Manual:Job queue/de

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Die Job queue („Warteschlange“) wurde in MediaWiki 1.6 eingeführt, um längerfristige Prozesse asynchron (zeitversetzt und besser verteilt) auszuführen. Sie wurde mit dem Ziel entwickelt, viele kleine Prozesse zu handhaben (Stapelverarbeitung).

Set up

Wenn möglich, sollte $wgJobRunRate auf 0 gesetzt werden und die Jobs z.B. mittels Cron regelmäßig abgearbeitet werden. Wenn z.B. cron dafür genutzt werden soll, die Jobs jeden Tag um Mitternacht abzuarbeiten, ist folgendes in die crontab-Datei einzutragen:

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

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 runJobs.php 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.

Simple service to run jobs

If you have shell access and the possibility to create init scripts, you can create a simple service to run jobs as they become available, and also throttle them to prevent the job runner to monopolize the CPU resources of the server:

Create a bash script, for example at /usr/local/bin/mwjobrunner:

#!/bin/bash
# Put the MediaWiki installation path on the line below
IP=/home/www/www.mywikisite.com/mediawiki
RJ=$IP/maintenance/runJobs.php
echo Starting job service...
# Wait a minute after the server starts up to give other processes time to get started
sleep 60
echo Started.
while true; do
	# Job types that need to be run ASAP mo matter how many of them are in the queue
	# Those jobs should be very "cheap" to run
	php $RJ --type="enotifNotify"
	php $RJ --type="htmlCacheUpdate" --maxjobs=50
	# Everything else, limit the number of jobs on each batch
	# The --wait parameter will pause the execution here until new jobs are added,
	# to avoid running the loop without anything to do
	php $RJ --wait --maxjobs=10
	# Wait some seconds to let the CPU do other things, like handling web requests, etc
	echo Waiting for 10 seconds...
	sleep 10
done

Depending on how fast the server is and the load it handles, you can adapt the number of jobs to run on each cycle and the number of seconds to wait on each cycle.

Make the script executable (chmod 755).

If using systemd, create a new service unit by creating the file /etc/systemd/system/mw-jobqueue.service. Change the User parameter to the user that runs PHP on your web server:

[Unit]
Description=MediaWiki Job runner

[Service]
ExecStart=/usr/local/bin/mwjobrunner
Nice=10
ProtectSystem=full
User=php-fpm
OOMScoreAdjust=200
StandardOutput=journal

[Install]
WantedBy=multi-user.target

Enable it and start it with those commands:

sudo systemctl enable mw-jobqueue
sudo systemctl start mw-jobqueue
sudo systemctl status mw-jobqueue


Job-Erledigung bei Seitenaufrufen

Standardmäßig wird bei jedem Seitenaufruf ein Job aus der Job-Schlange genommen und ausgeführt. Die Anzahl der bearbeiteten Jobs kann mittels $wgJobRunRate konfiguriert werden. Das Setzen dieser Variable auf 1 führt zu einer Jobabarbeitung pro Seitenaufruf. 0 deaktiviert die Ausführung von Jobs bei Seitenaufrufen, dann sollte runJobs.php per Hand oder automatisiert periodisch ausgeführt werden.

MediaWiki version: 1.23

Wenn aktiviert, wird zur Jobausführung ein Socket geöffnet und eine interne HTTP-Anfrage an special page: SpecialRunJobs.php geschickt. Siehe Abschnitt asynchronous. Performance Probleme

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 $wgJobRunRate to a number between 1 and 0. Werte zwischen 1 und 0 sorgen dafür, dass "im Durchschnitt" alle 1 / $wgJobRunRate Seitenaufrufe ein Job erledigt wird.

$wgJobRunRate = 0.01;

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/runJobs.php maintenance script. Zum Beispiel:

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

Geschichte

Asynchronous

The configuration variable $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

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 MediaWiki 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.php in the background. It will only work if $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:

  • If $wgPhpCli is set to an incompatible version of PHP (e.g.: an outdated version) jobs may fail to run (fixed in 1.23).
  • PHP open_basedir restrictions are in effect, and $wgPhpCli is disallowed (task T62208, fixed in 1.23).
  • Performance: even if the job queue is empty, the new PHP process is started anyway (task T62210, 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 (task T60719, 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 (task T62698, fixed in 1.23)
  • Normal shell limits like $wgMaxShellMemory , $wgMaxShellTime and $wgMaxShellFileSize 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 $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.

Änderungen 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 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

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

Changes introduced in MediaWiki 1.28

Between MediaWiki 1.23 and MediaWiki 1.27, use of $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

In MediaWiki 1.27.0 to 1.27.3 and 1.28.0 to 1.28.2, when $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.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

Aktualisierung der Linktabellen nach Vorlagenänderungen

Wird im Wiki eine Vorlage bearbeitet, fügt MediaWiki für jede Seite, die diese Vorlage verwendet, einen Job der Job queue hinzu. Jeder Job ist dabei der Befehl, die entsprechende Seite aufzurufen, die Vorlage neu zu expandieren und die Linktabelle entsprechend zu aktualisieren. Previously, the host articles would remain outdated until either their parser cache expires or until a user edits the article.

Außerkraftsetzen des HTML-Caches

Weitere Vorgänge können darüberhinaus für eine große Seitenanzahl zum Ungültigwerden des jeweiligen HTML-Caches führen:

  • Änderung eines Bildes (alle Vorschaubilder, die „Thumbnails“, müssen neu gerendert und ihre Größen neu berechnet werden)
  • Löschung einer Seite (alle Links auf anderen Seiten müssen von blau nach rot geändert werden)
  • Neuanlage oder Wiederherstellung einer Seite (wie vorstehend, aber von rot nach blau)
  • Änderung einer Vorlage (alle Seiten, die die Vorlage einbinden, müssen aktualisiert werden)

Abgesehen von Vorlagenänderungen betreffen diese Aktionen zwar nicht die Linktabellen, sie machen jedoch den HTML-Cache aller Seiten, auf denen die fragliche Seite verlinkt oder in denen ein Bild eingebunden ist, ungültig. Die Außerkraftsetzung des Caches einer Seite ist ein kurzer Vorgang; dazu wird nur die Aktualisierung eines einzigen Datenbankfelds und das Senden eines Multicast-Datenpakets zur Cacheleerung benötigt.

Es dauert allerdings länger, wenn mehr als 1000 Aktualisierungen vorzunehmen sind. Standardmäßig werden Jobs hinzugefügt, wenn mehr als 500 Seiten betroffen sind; ein Job pro 500 Vorgänge.

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 webVideoTranscode and webVideoTranscodePrioritized 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.

Typische Werte

In Zeiten geringer Serverlast kann die Job queue leer sein und den Wert Null haben. In der Praxis ist die Job queue – zumindest in den großen Wikimediaprojekten wie zum Beispiel der deutschsprachigen Wikipedia – fast nie leer. Außerhalb der Tagesspitzenzeiten kann sie einige hundert bis tausende anstehende Jobs umfassen, in Spitzenzeiten auch mehrere Millionen. Der Zahlenwert kann sehr kurzfristig um 10 Prozent oder mehr schwanken. [1]

Special:Statistics

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: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.

Für Entwickler

Siehe auch