Update on this epic:
Since we moved to Doctrine DBAL Master/slave should be configurable via Doctrine Bundle config. Example: https://github.com/doctrine/DoctrineBundle/blob/master/Tests/DependencyInjection/Fixtures/config/yml/dbal_service_single_master_slave_connection.yml
But we might need to change eZ Platform specific connection logic to adapt for it, this needs testing basically.
Besides that one missing piece is connection healtcheck / re-connection as it is especially needed for any kind of micro services setup (Docker, Platform.sh, ..) where we need to assume service might not be up as it is getting rebooted or swapped for updates, and/or is booting and inserting data, in both cases meaning we need to wait a tiny bit and re-try the connection. This is possible as of Doctrine DBAL 2.5: https://github.com/doctrine/dbal/issues/1454 And should be fairly simple to add support for in our connection classes.