The following setup has one master and two slaves with cross failover. If master fails, system will continue in read-only mode. If both slaves fail, system will continue only using the master.

<?php
$databases['default']['master'] = array (
  'database' => 'mydatabase',
  'username' => 'readwrite',
  'password' => 'readwrite',
  'host' => 'master',
  'port' => '',
  'driver' => 'mysql',
  'prefix' => '',
);
$databases['default']['slave'][] = array (
  'database' => 'mydatabase',
  'username' => 'readonly',
  'password' => 'readonly',
  'host' => 'slave1',
  'port' => '',
  'driver' => 'mysql',
  'prefix' => '',
  'readonly' => TRUE,
);
$databases['default']['slave'][] = array (
  'database' => 'mydatabase',
  'username' => 'readonly',
  'password' => 'readonly',
  'host' => 'slave2',
  'port' => '',
  'driver' => 'mysql',
  'prefix' => '',
  'readonly' => TRUE,
);

$databases['default']['default'] = array (
  'driver' => 'autoslave',
  'master' => array('master', 'slave'),
  'slave' => array('slave', 'master'),
);
?>

Comments

simanta’s picture

simanta commented

In Drupal 8 Master-Slave database setup can be configured using the suggestion provided in link: https://www.drupal.org/docs/8/api/database-api/database-configuration. While checking setup in local environment, this approach is not working as per expectation. So there is an alternative snippet which actually worked and tested on Drupal version 8.8.0.

// Master database
$connect_db[] = [
  'database' => 'databasename',
  'username' => 'sqlusername',
  'password' => 'sqlpassword',
  'prefix' => '',
  'host' => 'masterhostname',
  'port' => '3306',
  'namespace' => 'Drupal\\Core\\Database\\Driver\\mysql',
  'driver' => 'mysql',
];
// Salve database
$connect_db[] = [
  'database' => 'databasename',
  'username' => 'sqlusername',
  'password' => 'sqlpassword',
  'prefix' => '',
  'host' => 'slavehostname',
  'port' => '3306',
  'namespace' => 'Drupal\\Core\\Database\\Driver\\mysql',
  'driver' => 'mysql',
  'readonly' => TRUE,
];
foreach($connect_db as $conn) {
  $dsn = "mysql:host=".$conn['host'].";port=".$conn['port'].";dbname=".$conn['database'];
  try{
    $pdo = new PDO($dsn, $conn['username'], $conn['password']);
    $databases['default']['default'] = $conn;
    break;
  }
  catch (PDOException $e) {
    //Do nothing
  }
}