A migration source provides the initial rows that are passed to any registered processors and then ultimately to the destination entity. Drupal 8 core provides a SqlBase class that serves as the base class for migrations that read from the database. The migrate_example module contains the following:

<?php 
 
/** 
 * @file 
 * Contains  
 * \Drupal\migrate_example\Plugin\migrate\source\BeerNode. 
 */ 
 
namespace Drupal\migrate_example\Plugin\migrate\source; 
 
use Drupal\migrate\Plugin\migrate\source\SqlBase; 
use Drupal\migrate\Row; 
 
/** 
 * Source plugin for beer content. 
 * 
 * @MigrateSource( 
 *   id = "beer_node" 
 * ) 
 */ 
class BeerNode extends SqlBase { 
 
  /** 
   * {@inheritdoc} 
   */ 
  public function query() { 
    /** 
     * An important point to note is that your query *must* return 
     * a single row for each item to be imported. Here we might be 
     * tempted to add a join to migrate_example_beer_topic_node in  
     * our query, to pull in the relationships to our categories. 
     * Doing this would cause the query to return multiple rows  
     * for a given node, once per related value, thus processing  
     * the same node multiple times, each time with only one of  
     * the multiple values that should be imported. To avoid that,  
     * we simply query the base node data here, and pull in the  
     * relationships in prepareRow() below. 
     */ 
    $query = $this->select('migrate_example_beer_node', 'b') 
      ->fields('b', ['bid', 'name', 'body', 'excerpt', 'aid', 
               'countries', 'image', 'image_alt', 'image_title', 
               'image_description']); 
    return $query; 
  } 
 
  /** 
   * {@inheritdoc} 
   */ 
  public function fields() { 
    $fields = [ 
      'bid' => $this->t('Beer ID'), 
      'name' => $this->t('Name of beer'), 
      'body' => $this->t('Full description of the beer'), 
      'excerpt' => $this->t('Abstract for this beer'), 
      'aid' => $this->t('Account ID of the author'), 
      'countries' => $this->t('Countries of origin. Multiple values, delimited by pipe'), 
      'image' => $this->t('Image path'), 
      'image_alt' => $this->t('Image ALT'), 
      'image_title' => $this->t('Image title'), 
      'image_description' => $this->t('Image description'), 
      // Note that this field is not part of the query above - it  
      // is populated by prepareRow() below. You should document  
      // all source properties that are available for mapping  
      // after prepareRow() is called. 
      'terms' => $this->t('Applicable styles'), 
    ]; 
 
    return $fields; 
  } 
 
  /** 
   * {@inheritdoc} 
   */ 
  public function getIds() { 
    return [ 
      'bid' => [ 
        'type' => 'integer', 
        'alias' => 'b', 
      ], 
    ]; 
  } 
 
  /** 
   * {@inheritdoc} 
   */ 
  public function prepareRow(Row $row) { 
    /** 
     * As explained above, we need to pull the style relationships  
     * into our source row here, as an array of 'style' values  
     * (the unique ID for the beer_term migration). 
     */ 
    $terms = $this->select('migrate_example_beer_topic_node',  
             'bt') 
             ->fields('bt', ['style']) 
             ->condition('bid', $row->getSourceProperty('bid')) 
             ->execute() 
             ->fetchCol(); 
    $row->setSourceProperty('terms', $terms); 
 
    // As we did for favorite beers in the user migration, we need  
    // to explode the multi-value country names. 
    if ($value = $row->getSourceProperty('countries')) { 
      $row->setSourceProperty('countries', explode('|', $value)); 
    } 
    return parent::prepareRow($row); 
  } 
} 

A migration source implements \Drupal\migrate\Plugin\MigrateSourceInterface, which specifies the following functions:

• fields(): It returns an array of fields that are present in each row
• getIds(): It returns an array of fields that uniquely identify each row
• prepareRow(): It returns the modified row prior to being passed to the processors; if FALSE is returned, the row is skipped