+ * Available configuration keys:
+ * - database_state_key: (optional) Name of the state key which contains an
+ * array with database connection information.
+ * - key: (optional) The database key name. Defaults to 'migrate'.
+ * - target: (optional) The database target name. Defaults to 'default'.
+ * - batch_size: (optional) Number of records to fetch from the database during
+ * each batch. If omitted, all records are fetched in a single query.
+ * - ignore_map: (optional) Source data is joined to the map table by default.
+ * If set to TRUE, the map table will not be joined.
+ *
+ * For other optional configuration keys inherited from the parent class, refer
+ * to \Drupal\migrate\Plugin\migrate\source\SourcePluginBase.
+ *
+ * About the source database determination:
+ * - If the source plugin configuration contains 'database_state_key', its value
+ * is taken as the name of a state key which contains an array with the
+ * database configuration.
+ * - Otherwise, if the source plugin configuration contains 'key', the database
+ * configuration with that name is used.
+ * - If both 'database_state_key' and 'key' are omitted in the source plugin
+ * configuration, the database connection named 'migrate' is used by default.
+ * - If all of the above steps fail, RequirementsException is thrown.
+ *
+ * Drupal Database API supports multiple database connections. The connection
+ * parameters are defined in $databases array in settings.php or
+ * settings.local.php. It is also possible to modify the $databases array in
+ * runtime. For example, Migrate Drupal, which provides the migrations from
+ * Drupal 6 / 7, asks for the source database connection parameters in the UI
+ * and then adds the $databases['migrate'] connection in runtime before the
+ * migrations are executed.
+ *
+ * As described above, the default source database is $databases['migrate']. If
+ * the source plugin needs another source connection, the database connection
+ * parameters should be added to the $databases array as, for instance,
+ * $databases['foo']. The source plugin can then use this connection by setting
+ * 'key' to 'foo' in its configuration.
+ *
+ * For a complete example on migrating data from an SQL source, refer to
+ * https://www.drupal.org/docs/8/api/migrate-api/migrating-data-from-sql-source
+ *
+ * @see https://www.drupal.org/docs/8/api/database-api
+ * @see \Drupal\migrate_drupal\Plugin\migrate\source\DrupalSqlBase