop Distributed File System) had a single point of failure in its only NameNode. In other words, if that machine were to fail or be brought down due to maintenance, the entire cluster would be rendered inaccessible until it was brought back up. In newer versions, there is an option to set up as many redundant secondary NameNodes as you want. With this feature, these nodes effectively act as passive slaves to their master, getting sufficient state (via external services called journal nodes) to provide a fast failover if required (shown in Figure 1-5).

As a side-note, all DataNodes need to send heartbeats to all NameNodes to ensure that if a failover is required, it will happen as fast as possible.

As you can see, the slave-master approach can have slightly different implementations, but if you dig deep enough, they all end-up being the same.

Let’s take a look now at fault tolerance, to understand how it differs from HA.

Fault Tolerance

You can think of fault tolerance as a less strict version of HA. The latter was all about keeping the offline time of your platform to a minimum and always trying to keep performance unaffected. With FT, we will again try to minimize downtime, but performance will not be a concern—in fact, you could say that degraded performance is to be expected.

That being said, the most important difference between these two is that if an error occurs during an action, a highly available system does not ensure the correct end state of that action, while a fault-tolerant one does.

For example, if a web request is being processed by your highly available platform, and one of the nodes crashes, the user making that request will probably get a 500 error back from the API, but the system will still be responsive for following requests. In the case of a fault-tolerant platform, the failure will somehow (more on this in a minute) be worked-around and the request will finish correctly, so the user can get a valid response. The second case will most likely take longer, because of the extra steps.

This distinction is crucial because it will be the key to understanding which approach you’ll want to implement for your particular use case.

Usually fault-tolerant systems try to catch the error at its source and find a solution before it becomes critical. An example of this is having mirrored hard drives in case one of them fails, instead of letting a single drive fail. That would require replacing the entire server, affecting whatever actions the server could have been performing at the time.

Hardware-level fault tolerance is beyond the scope of this book; here, I will cover some of the most common techniques used to ensure FT at a software level.

Redundancy

One way to design fault-tolerant architectures is by incorporating redundancy into your key components. Essentially, this means that you have one or more components performing the same task and some form of checking logic to determine when one of them is has failed and its output needs to be ignored.

This is a very common practice for mission-critical components, and it can be applied to many scenarios.

For example, in 2012, SpaceX sent its Dragon capsule to berth with the International Space Station. During the ascent, the Falcon9 rocket used suffered a failure on one of its nine Merlin engines; and thanks to the implemented redundancy, the onboard computer was able to reconfigure the other eight engines to ensure the success of the mission.

Because these systems are so complex to code and to test, the cost-benefit ratio is not always something the normal software project can handle. Instead, these types of systems are usually present in critical applications, where human lives might be at risk (such as air traffic controllers, rocket guidance systems, and nuclear power plants).

Let’s go over some techniques to provide software redundancy and fault tolerance.

Triple Modular Redundancy

Also known as triple mode redundancy, TMR is a form of redundancy in which three systems perform the same process and their results are checked by a majority voting system that in turn produces a single output (see Figure 1-6). If one of the three systems fails, then the other two will correct it by providing the accurate output to the voting system.

This is a particular implementation of the N-modular redundancy systems, where as you might’ve guessed, you can add as many parallel systems as you see the need for, in order to provide a higher degree of fault tolerance for a given component. A particularly interesting real-world use case for this type of solution (in this case a 5-modular redundancy system) is the FlexRay² system.

FlexRay is a network communication protocol used in cars; it was developed by the FlexRay Consortium to govern onboard car computing. The consortium disbanded in 2009, but the protocol became a standard. Cars such as the Audi A4 and BMW 7 series use FlexRay. This protocol uses both data redundancy, sending extra information for problem detection purposes as metadata in the same messages, and structural redundancy in the form of a redundant communication channel.

Forward Error Correction

Yet another way to add a form of redundancy to the system, Forward Error Correction (FEC) adds redundancy into the message itself. That way, the receiver can verify the actual data and correct a limited number of detected errors caused by noisy or unstable channels.

Depending on the algorithm used to encode the data, the degree of redundancy on the channel may vary and with it, the amount of actual data that can be transferred through it.

There are two main types of encoding algorithms: block codes and convolutional codes. The first kind deals with fixed-length blocks of data, and one of the most common algorithms is Reed-Solomon. A classic example of this is two-dimensional bar codes, which are encoded in such a way that the reader can withstand a certain number of missing bits from the code.

Another very interesting real-world example of this type of redundancy can be found on the messages sent by the Voyager space probe and similar probes. As you can imagine, the communication with these devices can’t really afford retransmissions due to a faulty bit, so this type of encoding is used to ensure that the receiving end takes care of solving as many errors caused by a problematic channel as it can.

By contrast, convolutional codes deal with streams of arbitrary length of data, and the most common algorithm used for this is the Viterbi algorithm. This algorithm is used for CDMA (Code Division Multiple Access) and GSM (Global System for Mobiles) cellular networks, dial-up models, and deep-space communications (sometimes it’s even used in combination with Reed-Solomon to ensure that whatever defect can’t be fixed using Viterbi is fixed using R-S).

