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+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 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.
+
+Currently there are a few different types of jobs:
+
+ refreshLinks
+ Used to refresh the database tables that store the links between pages.
+ When a page is changed, all pages using that page are also cleared by
+ inserting a new job for all those pages. Each job refreshes only one page.
+
+ htmlCacheUpdate
+ Clear caches when a template is changed to ensure that changes can be seen.
+ Each job clears $wgUpdateRowsPerJob pages (300 by default).
+
+ enotifNotify
+ Used to send mail using the job queue.