MessageCache: Avoid 'contributions' replicas

On WMF wikis, the partitioning of the revision table on the
'contributions' replicas makes the query here perform really poorly.
Specify 'api' as a hack for now to avoid those replicas.

This query happens once per day per wiki, plus when someone edits the
MediaWiki namespace, so it shouldn't be much additional load.

Bug: T164666
Change-Id: I5ae74d1702144f6475e9cfb13effc43389d66233

diff --git a/includes/cache/MessageCache.php b/includes/cache/MessageCache.php
index c992112..8f88ee9 100644 (file)
--- a/includes/cache/MessageCache.php
+++ b/includes/cache/MessageCache.php
@@ -464,7 +464,11 @@ class MessageCache {
        protected function loadFromDB( $code, $mode = null ) {
                global $wgMaxMsgCacheEntrySize, $wgLanguageCode, $wgAdaptiveMessageCache;
 
-               $dbr = wfGetDB( ( $mode == self::FOR_UPDATE ) ? DB_MASTER : DB_REPLICA );
+               // (T164666) The query here performs really poorly on WMF's
+               // contributions replicas. We don't have a way to say "any group except
+               // contributions", so for the moment let's specify 'api'.
+               // @todo: Get rid of this hack.
+               $dbr = wfGetDB( ( $mode == self::FOR_UPDATE ) ? DB_MASTER : DB_REPLICA, 'api' );
 
                $cache = [];