Byzantine Fault-Tolerance

I intentionally left this one for last, because it could be considered the sum of all of the above. What we have here is the “Byzantine Generals Problem”, basically a distributed system where some components fail, but the majority of the monitoring modules can’t reach a consensus. In other words, you’re in trouble.

Figure 1-7 shows a basic and high-level example of what this problem means for a platform architecture. In it, you have five replicas of the component C, which send their output to four different status checkers (A, R, M and Y), they in turn, exchange “notes” and try to reach a consensus regarding the data they all received. But because there is a problem, maybe with the data channel or with the fifth component, different values are sent to different checkers, so in this case a majority consensus can’t be reached.

There are different approaches to tackle this type of problem; in fact, there are too many out there to cover in a single chapter, so I’ll just go over the most common ones, to try to give you an idea of where to start.

The simplest approach, and it is not so much a solution as a workaround, is to let your status checkers default to a specific value whenever consensus can’t be reached. That way, the system is not stalled, and the current operation can continue.

Another possible solution, especially when the fault is on the data channel and not on the component generating the message itself, is to sign the messages with some sort of CRC algorithm, so that faulty messages can be detected and ignored.

Finally, yet another approach to ensure the authenticity of the message sent is to use blockchain, just as Bitcoin does, with a Proof of Work approach, in which each node that needs to send a message must authenticate it by performing a heavy computation. I’m simply mentioning this approach, since it could be the subject of an entire book, but the idea behind this approach is that it solves the Byzantine Generals problem without any inconvenience.

Layered Architecture

Figure 2-1 shows a very high-level overview of how this pattern expects the layers to be organized. The level of abstraction, as shown in the image, refers to the specifics of the business that the logic for your application is built around. In other words, the deeper you go into your layers, the more detail you’ll have to deal with regarding your specific business data model and business rules.

One common misconception about this pattern is that people confuse it with MVC (Model-View-Controller), thinking the latter to be the 3-tier version of this one. In the next pattern, I’ll go over the main differences between the two.

As a side-effect of using a layered architecture, the code inside a layer is all related to the same functionality (or at least, follows a set of standards common to the rest). This in turn helps developers work independently from each other on different layers. This allows the teams to make internal changes to the layers without affecting anything else (as long as the actual interface remains the same).

Some people also split the business layer into two, one in which the business knowledge remains, and another, often called the application layer, which owns the interaction logic between the UI and the business layer and also provides some common services useful for that interaction.

MVC Is Not Layered

If you’ve been doing any kind of web development for the last few years, you’ve probably heard or read about Model-View-Controller (MVC) . This pattern is one that many web frameworks have adopted (Ruby on Rails, Django, and Sails.js, to name a few), because the structure of most web projects resembles this approach.

That is, in most web projects you have a UI (or View); you most likely will want to handle the requests of your UI somewhere (the Controller); and finally, you most likely have a storage engine, inside of which you can probably force your data to fit into a set of Models that represent your resources. You’ll also want to perform some transformations on this.

Now, don’t get me wrong; MVC is not web-specific. In fact, you can use this pattern on any application that handles some sort of domain knowledge, has a representation for it, and is capable of performing actions on it. Let me summarize what the definition says for this pattern:

• The model is the boss: here’s where the domain knowledge or business logic is stored, here is where the actual data is handled, and here is where all the business specific coding should take place.

• The view is a simple representation of the model; there can even be several views for the same model. Remember that “representation” doesn’t mean “web page”; it means anything that can be read and understood by another system. A JSON object can be a view; this pattern could be applied to RESTful APIs just as much as it could be implemented to the front-end architecture of your very complex SPA.

• Finally, the controller is the poor guy who drew the short stick and is in charge of interconnecting the model and the view. It basically takes input from the view and passes it along to the model, while executing whatever command is needed on that input. Then, once the controller gets something back from the model, it sends it back to the view, updating it. In some cases, the controller can even avoid the last part (as seen in Figure 2-2), letting the model directly update the view.

We now have enough information to answer the question of how this is different from a 3-layered architecture. You have seen that the layered architectural pattern does not allow jumping layers when sending messages between them. You could say that for the n-tier architecture, the communication is completely linear (it needs to go through all intermediate layers in all cases), while for the MVC pattern, you can work around that (as seen in Figure 2-2), since it is more of a triangular setup. This flexibility can potentially be a negative for this pattern if performance is a big concern, since every extra layer that you add to it will definitely add latency to the communication (no matter how fast you make it work).

Finally, I want to mention some of the variations on this pattern. Over the years, many adaptations have been created to improve on it, depending on whether the goal is to have less component coupling, better testing capabilities, or simply to follow a similar logic but adapted to particular needs. For instance, the MVP (Model, View, Presenter) pattern aims to remove that (normally) unwanted interaction between Model and View, making the Presenter the sole man-in-the-middle taking care of passing information between its associated View (there is only one view for each presenter) and it’s Model. Another very common variation is the MVVM pattern (Model, View, ViewModel), which aims for a two-way data binding between the View and ViewModel. This in turns allows for automatic updates on the view, based on changes in the model.

Client-Server

The client-server pattern is a very simple yet powerful one. It consists of having a powerful server that provides meaningful services to many clients.

This pattern should sound very familiar to anyone who’s done any web development, since it’s the basic pattern for the World Wide Web. Browsers act as the many clients, which in turn request resources (web pages) from the different servers they interact with. Figure 2-3 shows what this looks like.

