As of 950cf6016c, the mediawiki/core repo was updated to use DB_REPLICA instead of DB_SLAVE, with the old constant left as an alias. This is part of a string of commits that cleaned up the mixed use of "replica" and "slave" by sticking to the former. Extensions have not been mass converted. Please use the new constant in any new code.

The word "replica" is a bit more indicative of a broader range of DB setups*, is used by a range of large companies**, and is more neutral in connotations.

Drupal and Django made similar updates (even replacing the word "master"): * https://www.drupal.org/node/2275877 * https://github.com/django/django/pull/2692/files & https://github.com/django/django/commit/beec05686ccc3bee8461f9a5a02c607a0235...

I don't plan on doing anything to DB_MASTER, since it seems fine by itself, like "master copy", "master tape" or "master key". This is analogous to a master RDBMs database. Even multi-master RDBMs systems tend to have a stronger consistency than classic RDBMs slave servers, and present themselves as one logical "master" or "authoritative" copy. Even in it's personified form, a "master" database can readily be thought of as analogous to "controller", "governer", "ruler", lead "officer", or such.**

* clusters using two-phase commit, galera using certification-based replication, multi-master circular replication, ect... ** https://en.wikipedia.org/wiki/Master/slave_(technology)#Appropriateness_of_u... *** http://www.merriam-webster.com/dictionary/master?utm_campaign=sd&utm_med...