This is true if the application needs to talk to the slaves directly. If the application doesn't care, then you could do the smarts in the proxy layer underneath the application. I see three scenarios:

* Stateless or Transparent master/master backend

  Example: Memcached cluster

  Use load balancing in the proxy layer

* Failover backend with failover on server side

  Examples: Mysql master/slave

  Use failover logic in the proxy layer

* Failover backend with failover on the client side

  Example: HA RabbitMQ cluster

  Above suggestion from polvi is needed