This pattern favors the off-loading of application logic into the server, where more hardware resources can easily be allocated. It also keeps the clients “thin” and “dumb,” in the sense that they usually don’t have a lot of business logic knowledge; instead, they simply know what to request from the server, and it is in the latter where all the heavy business-related computation will take place.

One of the main benefits of this approach is that making changes and fixes to either side does not necessarily mean affecting the other. That is, fixing a server bug doesn’t mean you need to even touch the client code, and vice-versa. Inherent security is another plus that comes out of this setup, since any core security check can be done on the server side, making sure any clients that are tampered with can still remain secure. (Think of a multiplayer game client, for example, where hackers can modify a player’s position; if the server is still checking for that, then the modification has no effect.) The clear separation of concerns between client and server is what give you that ability.

Another benefit from thin and dumb clients is that they’re easier to distribute (you don’t need a 2GB client when a simple 10MB will do, and you can keep the rest of the 2GB code in the server).

Finally, consider that even though Figure 2-3 shows a single server dealing with all the clients (essentially representing a monolithic approach), your server “box” can actually be expanded into a set of microservices or any other distributed architecture you might find more useful, where there is a single point of contact between dumb clients and business logic.

In fact, if you think about it, doing that you could very well end-up with a layered pattern, in which every layer is usually a different tier (physically separated from each other). As long as your client and server layers are physically separated, your client-server architecture is essentially a 2-tier one, in which the two constraints described earlier apply.

Master-Slave

The master-slave pattern at first glance could be confused with the previous one, since it implies a single channel of communication between two parties. But conceptually they’re very different, because instead of having one centralized hub for the business logic and heavy processing of data, you have a one-way controlled communication between a controlling node (the master) and many decentralized nodes (the slaves). Figure 2-4 shows an example of this architecture.

Event-Bus or Event-Driven Architectures

This pattern is a very interesting one in the sense that it breaks the mold of what I’ve been telling you about so far. Event-driven architectures don’t have the one-to-many or many-to one type of relationship between components; instead, they are usually many-to-many relationships. Let’s first take a look at Figure 2-5, which shows a basic example of an event-driven architecture and then I’ll go into more detail about it.

This pattern is extremely powerful for providing a highly available platform, or even when trying to scale up—as long as your architecture follows the guidelines of the diagram in Figure 2-5. In other words, you need to avoid component-to-component communication and allow them to interact with each other only through the event bus.

If you do that, you can potentially replace crashed nodes with new instances in the time it takes them to boot up. The same happens if you need more processing power; you simply add new consumer or sources and connect them to the event bus, and that’s all.

A good idea when dealing with this type of architecture is to use a third-party data bus (as long as that’s an option), because that will allow you to focus on creating the event sources and event consumers, while at the same time using a tried-and-tested bus, one that can reliably transmit the data and scale when needed.

A classic example of this approach consists of using a message queue as the event bus. In this case you wouldn’t want to create your own bus; you would most likely want to use one of the many existing solutions such as RabbitMQ, Kafka, ZeroMQ or any other.

Microservices Architecture

This pattern is one of the best-known , since it’s been growing in popularity in the last few years. Everybody and their mother is jumping into the microservices bandwagon, whether they have use for them or not. Just like with anything in our industry, there is no silver bullet solution; so hoping that microservices will solve all your problems without taking into consideration its pros and cons is reckless, at best.

The core behind this pattern is your understanding of the different features your platform is supposed to have (that is, the different services it needs to provide). If you properly understand your platform, you can probably split the services into a set of smaller services that, when used together by the client app, yield the same result you would get from a single block of code with all features bundled together. Figure 2-6 shows an example.

By splitting your platform into individual services, you gain a new level of control over it that you never had with a monolithic approach. Figure 2-6 shows an example of a fake platform that takes care of payments. It also needs a proper authentication strategy, so it also has a dedicated Auth service. Every user needs to be authenticated against it to start using the platform, and then every request is authorized against the service. Users also have a way to register into the system, which is why there is a UserMgnmt service .

This pattern might sound like the best solution for most problems, but you need to take into account that, as with any other option, you might run into problems because you’re trying to meet your needs with the wrong architecture.

In the end, it’ll be a matter of picking the right tool for the job, as with everything else.

The Broker Pattern

You can think of the broker pattern as a specialization of the microservices architecture. One of the pain points for the latter was that given a high enough number of microservices, you begin to need a form of orchestration; otherwise, your clients start to lose the ability to communicate easily with your platform. You start to burden them with the knowledge of where everything is in your system, and that should not be the case. Ideally, clients should be able to discover your services organically and with minimal previous knowledge.

Here is where the broker pattern comes into play. Its main component is a node called broker, whose purpose is to centralize and redistribute requests among different services.

Another key characteristic of this pattern is that by default it is not the broker that “knows” about its servers; instead, it’s the servers that register with the broker once they come online, and provide all the information it needs to understand the services they provide.

Figure 2-7 shows an example of a broker providing services from three different servers. This pattern is very common among message queues, such as RabbitMQ,¹ Apache Kafka,² and Apache ActiveMQ.³

Vertical or Horizontal Scaling?

