Manual:Job queue

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In MediaWiki 1.6, a job queue was introduced to perform long-running tasks asynchronously. The job queue is designed to hold many short tasks using batch processing.

Set up[edit]

By default, jobs are run at the end of a web request. If possible, it is recommended that you disable this default behaviour by setting $wgJobRunRate to 0, and instead schedule the running of jobs completely in the background, via the command line. For example, you could use cron to run the jobs every day at midnight by entering the following in your crontab file:

0 0 * * * /usr/bin/php /var/www/wiki/maintenance/runJobs.php > /var/log/runJobs.log 2>&1

Job execution on page requests[edit]

By default, at the end of each web request, one job is taken from the job queue and executed. This behavior is controlled by the $wgJobRunRateManual:$wgJobRunRate configuration variable. Setting this variable to 1, will run a job on each request. Setting this variable to 0 will disable the execution of jobs during web requests completely, so that you can instead run runJobs.phpManual:runJobs.php manually or periodically from the command line.

MediaWiki version: 1.23

When enabled, jobs will be executed by opening a socket and making an internal HTTP request to an unlisted special page: Special:RunJobs. See also the asynchronous section.

Performance issue[edit]

If the performance burden of running jobs on every web request is too great but you are unable to run jobs from the command line, you can reduce $wgJobRunRateManual:$wgJobRunRate to a number between 1 and 0. This means a job will execute on average every 1 / $wgJobRunRate requests.

$wgJobRunRate = 0.01;

Manual usage[edit]

There is also a way to empty the job queue manually, for example after changing a template that's present on many pages. Simply run the maintenance/runJobs.phpManual:runJobs.php maintenance script. For example:

/path-to-my-wiki/maintenance$ php ./runJobs.php

History[edit]

Asynchronous[edit]

The configuration variable $wgRunJobsAsyncManual:$wgRunJobsAsync has been added to force the execution of jobs synchronously, in scenarios where making an internal HTTP request for job execution is not wanted.

When running jobs asynchronously, it will open an internal HTTP connection for handling the execution of jobs, and will return the contents of the page immediately to the client without waiting for the job to complete. Otherwise, the job will be executed in the same process and the client will have to wait until the job is completed. When the job does not run asynchronously, if a fatal error occurs during job execution, it will propagate to the client, aborting the load of the page.

Note that even if $wgRunJobsAsync is set to true, if PHP can't open a socket to make the internal HTTP request, it will fallback to the synchronous job execution. However, there are a variety of situations where this internal request may fail, and jobs won't be run, without falling back to the synchronous job execution. Starting with MediaWiki 1.28.1 and 1.27.2, $wgRunJobsAsync now defaults to false.

Deferred updates[edit]

The deferred updates mechanism was introduced in MediaWiki 1.23 and received major changes during MediaWiki 1.27 and 1.28. It allows the execution of some features at the end of the request, when all the content has been sent to the browser, instead of queuing it in the job, which would otherwise be executed potentially some hours later. The goal of this alternate mechanism is mainly to speed up the main MediaWiki requests, and at the same time execute some features as soon as possible at the end of the request.

Some deferrable updates can be both deferrable updates and jobs, if specified as such.

Changes introduced in MediaWiki 1.22[edit]

In MediaWiki 1.22MediaWiki 1.22, the job queue execution on each page request was changed (Gerrit change 59797) so, instead of executing the job inside the same PHP process that's rendering the page, a new PHP cli command is spawned to execute runJobs.phpManual:runJobs.php in the background. It will only work if $wgPhpCliManual:$wgPhpCli is set to an actual path or safe mode is off, otherwise, the old method will be used.

This new execution method could cause some problems:

There's no way to revert to the old on-request job queue handling, besides setting $wgPhpCliManual:$wgPhpCli to false, for example, which may cause other problems (task T63387). It can be disabled completely by setting $wgJobRunRate = 0;, but jobs will no longer run on page requests, and you must explicitly run runJobs.php to periodically run pending jobs.

