Some information about database access in MediaWiki.
By Tim Starling, January 2006.

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    Database layout
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For information about the MediaWiki database layout, such as a 
description of the tables and their contents, please see:
  http://www.mediawiki.org/wiki/Manual:Database_layout
  http://svn.wikimedia.org/viewvc/mediawiki/trunk/phase3/maintenance/tables.sql?view=markup


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    API
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To make a read query, something like this usually suffices:

$dbr = wfGetDB( DB_SLAVE );
$res = $dbr->select( /* ...see docs... */ );
while ( $row = $dbr->fetchObject( $res ) ) {
	...
}
$dbr->freeResult( $res );

Note the assignment operator in the while condition.

For a write query, use something like:

$dbw = wfGetDB( DB_MASTER );
$dbw->insert( /* ...see docs... */ );

We use the convention $dbr for read and $dbw for write to help you keep
track of whether the database object is a slave (read-only) or a master
(read/write). If you write to a slave, the world will explode. Or to be
precise, a subsequent write query which succeeded on the master may fail
when replicated to the slave due to a unique key collision. Replication
on the slave will stop and it may take hours to repair the database and
get it back online. Setting read_only in my.cnf on the slave will avoid
this scenario, but given the dire consequences, we prefer to have as
many checks as possible.

We provide a query() function for raw SQL, but the wrapper functions
like select() and insert() are usually more convenient. They take care
of things like table prefixes and escaping for you. If you really need
to make your own SQL, please read the documentation for tableName() and
addQuotes(). You will need both of them.


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    Basic query optimisation
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MediaWiki developers who need to write DB queries should have some
understanding of databases and the performance issues associated with
them. Patches containing unacceptably slow features will not be
accepted. Unindexed queries are generally not welcome in MediaWiki,
except in special pages derived from QueryPage. It's a common pitfall
for new developers to submit code containing SQL queries which examine
huge numbers of rows. Remember that COUNT(*) is O(N), counting rows in a
table is like counting beans in a bucket.


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    Replication
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The largest installation of MediaWiki, Wikimedia, uses a large set of
slave MySQL servers replicating writes made to a master MySQL server. It
is important to understand the issues associated with this setup if you
want to write code destined for Wikipedia.

It's often the case that the best algorithm to use for a given task
depends on whether or not replication is in use. Due to our unabashed
Wikipedia-centrism, we often just use the replication-friendly version,
but if you like, you can use $wgLoadBalancer->getServerCount() > 1 to
check to see if replication is in use.

=== Lag ===

Lag primarily occurs when large write queries are sent to the master.
Writes on the master are executed in parallel, but they are executed in
serial when they are replicated to the slaves. The master writes the
query to the binlog when the transaction is committed. The slaves poll
the binlog and start executing the query as soon as it appears. They can
service reads while they are performing a write query, but will not read
anything more from the binlog and thus will perform no more writes. This
means that if the write query runs for a long time, the slaves will lag
behind the master for the time it takes for the write query to complete.

Lag can be exacerbated by high read load. MediaWiki's load balancer will
stop sending reads to a slave when it is lagged by more than 30 seconds.
If the load ratios are set incorrectly, or if there is too much load
generally, this may lead to a slave permanently hovering around 30
seconds lag.

If all slaves are lagged by more than 30 seconds, MediaWiki will stop
writing to the database. All edits and other write operations will be
refused, with an error returned to the user. This gives the slaves a
chance to catch up. Before we had this mechanism, the slaves would
regularly lag by several minutes, making review of recent edits
difficult.

In addition to this, MediaWiki attempts to ensure that the user sees
events occurring on the wiki in chronological order. A few seconds of lag
can be tolerated, as long as the user sees a consistent picture from
subsequent requests. This is done by saving the master binlog position
in the session, and then at the start of each request, waiting for the
slave to catch up to that position before doing any reads from it. If
this wait times out, reads are allowed anyway, but the request is
considered to be in "lagged slave mode". Lagged slave mode can be
checked by calling $wgLoadBalancer->getLaggedSlaveMode(). The only
practical consequence at present is a warning displayed in the page
footer.

=== Lag avoidance ===

To avoid excessive lag, queries which write large numbers of rows should
be split up, generally to write one row at a time. Multi-row INSERT ...
SELECT queries are the worst offenders should be avoided altogether.
Instead do the select first and then the insert.

=== Working with lag ===

Despite our best efforts, it's not practical to guarantee a low-lag
environment. Lag will usually be less than one second, but may
occasionally be up to 30 seconds. For scalability, it's very important
to keep load on the master low, so simply sending all your queries to
the master is not the answer. So when you have a genuine need for
up-to-date data, the following approach is advised:

1) Do a quick query to the master for a sequence number or timestamp 2)
Run the full query on the slave and check if it matches the data you got
from the master 3) If it doesn't, run the full query on the master

To avoid swamping the master every time the slaves lag, use of this
approach should be kept to a minimum. In most cases you should just read
from the slave and let the user deal with the delay.


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    Lock contention
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Due to the high write rate on Wikipedia (and some other wikis),
MediaWiki developers need to be very careful to structure their writes
to avoid long-lasting locks. By default, MediaWiki opens a transaction
at the first query, and commits it before the output is sent. Locks will
be held from the time when the query is done until the commit. So you
can reduce lock time by doing as much processing as possible before you
do your write queries. Update operations which do not require database
access can be delayed until after the commit by adding an object to
$wgPostCommitUpdateList.

Often this approach is not good enough, and it becomes necessary to
enclose small groups of queries in their own transaction. Use the
following syntax:

$dbw = wfGetDB( DB_MASTER );
$dbw->immediateBegin();
/* Do queries */
$dbw->immediateCommit();

There are functions called begin() and commit() but they don't do what
you would expect. Don't use them.

Use of locking reads (e.g. the FOR UPDATE clause) is not advised. They
are poorly implemented in InnoDB and will cause regular deadlock errors.
It's also surprisingly easy to cripple the wiki with lock contention. If
you must use them, define a new flag for $wgAntiLockFlags which allows
them to be turned off, because we'll almost certainly need to do so on
the Wikimedia cluster.

Instead of locking reads, combine your existence checks into your write
queries, by using an appropriate condition in the WHERE clause of an
UPDATE, or by using unique indexes in combination with INSERT IGNORE.
Then use the affected row count to see if the query succeeded.