When it’s time to start scaling, the most basic things you can do are either improving the hardware your code is running on, or simply adding more computers to distribute the load among them.

That is the essence of vertical and horizontal scaling (as seen in Figure 3-1). You can think of this as changing your old car’s tires compared to buying a new Ferrari when trying to get more horsepower. With the first method you’re always trying to add resources to your existing hardware: memory, processing power, disk, anything that might help out depending on your needs.

This approach normally requires some downtime because of the physical changes that need to take place. Whatever benefits you might get from doing this are limited by the resources you must add to the hardware. There is, of course, a sort of improved vertical scaling option, which is available if you’re in a cloud-based environment. In these situations, your cloud provider will allow you to switch your application from one type of hardware to another. This can be done with a few clicks. Options are limited, but at least you’re not constrained by your original hardware’s specs. For example, Amazon will let you switch your t2.nano instance that only has half a Gigabyte worth of RAM for a t2.2xlarge one, which actually has 32 GB of RAM and 8 virtual CPUs. If memory or CPU power was your bottleneck, this would solve it).

The main problem with vertical scaling, though, is that sooner or later you’re going to hit the cap for your scaling capacity. No matter what service you’re paying for, eventually you’ll run out of specs to improve.

And here is where horizontal scaling enters. Instead of improving a single server, you’re now working with a pool of them, and adding to that pool is as simple as connecting the new server to it (or removing one from it). You can design your application to withstand such changes and avoid any real downtime when scaling. With that you gain “elasticity” in your capacity, and control over how much money you want to spend, during both normal operation and peak hours.

Taking Advantage of the Cloud

The Power of a Multi-Zone Deployment

Although it is not strictly related to scaling your architecture, taking advantage of a multi-zone deployment on the cloud is a must if you have the chance. This is usually something that most of the major cloud providers already allow, so you should consider, when planning your architecture, deploying and even scaling into multiple zones.

By doing this, you gain tolerance for big network outages that tend to affect an entire geographical zone. These types of network problems aren’t very common, but when they happen, big sections of the internet are isolated from the world, and this is something you want to avoid.

There are two types of resources to consider this technique for: services and data. If it’s services you’re deploying, then all you have to do is make sure you’re deploying behind a load balancer (such as Elastic Load Balancing for AWS). Figure 3-2 shows an example of such a case. By doing so, you’ve made sure that no matter what happens, your services will always be available.

But if you’re actually deploying data, or a data storage platform such as a database, this is also a good idea for you. To make it work, however, requires some extra work. In the case of data, what you want is to make it available no matter what. To accomplish this, you must make sure you’re properly replicating your data. Figure 3-3 shows one example of what such a deployment might look like.

Clustering with PM2

The cluster module is great, because it allows you to make the most of your CPU with minimum effort; but that’s about all it can do for you. If you want more control over how it’s done, or need to know a bit more about what exactly is happening with your cluster, there is very little you can do out of the box.

With that command, you’ll be starting up your application, naming it “my app” in PM2’s list of processes, and taking advantage of all your CPU cores (thanks to the -i max modifier). Figure 3-5 shows the output from the start command .

Figure 3-6 shows the possible output from the logs. As you can see, the same line is repeated four times, thanks to the four processes running in parallel (because of my four cores).

In Conclusion

Clustering is and should be your first step toward scaling whatever Node.js application you might be working on. Depending on your needs, you might want to go with the cluster module. You don’t need anything extra; simply add a few lines of code and you’re done. On the other hand, if you’re looking to avoid changing your code, and you need extra support for your production environment, then PM2 or similar solutions should definitely be your choice.

In Conclusion

Microservices is a topic that you’ll read about throughout this book because it is a very helpful pattern for scaling Node.js platforms (since Node is usually used to create APIs).

That being said, and as I’ve already covered in previous chapters, it is not a silver bullet and will not work for you every time. You need to remember when this pattern is helpful and how you need to prepare your code and your team to be able to get the most out of it.

You’re Just Logging into stdout and stderr

And what makes it even worse is that you’re not wrapping the output function/method of your language of choice into a construct under your control. In other words, and in the Node.js universe, you’re logging using console.log and console.error.

This is great for small projects and quick PoC (Proof of Concept) , but if you’re interested in getting anything out of your logs, then you need to do something about it. In particular, because both the stdout and stderr are local to each server instance, if you start scaling your application into multiple servers, you’ll have to deal with distributed logs that aren’t being saved anywhere (or maybe they are, depending on your setup).

This will work, even if you’re not saving the data anywhere, since PM2 will catch all your output and save it automatically, just in case. But that will only get you halfway, since we also need to send those log files into a centralized location.

Because there are so many options, and so many variations of similar solutions, I’m just going to cover a simple one, assuming you have an ELK⁴ (Elastic, Logstash, and Kibana) cluster configured and ready to receive logs somewhere on your architecture. This will act as the centralized logging and analytics system from Figure 4-1.

What you want to do in this situation is configure something that will ship the log files stored by PM2, into Logstash, which in turn will apply any transformation you might need to the data and then send it and index it into Elastic for your consumption using Kibana.

