Manual:Shared database

This page provides a brief overview of using shared databases in MediaWiki. Most of the information here should work with a plain installation of MediaWiki (no extensions). Any extension requirements are noted. Note that this is designed around MySQL databases. Other database engines may not support shared databases in this way.

Basics
Shared databases are configured with 3 main global configuration variables:
 * $wgSharedDB
 * $wgSharedTables
 * $wgSharedPrefix

Depending on your needs and environment, you may not need to use all of these.

The simplest setup: A shared user table
A shared user table can be used to have multiple wikis that have shared user registrations, so that users need only sign up to one wiki. Setting $wgSharedTables is not necessary, as this defaults to the user table.

Sharing more tables
You can share tables other than the user table, but be careful when doing so. If a table contains any data specific to one wiki, sharing may cause problems. Note that each of the subsections here assume that you are also sharing the user table. Whenever adding other tables, be sure to either append to the array or include the user table in the new array definition.

The ipblocks table
By sharing the ipblocks table, you can have "global blocks" so that a block on one wiki will block the user or IP on all other wikis using the shared database.

Caveats
There can be some minor issues when sharing the ipblocks table:
 * The  field is set as the "reason" on Special:Block when blocking a user. When a blocked user sees the "You are blocked" message, this is parsed as wikitext, and links are parsed on Special:BlockList, you will need to make sure when blocking a user that the message makes sense on all wikis.
 * Block logs will not be shared (sharing the logging table is not recommended).

The user_groups table
Sharing the user_groups table will allow you to have global user groups.

Caveats

 * As with the block log, the user rights log is not shared.
 * All user groups will be global. You can work around this somewhat by customizing your user groups. For example, to have admins on one wiki be separate from admins on another, you could do something like the following:
 * In the config for wiki 1:


 * In the config for wiki 2:


 * To prevent bureaucrats on one wiki from assigning themselves rights on other wikis that they shouldn't have, you can do a similar configuration to the above, but remove the 'userrights' right that allows them to set any group, and use $wgAddGroups/$wgRemoveGroups instead.

Other tables
This section covers other tables, with less common reasons to share, as well as which tables not to share.


 * The interwiki table contains mostly static data; it may be useful to share if you have many custom interwikis.
 * The site_stats table could possibly be shared, to aggregate data over all your wikis.


 * By default, the user_properties table is included in the list of shared tables. If your wikis were started using MediaWiki 1.15 or older, you should probably keep this for backward compatibility, as user preferences will be silently automatically migrated from the user table to the user_properties table. Otherwise, you can remove this from the array to allow users to have different preferences on each wiki (if desired)

Most other tables should not be shared, as they include wiki-specific data, typically connections to pages via a pageid or a namespace/title combination. This includes (but is not limited to):
 * Any of the links tables (pagelinks, templatelinks, etc.)
 * The page table
 * The revision table
 * The image table