Changes introduced in MediaWiki 1.23[edit]

In MediaWiki 1.23, the 1.22 execution method is abandoned, and jobs are triggered by MediaWiki making an HTTP connection to itself.

It was first designed as an API entry point (Gerrit change 113038) but later changed to be the unlisted special page Special:RunJobs (Gerrit change 118336).

While it solves various bugs introduced in 1.22, it still requires loading a lot of PHP classes in memory on a new process to execute a job, and also makes a new HTTP request that the server must handle.

Changes introduced in MediaWiki 1.27[edit]

In MediaWiki 1.25 and MediaWiki 1.26, use of $wgRunJobsAsyncManual:$wgRunJobsAsync would sometimes cause jobs not to get run if the wiki has custom $wgServerNameManual:$wgServerName configuration. This was fixed in MediaWiki 1.27. task T107290

Changes introduced in MediaWiki 1.28[edit]

Between MediaWiki 1.23 and MediaWiki 1.27, use of $wgRunJobsAsyncManual:$wgRunJobsAsync would cause jobs not to get run on if MediaWiki requests are for a server name or protocol that does not match the currently configured server name one (e.g. when supporting both HTTP and HTTPS, or when MediaWiki is behind a reverse proxy that redirects to HTTPS). This was fixed in MediaWiki 1.28. task T68485

Changes introduced in MediaWiki 1.29[edit]

In MediaWiki 1.27.0 to 1.27.3 and 1.28.0 to 1.28.2, when $wgJobRunRateManual:$wgJobRunRate is set to a value greater than 0, an error like the one below may appear in error logs, or on the page:

PHP Notice: JobQueueGroup::__destruct: 1 buffered job(s) never inserted

As a result of this error, certain updates may fail in some cases, like category members not being updated on category pages, or recent changes displaying edits of deleted pages - even if you manually run runJobs.phpManual:runJobs.php to clear the job queue. It has been reported as a bug (task T100085) and was solved in 1.27.4 and 1.28.3.

Job examples[edit]

Updating links tables when a template changes[edit]

When a template changes, MediaWiki adds a job to the job queue for each article transcluding that template. Each job is a command to read an article, expand any templates, and update the link table accordingly. Previously, the host articles would remain outdated until either their parser cache expires or until a user edits the article.

HTML cache invalidation[edit]

A wider class of operations can result in invalidation of the HTML cache for a large number of pages:

  • Changing an image (all the thumbnails have to be re-rendered, and their sizes recalculated)
  • Deleting a page (all the links to it from other pages need to change from blue to red)
  • Creating or undeleting a page (like above, but from red to blue)
  • Changing a template (all the pages that transclude the template need updating)

Except for template changes, these operations do not invalidate the links tables, but they do invalidate the HTML cache of all pages linking to that page, or using that image. Invalidating the cache of a page is a short operation; it only requires updating a single database field and sending a multicast packet to clear the caches. But if there are more than about 1000 to do, it takes a long time. By default, one job is added per 500 operations (see $wgUpdateRowsPerJobManual:$wgUpdateRowsPerJob)

Typical values[edit]

During a period of low load, the job queue might be zero. At Wikimedia, the job queue is, in practice, almost never zero. In off-peak hours, it might be a few hundred to a thousand. During a busy day, it might be a few million, but it can quickly fluctuate by 10% or more. [1]

Special:Statistics[edit]

Up to MediaWiki 1.16, the job queue value was shown on Special:Statistics. However, since 1.17 (rev:75272) it's been removed, and can be seen now with API:SiteinfoAPI:Siteinfo:

The number of jobs returned in the API result may be slightly inaccurate when using MySQL, which estimates the number of jobs in the database. This number can fluctuate based on the number of jobs that have recently been added or deleted. For other databases that do not support fast result-size estimation, the actual number of jobs is given.

For developers[edit]

See also[edit]