This might sound like a lot at first glance, especially if this is your first time dealing with something like this, but it is a scalable way of going about it. If you do it right, you gain the ability to support failures and downtimes on your Elastic cluster, you get back-pressure, on your ogging pipeline, making sure you’re not overwhelming your analytics platform, and so on.

To achieve this, you’ll install and configure Filebeat in all your servers (all those that need to send data out). Filebeat is essentially a log shipper that follows a standard protocol called Beat. This shipper (and its associated protocol) is the result of several years of iteration by the team of Elastic to get the best and most lightweight log shipper possible.

That configuration will pull the contents of the log files for your app (stored in a default location by PM2) and into a Logstash server. You need to replace the constants YOURHOMEFOLDER and LOGSTASH-HOST by the actual values to make it work.

With that, you’re ready to retrieve your log files (if you’re crazy enough to log using only console.log). But you still need to configure Logstash to make sure you can parse these logs, transform them (if needed) and then index them into Elastic. So stop smiling, and keep reading.

Note that the configuration from Listing 4-3 will only receive and index data; it will not transform (which is one of the key benefits of Logstash) anything. So if you wanted to do some extra tweaking of your logs before indexing them, I recommend looking at the full documentation for Logstash.⁶ Also, make sure to match the port under the configuration for the Beat plugin with the port specified on the Filebeat config file (Listing 4-1).

You’re now set. Congratulations, you’ve managed to avoid a major problem by using the right set of tools.

If, on the other hand, you weren’t using anything like PM2 that would catch the output of your process and save it into a file, you’re out of luck. You’ve lost your logs to the black hole that lives inside every server and there is no way for you to retrieve them. So don’t do it like this.

You’re Logging into a Single File

You could potentially take care of both from inside your own code, by adding extra logic to your logger and make sure you properly keep the size and history of your logs in check. You can also use an external tool, such as the classic logrotate⁷ command line utility, which is already part of most (if not all) Linux distributions.

This is definitely the preferred way of handling this logic instead of writing it directly into your own logger’s code. But you’re not there yet. You now have your own log file, and you’re properly making sure it doesn’t grow out of hand, but you still need to send its content into a centralized location. You can look at the previous point in order to understand how to configure Filebeat and Logstash.

With that last bit of configuration, you’re ready to move on with your development, because you again have a stable logging system within your platform.

Single Points of Failure? No Thank You!

It may seem obvious, but you should avoid having single points of failure (SPOFs) whenever possible. They are basically your weakest link and if they break, your entire application/platform/system will be rendered useless.

That being said, they also have the habit of sneaking into some architectures when you’re not looking for them. This is especially true when systems start growing organically instead of systematically, and you start adding new bits and pieces based on your most recent needs, without thinking about the future. For example, without going too far from what we’ve been talk about, imagine having to start sharing memory between processes. You’ve followed my earlier advice, but miss the very crucial part about setting up your external memory in cluster mode, so you end up with something that looks like Figure 4-3.

In this scenario, all services depend on Redis, but Redis is not set up to be able to handle any type of problems; in fact, there is only one instance of Redis running. If it fails, your entire system will be rendered useless. The fix for this one is easy enough; just change the way you have your Redis running. You’d probably set it up as a cluster, or with Sentinel,¹³ so your master nodes will have a slave assigned just in case.

Getting rid of single points of failure can be as easy as setting up a cluster or as hard as adding redundancy to your entire platform. It mostly depends on how far you can take the process until it no longer provides enough bang for your buck.

Let me give you another example, going back to Figure 4-3. After you set up your minimum Redis cluster (which by definition will already contain three master nodes and three slave nodes) you realize that all nodes could fail, because they’re servers you actually have inside your own datacenter. So you decide to move to the cloud and deploy your own Redis cluster in AWS. You then realize that all those servers could still be unreachable if there is a region-level outage, so you again redeploy on a multi-region setup. Great, but what if Amazon goes out of business (unlikely, I know, but bear with me for a second)? You now redeploy your multi-zone Redis cluster in Azure as well, and you have both multi-zone load balancers and extra logic in your code to determine when to pick each one.

You’re now paying a whole lot more money for a level of stability that you might never need.

Alerting

A final concept for monitoring in general, before we start delving deeper into each of the levels I just mentioned, is alerting . This can mean anything; it can be sending an email to the user or it can be notifying some system that can it perform predefined actions. Whatever it might mean to you, it’s the whole point of this practice: you monitor and then you react to the values received.

Deciding when to send those alerts is also very important; both the triggering condition and the periodicity of the messages need to be taken into account to provide a good user experience and add value. Alerting for the sake of alerting is the worst thing you can do; the only thing you’ll achieve by doing that is overwhelming your users to the point where received alerts will be ignored.

Monitoring Your Apps

Specifically when it comes to monitoring Node.js applications, there are too many possibilities to cover them all in this book, and eventually we would start seeing repeated features. And we’re not just looking for a monitoring tool, remember that we’re trying to set up an environment that can scale up and down, according to our needs. So we need a solution that can monitor and react to the values receive by triggering a set of scaling rules.

So instead of reviewing products such as PM2 Enterprise,¹ or even Prometheus,² I’m going to review how to set up AWS CloudWatch³ to monitor your application and your infrastructure at the same time (so you’ll be able to cover the entire pyramid from Figure 5-1) and on top of that, how to make it react and trigger new instances of your application. Figure 5-2 shows the base architecture we’ll use for this example.

