Manual:$wgSessionCacheType

Details
The storage mechanism for session data. Note that despite the name saying "cache" this is the primary storage mechanism for sessions; if it doesn't work, it will be impossible to log in to the wiki.

From MediaWiki 1.20 to 1.26, it was used only if   was true, otherwise MediaWiki fell back to standard PHP session handling.

Since MediaWiki 1.27, sessions are always stored in the object cache.

For available types see  .

For proper operation this must be set to a persistent cache, and if there are multiple servers that might serve a single client's requests the cache must be shared by all of them.

If   is set to  and this is left at its default , the cache used may not meet these requirements.

The solution is to set this to an appropriate cache, such as.

Note that if   is used then users cannot log in when the respective wiki's database was locked with e.g.  .