doctrine:
    dbal:
        default_connection: default
        connections:
            default:
                driver:   pdo_mysql
                host:     "%env(DATABASE_HOST)%"
                dbname:   "db1"
                user:     "%env(DATABASE_USER)%"
                password: "%env(DATABASE_PASS)%"
                charset:  UTF8
            ds:
                driver:   pdo_mysql
                host:     "%env(DS_DATABASE_HOST)%"
                dbname:   "db2"
                user:     "%env(DS_DATABASE_USER)%"
                password: "%env(DS_DATABASE_PASS)%"
                slaves:
                    slave1:
                        host:     "%env(DS_DATABASE_SLAVE1_HOST)%"
                        user:     "%env(DS_DATABASE_USER)%"
                        password: "%env(DS_DATABASE_PASS)%"
                        dbname:   "db2"
                    slave2:
                        host:     "%env(DS_DATABASE_SLAVE2_HOST)%"
                        user:     "%env(DS_DATABASE_USER)%"
                        password: "%env(DS_DATABASE_PASS)%"
                        dbname:   "db2"

orm:
    default_entity_manager: default
    entity_managers:
        default:
            connection: default
            mappings:
                Main:
                    is_bundle: false
                    type: annotation
                    dir: '%kernel.project_dir%/src/Entity/Main'
                    prefix: 'AppEntityMain'
                    alias: Main
        ds:
            connection: ds

# Entity managers
AppServiceDatabaseMainEntityManager:
    arguments:
        $wrapped: '@doctrine.orm.default_entity_manager'
AppServiceDatabaseDSEntityManager:
    arguments:
        $wrapped: '@doctrine.orm.ds_entity_manager'

$result = $this->em->getConnection()->prepare($sql);
    $result->execute($args);

ds:
            driver:   pdo_mysql
            host:     "%env(DS_DATABASE_HOST)%"
            dbname:   "db2"
            user:     "%env(DS_DATABASE_USER)%"
            password: "%env(DS_DATABASE_PASS)%"
            wrapper_class: "%env(DS_DATABASE_PASS)%"
            slaves: AppServiceDatabaseDSWrapper
                slave1: ...

# DBAL connections
AppServiceDatabaseDSWrapper: '@doctrine.dbal.ds_connection'

class DSWrapper extends MasterSlaveConnection
{
public function prepareSlave($statement)
{
    $this->connect('slave');

    try {
        $stmt = new Statement($statement, $this);
    } catch (Exception $ex) {
        throw DBALException::driverExceptionDuringQuery($this->_driver, $ex, $statement);
    }

    $stmt->setFetchMode($this->defaultFetchMode);

    return $stmt;
}

public function querySlave()
{
    $this->connect('slave');

    $args = func_get_args();

    $logger = $this->getConfiguration()->getSQLLogger();
    if ($logger) {
        $logger->startQuery($args[0]);
    }

    $statement = $this->_conn->query(...$args);

    if ($logger) {
        $logger->stopQuery();
    }

    return $statement;
}
}