The idea behind this example is that CloudWatch will be gathering metrics both from our own application (ad-hoc metrics) and from AWS services that already provide them automatically (such as EC2). These metrics will be monitored by AWS CloudWatch Alerts and displayed and charted by AWS CloudWatch Dashboard. The alerts will trigger actions, which can be anything ranging from a notification (via email, SMS, or even an SQS queue) to a call to the AWS AutoScaling service.

In order to look at this, we’ll use a sample application, so you can see how the ad-hoc metrics can be provided to AWS using their own libraries. This sample app is going to be mind-numbingly simple, but the point is to avoid distracting you by looking at unimportant code, so bear with me.

To start the refactoring process, we’ll begin by installing the SDK module from AWS, and with it, we’ll create a basic metric reporter, which will let us send custom metrics back to CloudWatch.

That file should be stored in ~/.aws/credentials (or your usual Windows equivalent), and you can get that information from AWS Console, while creating your users or by regenerating these keys (for security reasons the AWS system does not allow you to download existing keys).

Let’s break down the code from Listing 5-6, because there is a lot in there, even though it might not seem like it.

I’ve added all the code inside the request handler; this is for simplicity, but you can refactor it as you see fit. In it you can see I’m just calculating the time difference between the previous line actually getting the current date and the line after it. In other words, I’m just going to be reporting the performance of this endpoint and the number of requests it received (simply by sending the first of the two report calls).

Because this is clearly a very simple endpoint, I’m adding a few random numbers to the time difference calculation, just to add some variability to our results (you’ll want to omit that random bit in your own code, obviously).

I’m reporting the data using our wrapper module from Listing 5-4. The parameters seen here are what are important. In both cases, the first one is the namespace. This is key because it allows you to group your metrics in CloudWatch’s console. As you’ll see in Figure 5-3, both of our metrics will be saved in the same API/TRAFFIC namespace. The second one is the actual metric itself; the first report simply sends a 1, as a count, but the second report call sends the time difference, on milliseconds (see the unit parameter). Finally the third parameter is the dimension; you can have up to 10 dimensions for each namespace and they will allow you to further group your metrics.

After you’re ready with your code and you’re properly sending the data up to CloudWatch, you can begin creating your plots Figure 5-4 shows the first one, which is plotting the number of requests received by our API.

We do the same for the other metric (see Figure 5-5), but instead of adding up the numbers, we leave the Statistic column at Average, because in this case, we want the average of the delay for all requests received within a minute.

Adding AWS Metrics into Your Dashboard

Compared to adding your own custom metrics, this should be a walk in the park; but nevertheless it’s also something you should be aware of, since it will allow you to monitor your infrastructure.

The great thing about using CloudWatch is that it is very straightforward. Remember the earlier section on the AWS/ namespace? Well, for every one of its services you use, several namespaces will pop-up under this section, letting you pick the metrics you care most about. Maybe you want to look at CPU utilization, or memory consumption, disk read operations, network traffic or a lot of other indicators.

Again, this will happen automatically, so all you have to do is start using your services, and wait a few minutes until those resources finish booting up and connecting (usually no more than 5 minutes). After that, you can go to the CloudWatch console inside the metrics section, and you’ll find all the new namespaces waiting for you. Figure 5-6 shows how after starting a single EC2 instance and setting up an RDS MySQL instance, three different official namespaces have appeared.

As you can see, AWS provides a lot of metrics for each service, so you can make sure you are aware of everything you need for your infrastructure-related metrics. And based on that, you can create whatever scaling rules you need (more on that in a bit).

Reacting to Your Metrics

This is the last step in the process. So far, you’ve being setting up your monitoring system to look for both custom and default metrics received by your infrastructure and your own Node.js application. You’ve created at least one dashboard (maybe more), and you can log in into AWS Console any time you want and look at those numbers.

But we want that process to also be automated; it makes no sense to have you, or anyone else, systematically logging in and checking numbers, does it? So instead, let’s look at how you can use AWS Console and its services to understand how to automate that process.

That is the basic process you need to go through to automate the reaction to your metrics. Let’s now go into a bit more detail on each step.

Step 1: Creating Your Alerts

From within the CloudWatch console, you can create a new Alert, first by specifying the metrics to use, and then by specifying how to treat those numbers (as we did for the charts), as seen in Figure 5-7. You’ll have to select the namespace, which will have your own ones and the default ones from AWS. After that, you’ll get the list of metrics within that namespace. Simply select the one you want to alert on and the chart below it will update, so you can verify the data.

You can play around with the chart in Figure 5-7 as much as you like, even changing the operation to perform on the data (as we did for the Statistic column a while ago) and the time period for the sampling of these numbers. After you’re happy with the results, you can move on to the final step, which will take care of setting up the actions for this alert.

Figure 5-8 shows the second step of this process, where you can set the name for the alert and the values to react to (the threshold). Finally, you can select a notification list, or create one on the spot.

Note

After you’re done with the alert, it will appear on your alert list (under the CloudWatch console). Initially this alarm will appear on the INSUFFICIENT list, simply because this alert will take a few minutes to start reporting the data needed. Just wait 5 minutes and the alert should go into another section (either OK or ALARM if it’s being triggered).

