How to connect multiple database to Drupal

 Multiple databases can be connected to Drupal using the Database API and configuring settings.php file.

     Use the Database API to connect to additional databases

     Add the database connection details to settings.php file

     Use the db_set_active() function to switch between databases

     Use the db_query() function to execute queries on the selected database

#Drupal 8

#Adding additional databases to your configuration

Preferably you would add your configuration to the settings.php file for your site so that all modules can interact with the new database.

In your settings.php:

$databases['default']['default'] = array (
  'database' => 'drupal8_default',
  'username' => 'root',
  'password' => 'root',
  'prefix' => '',
  'host' => 'localhost',
  'port' => '3306',
  'namespace' => 'Drupal\\Core\\Database\\Driver\\mysql',
  'driver' => 'mysql',
);

$databases['second']['default'] = array (
  'database' => 'drupal8_second',
  'username' => 'root',
  'password' => 'root',
  'prefix' => '',
  'host' => 'localhost',
  'port' => '3306',
  'namespace' => 'Drupal\\Core\\Database\\Driver\\mysql',
  'driver' => 'mysql',
);

#Connect to the correct data base:

//To get the default database key "default"
$con = \Drupal\Core\Database\Database::getConnection();

//To get another database (here : 'second')
$con = \Drupal\Core\Database\Database::getConnection('default','second');

//To set the active connection
$conn = \Drupal\Core\Database\Database::setActiveConnection('second');