Manual:Job queue

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
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 to , 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:

Warning: Running jobs once a day like that could be very problematic for consistency and responsiveness -- you should instead run jobs as soon as possible after they are queued, using a wrapper script that waits for jobs to be queued and runs the appropriate job runners.

@FIXME: detail the actual wrappers Wikimedia uses in production to run jobs.

Note: you should run  as the same user as the web server runs as, to ensure that permissions to the filesystem are correctly accounted for if jobs touch uploaded files.

Job execution on page requests
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  configuration variable.

Setting this variable to, will run a job on each request.

Setting this variable to  will disable the execution of jobs during web requests completely, so that you can instead run  manually or periodically from the command line.

When enabled, jobs will be executed by opening a socket and making an internal HTTP request to an unlisted man>Special:MyLanguage/Manual:Special pages|special page: Special:RunJobs.

See also the asynchronous section.

Performance issue
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  to a number between  and.

This means a job will execute on average every   requests.

Manual usage
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 script.

For example:

Asynchronous
The configuration variable  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
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
In </>, the job queue execution on each page request was changed (<tvar|gerrit></>) 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 <tvar|runJobs></> in the background.

It will only work if <tvar|PhpCli></> 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:


 * If <tvar|PhpCli></> is set to an incompatible version of PHP (e.g.: an outdated version) jobs may fail to run (fixed in 1.23).


 * PHP  restrictions are in effect, and <tvar|PhpCli></> is disallowed (<tvar|task></>, fixed in 1.23).


 * Performance: even if the job queue is empty, the new PHP process is started anyway (<tvar|task></>, fixed in 1.23).


 * Sometimes the spawning PHP process cause the server or only the CLI process to hang due to stdout and stderr descriptors not properly redirected (<tvar|task></>, fixed in 1.22)


 * It does not work for shared code (wiki farms), because it doesn't pass additional required parameters to runJobs.php to identify the wiki that's running the job (<tvar|task></>, fixed in 1.23)


 * Normal shell limits like <tvar|MaxShellMemory></>, <tvar|MaxShellTime></> and <tvar|MaxShellFileSize></> are enforced on the runJobs.php process that's being executed in the background.

There's no way to revert to the old on-request job queue handling, besides setting <tvar|PhpCli></> to, for example, which may cause other problems (<tvar|task></>).

It can be disabled completely by setting <tvar|code> </>, 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
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 man>Special:MyLanguage/Manual:API</>|API entry point (<tvar|gerrit1></>) but later changed to be the unlisted special page Special:RunJobs (<tvar|gerrit2></>).

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
In MediaWiki 1.25 and MediaWiki 1.26, use of <tvar|RunJobsAsync></> would sometimes cause jobs not to get run if the wiki has custom <tvar|wgServerName></> configuration. This was fixed in MediaWiki 1.27.

Changes introduced in MediaWiki 1.28
Between MediaWiki 1.23 and MediaWiki 1.27, use of <tvar|RunJobsAsync></> 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.

Changes introduced in MediaWiki 1.29
In MediaWiki 1.27.0 to 1.27.3 and 1.28.0 to 1.28.2, when <tvar|JobRunRate></> 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 <tvar|runJobs></> to clear the job queue. It has been reported as a bug (<tvar|bug></>) and was solved in 1.27.4 and 1.28.3.

Updating links tables when a template changes
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
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 300 operations (see <tvar|1></>)

Audio and video transcoding
When using TimedMediaHandler to process local uploads of audio and video files, the job queue is used to run the potentially very slow creation of derivative transcodes at various resolutions/formats.

These are not suitable for running on web requests -- you will need a background runner.

It's recommended to set up separate runners for the  and   job types if possible. These two queues process different subsets of files -- the first for high resolution HD videos, and the second for lower-resolution videos and audio files which process more quickly.

Typical values
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.

Special:Statistics
Up to MediaWiki 1.16, the job queue value was shown on Special:Statistics.

However, since 1.17 (<tvar|rev>75272</>) it's been removed, and can be seen now with <tvar|api></>:

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.