The Definition

This example is still quite simple, but I’ve added a bit more complex internal logic to the function called myFunction by adding the ability to pass in the mathematical function to apply as the first parameter. With this new logic, the function we’re testing is bigger, and it does different things, so if we design our tests as in Listing 6-2, we can run into a problem: what happens if our test fails?

I haven’t really covered what it means to “run” our tests, but it should be pretty obvious by now: your code is executed and the assertions are verified; if they are true, then your test will succeed, but if they fail (your assertion stated that two values were to be equal and in practice, they aren’t), then your entire test fails. You can see the problem if you’re testing several things within the same test. Once you get the results back from the execution, you will have to dig deeper into the execution logs (if there are any) to understand where your problem lies.

Now whenever a test fails, you’ll get better details from your test runner, because you’ll know exactly which test failed, and thus, you’ll be able to determine immediately which block of code in your complex function failed.

The word unit is the most important one there, because if you go online, you’ll probably find a lot of people defining it as your functions (provided you’re using a procedural programming language) or your methods (if you’re using an OOP language). But as you can see from a simple generic and pseudocode-based example, a unit of code can actually be smaller than that. It’s true that in all these examples I didn’t really show the actual code of the function, and you could argue that for each test case of Listing 6-3, our function is actually calling other, smaller functions, and that is a very good point!

So putting both definitions together, you get a pretty accurate idea of what testing your code means and a good basis for the rest of this chapter.

The Tools

Now that we’ve covered what unit testing is, let’s review the tools provided by this methodology that will allow you to test your code.

These are not software tools; they aren’t libraries or frameworks you can use. We’re not there yet. What I’m trying to give you here are the concepts, the wheels you’ll use to build your car down the road.

Best Practices

I’ve already covered some of these briefly in the previous section, but it’s a good idea to review the full list of recommended practices when writing tests. Like anything in software development, it’s never a solo effort; even if you’re the only one writing code right now, you have to think about the future.

Testing without External Modules

Let me first talk about this option, it’s probably not the way to go, since the provided module is pretty basic, but if you’re looking for something that’s quick and dirty, this will do the job.

One of the major things you’ll notice this library is missing is the rest of the framework; with it, you only get the assertion support. The rest will have to come from you or someplace else, but let’s look into it anyway.

As I already mentioned, this module does not require any kind of installation steps, since it’s already provided with Node’s installation. All you have to do to use it is to require the module assert. After you do so, you’ll gain access to a set of assertion methods, which basically help you compare two values (or objects).

This method performs a deep comparison between two objects. That means Node will recursively compare (using the strictly equal operand) properties within the objects, and if that comparison fails it will throw an AssertionError.

As expected, because in JavaScript, the number 1 and the string literal ‘1’ aren’t strictly the same, the objects compared in Listing 6-8 aren’t equal.

The other method I wanted to highlight is this one, which will test your block of code for a thrown exception. The only mandatory parameter here, is (as the method signature indicates) the first one, but you can also add pretty interesting behaviors using the second one.

There are many other methods to use, but they’re simply variations on the three we’ve just covered, so I’ll let you browse the documentation. Let’s now look at adding tests in Node using one of the most common libraries, Mocha.

Mocha

When it comes to testing libraries for Node, the list is always growing. You have some that add assertions, others that are full testing frameworks for TDD, others provide the tools you need if you’re practicing BDD, and I could keep going. Here I’ll focus on the one most people in the community seem to be using these days, and see what testing with it looks like.

Mocha² is a testing framework (not just an assertion library, it actually provides a full set of tools for us) that allows both asynchronous and synchronous testing, so considering that asynchronous functions are quite common in Node.js, this is a great choice already.

Testing Asynchronous Code

Before going into the specifics of how to test our project, I’m going to talk about one more feature provided by Mocha, since it will come in handy: asynchronous tests.

In order to test asynchronous functions using Mocha, you simply add a parameter to the callback on the function. This will tell Mocha that the test is asynchronous, so it will know to wait until that parameter is called upon (it’s going to be a function indicating the end of the test). It is worth noting that this function can only be called once per test, so if you try to call it more than once (or do so by accident), your test will fail.

There are many other features for this library that I haven’t covered (and won’t) in this chapter, so I urge you to go to its main website and browse through its documentation. Let’s now look at what it would look like to add some tests to our API project.

Testing: a Practical Example

Let’s now apply everything I’ve discussed so far into a single example. Consider an API project, one that is designed for a bookstore. This is a very straightforward API, so it will have a lot of classic CRUD endpoints. Let’s assume this API is using the classic MVC pattern for its internal architecture.

All resources are grouped into controllers, with each one having its own set of actions to perform.

For the purpose of this exercise and to show how you can add tests yourself, I’m going to show how to create tests for the create method of the BookSales controller . This controller simply takes care of listing and creating new resources (new book sales if you will), nothing else, and these actions are simple interactions with the database; so again, there is nothing too complex, since the focus here is on the test cases to add, not the actual code to test.

