deferred.txt

A few of the database updates required by various functions here
can be deferred until after the result page is displayed to the
user.  For example, updating the view counts, updating the
linked-to tables after a save, etc.  PHP does not yet have any
way to tell the server to actually return and disconnect while
still running these updates (as a Java servelet could), but it
might have such a feature in the future.

We handle these by creating a deferred-update object (in a real
O-O language these would be classes that implement an interface)
and putting those objects on a global list, then executing the
whole list after the page is displayed. We don't do anything
smart like collating updates to the same table or such because
the list is almost always going to have just one item on it, if
that, so it's not worth the trouble.


Since 1.6 there is a 'job queue' in the jobs table, which is used
to update link tables of transcluding pages after edits; this
may be extended in the future to more general background tasks.

Job queue items are fetched out of the queue and run either
at a random rate during regular page views (by default) or by
a batch process which can be run via maintenance/runJobs.php.