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.  .

Session share hack
Before MediaWiki 1.27, some wiki farms used a single domain, shared user tables and a shared session cache directory as a poor man's single sign-on. Since 1.27 file-based session cache cannot be used; to achieve the same thing, use something like: which will make the session cache of all wikis use the same keyspace.

Note that this method is not officially supported and can lead to things breaking if you use an extension that tries to put different things in the session on different wikis; use at your own risk. Also, it only works in 1.30 or later.