View source for Copying or merging wiki user accounts

=Overview=
There are several use cases where user accounts need to be copied from one wiki to another, e.g.

;Merging two wikifarms
:Wikifarms store user accounts in the metawiki database, which is shared by all wikis in the wikifarm. If the wikis of a wikifarm (or a selection thereof) need to be moved to another wikifarm, then a merge of the user accounts becomes necessary.

;Moving a wikifarm to anothers server
:Strictly speaking, moving a wikifarm to another server can be done by copying the whole databse. In practice however, the new wikifarm will often be a separate branch of the original wiki, so a merge becomes necessary.

The process consists of a sequence of activities, described in more detail below:

# Backup the user database
# Resolve conflicts
# Merge the user accounts
# Anonymize user accounts which should be discarded
# Clean-up

* The source database server shall be refered to as "FROM"
* The sink database server shall be refered to as "TO"

==1. Backup==
Make a backup of the tables in TO that will be modified:
* user table in metawiki database
* user tables user_groups, user_newtalk, user_openid, user_properties, recentchanges, revision, watchlist in all individual wiki databases.

==2. Resolve conflicts==
User accounts can conflict if FROM and TO have accounts with the same ids but with different data. This can happen if:
* accounts in FROM have been updated after TO was copied.
* accounts have been created in TO, but not in FROM.

Resolving conflicts involves the following actions:
# Search for incomplete accounts in FROM (e.g. without an e-mail address). If incomplete accounts are found, stop.
# List all conflicting accounts in TO, i.e. which have no equivalent in FROM because they were created after TO was copied.
# Add a buffer of e.g. 2000 accounts in TO, to accomodate future account merges.
# Move conflicting accounts after the buffer.
# Update user ids in the individual wiki databases, in the tables user_groups, user_newtalk, user_openid, user_properties, recentchanges, revision, watchlist.
# Check that conflicts are resolved, else stop.

==3. Merge==
Merge the user accounts in TO with accounts in FROM.

# Copy the table user_groups from all individual wiki databases that will be merged.
# Update user data in the tables user_newtalk, user_openid, user_properties, recentchanges, revision, watchlist from all individual wiki databases that will be merged.
# Copy accounts which exist in FROM but not in TO, or which were updated in FROM after TO was copied.

==4. Anonymize==
In some cases, not all accounts shall be merged. However, to keep the page revisions, discussions etc., user accounts cannot simply be deleted, so they have to be anonymized: 

# Make a list of users which will be kept, e.g.
##Users which belong to a specific user group
##Users which are registered with a specific wiki in TO
##Users with certain E-mail addresses
##Administrators, bureaucrats
##Bots
#Anonymize all users which are not in the list obtained in the previous step:
##Erase E-mail address and related columns
##Replace login with some string, e.g. id
##Replace password and related columns by empty string or null
##Replace real name by id

=Script=
Merging wikifarms can be complicated, so it will generally be an iterative process. A script can automatize the most tedious tasks, and provide more security by allowing to (unit) test the merge on a test server.
 
There are several ways of implementing this:
* Using the Mediawiki http API. Using PHP, the [http://sourceforge.net/projects/snoopy/ Snoopy library] for simulating a browser can come in handy.
* Directly modifying the wiki database, e.g. using the [http://php.net/pdo PDO data abstraction layer] for PHP. This practice is not recommended by Mediawiki, but in this case, given the potentially large number of modifications necessary, it can be the most efficient solution.
* Using [http://www.mediawiki.org/wiki/Manual:Database_access the database abstraction layer] provided by Mediawiki.

A script was written for automatically merging the wikis of Museum für Naturkunde Berlin (MfN). The script implements the steps listed above in 4 classes: 
;Backuper
:Create a backup of the metawiki.user table on the TO DB.
;Resolver
:Detect incomplete user accounts on FROM database, i.e. without login, password, name or e-mail; Check for conflicting user accounts on TO-database; Add a number of dummy accounts; Try to resolve any conflicting accounts by moving the accounts at the end of the account list.
;Merger
:Copy tables user_groups in the FROM databases to the TO databases; Copy tables user_newtalk, user_openid, user_properties, recentchanges, revision, watchlist FROM->TO; Find users which have been created in the FROM database after the TO database was copied; Copy an array of users FROM->TO;
;Anonymizer
:Lists all accounts on the TO database, which will NOT be anonymized; Anonymizes data from all users which are not to be kept.

''The interaction between these classes is modelled in the following UML activity diagramm:''

[[File:Merge users of two wikifarms UML diagramm.png|UML activity diagramm of merging or copying the users from one wikifarm to another (FROM->TO).]]