Drupal 7 Module Development

Appendix A
Slave servers
Drupal also supports Master/slave database replication. Certain select queries can
be run against a slave server to offload the work to separate servers, allowing the site
to scale better. However, most sites do not have a slave server configured, so how do
we write code that will work properly either way?
Drupal lets us do so by specifying "targets". The third parameter to db_query() or
db_select() is an array of options that tweak the behavior of the query. We saw the
fetch key earlier. The other key of interest is target, which specifies which database
variant the system should try. Legal values are default (which is the default) and
slave. If "slave" is specified, Drupal will try to run the query against a slave server. If
one is not available, though, it will silently fall back to using the default server. Using
our previous example:
$result = db_query("SELECT name, filename FROM {system} WHERE type =
:type AND status = :status", array(':type' => 'module', ':status' =>
1), array('fetch' => PDO::FETCH_ASSOC, 'target' => 'slave'));
The way we handle the query is otherwise identical.
So why don't all Select queries use the slave server? Data on a slave server is always
a little behind the master server, by a fraction of a second or as much as a minute or
two depending on the configuration and traffic. Not all Select queries can handle
their data being slightly stale and there's no reliable way to detect automatically
which are which, so by default all queries use the master server.
However, there is another trick up our sleeve which we can use. After writing data,
we can call db_ignore_slave(). That will make a note in the active session to
disable the slave server for the current user only for a configurable period of time.
(The default is five minutes.) That way, Select queries run on subsequent page
requests by that user will always get the most up-to-date data but other users will
get the old data until the slave servers catch up. That can be very useful, for instance,
when a user posts a comment. They want to see the comment immediately but it's
OK if other users don't see it for 20-30 seconds.
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