Let’s look at the code in Listing 6-16 first, and then I’ll do a quick overview of what’s being presented.

const assert = require("assert");

const restifyErrors = require("restify-errors")

const sinon = require("sinon")

const mongoose = require("mongoose")

const lib = require("../lib");

describe("Controllers", function () {

        describe("BookSales", function() {

                describe("#create", function() {

                        let BookSales;

                        //setup all we need for the tests

                        beforeEach(function() {

                                BookSales = require("../controllers/booksales")(lib);

                                sinon.spy(BookSales, "writeHAL")

                        })

                        //and tear down whatever we changed

                        afterEach(function(){

                                BookSales.writeHAL.restore();

                        })

                        //tests

                        it("should return an InvalidArgument exception if no body is provided in the request", function (done) {

                                BookSales.create({}, {}, function(err) {

                                        assert.ok(err instanceof restifyErrors.InvalidArgumentError)

                                        done();

                                })

                        })

                        it("should call the save method for the booksale model", function() {

                                //we'll spy on this method to understand when and how we call it

                                sinon.spy(mongoose.Model.prototype, "save")

                                BookSales.create({body: {totalAmount: 1}}, {})

                                assert.ok(mongoose.Model.prototype.save.calledOnce)

                                mongoose.Model.prototype.save.restore();

                        })

                        it("should call the writeHAL method", function() {

                                //we stub the method, so it can actually succeed even without a valid connection

                                sinon.stub(mongoose.Model.prototype, "save").callsFake( cb => cb() )

                                //we create a simple fake "json" property that will be called by writeHAL

                                BookSales.create({body: {totalAmount: 1}}, {json: () => {} })

                                assert.ok(BookSales.writeHAL.calledOnce)

                                mongoose.Model.prototype.save.restore();

                        })

                })

        })

}        )

Listing 6-16

Unit tests for the BookSales controller’s create method.

We begin by creating the groups for our tests. As I mentioned before, these groups can be anything we want; in my case I felt that grouping as Controllers ➤ [Controller name] ➤ [Method name] would come in handy.

After that, for the specific method we’re testing here, we’ll test the following:

• Make sure it returns the correct type of error message whenever the body for a new book sale is not present.

• Make sure it calls the save method on the model being created.

• Check that after a successful data save on the database, the controller is actually calling the writeHAL method , to create the correctly formatted response.

All three tests have different mechanics. The first one shows you how to use the done callback optionally available within each test. If you’re dealing with an asynchronous function, that’s how you tell it when to stop waiting for a response.

The second test is actually creating a spy on a method, so we can tell whether it was called. Note that in order to create the spy, we’re using yet another module, called SinonJS; this particular library works together with Mocha (or any other unit testing framework) and provides the same tools we saw earlier in this chapter: mocks, spies and stubs.

Finally, the third test case is creating a stub, because we need to control exactly how the insertion into the database works. (In this case, we end with a returned value of success as if the database was actually there.) This particular test also does not directly create or restore the spy on the writeHAL method for the controller; instead, that happens in the beforeEach and afterEach function callbacks, which are part of the testing flow executed by Mocha. They’re provided to exclude from the test case’s code anything that needs to happen for every single test.

Now that we’ve covered the code, let’s quickly look at its output to understand what you should be aiming to see. First, you execute it with the following line, assuming you’ve added the code from Listing 6-16 in a folder called tests:

$mocha tests/

The output should be something like Figure 6-1.

Here you can see the other point of having the groups; namely that the results are much easier to understand if the tests are properly grouped. Also notice the error message; even though the tests are all green, we’re showing an error message, and that’s completely normal, since the very first test is actually testing for the error type.

Tip

Even if error messages are normal, you still might want to catch them somehow (depending on how you’re printing out the error, possibly using a try-catch block, or redirecting the output based on an environment variable) in order to show only messages related to the output of the test, and avoid any clutter related to actual error messages. This could be useful if you have too much clutter from your own code.

Testing Approaches

There are different ways to implement integration testing into your system, each with its pros and cons. The most common ones are the big bang, top-down, and bottom-up approaches.

Let’s take a closer look at each one to understand when to use them and when to avoid them.

The Big Bang Approach

This approach consists of testing the entire system integration in one go. Forget about module-to-module communication; this approach goes for the entire platform at once.

This is great for small systems, with not a lot of interaction between components, that way if something goes wrong and there is an issue, you can quickly find the root cause for it. If your platform is complex and consists of multi-step interactions between modules, on the other hand, then this approach is not really recommended.

Because you’re testing everything at once, finding the root cause of a problem becomes a challenge. you can’t simply look at the output of your test and know which module failed; all you’ll know is that something went wrong, AND then it’s dive-into-logfiles time to try to see where the error is.

Figure 6-2 will help you visualize when a big bang approach is actually useful and when it can become a pain in the neck.

With this approach, finding root causes for issues turned up in the second example of Figure 6-2 would require you to go through the logs of four different modules in the hope of finding out what went wrong.

Another big problem with this approach is that it requires the entire system to be present to start applying it. You can’t partially test a user flow if you’re going big bang; you have to test the entire thing, so you need to wait for your team (or teams) to have a working version of all required modules before you can start implementing the integration tests between them.

If you’re dealing with complex systems, there are other ways to go about integration testing that might better suit your needs.

Biggest Challenges

According to LinkedIn’s engineers, some of the biggest challenges they met while changing their entire architecture and tech stack were monitoring and profiling.