The Galera clustering technology in MariaDB deals with all that and more, with Drupal webservers able to just connect to any number of connected MariaDB servers, even across different datacentres. All active.
If you still have to work Drupal 6, that's also fine - with Galera you can still have read-only slaves, but you don't necessarily. So for an application environment that doesn't handle master/slave logic, Galera can be an excellent solution.
In this session Daniel describes how, in concept as well as the basic practical terms. Naturally each deployment is slightly different, but this information will get you on the right track!