Manual:Job queue/pl

W MediaWiki 1.6 została wprowadzona kolejka zadań (ang. job queue) do wykonywania długotrwałych zadań w sposób asynchroniczny. Celem kolejki zadań jest umożliwienie wykonywania wielu zadań przy wykorzystaniu przetwarzania wsadowego.

Set up
It is recommended that you instead schedule the running of jobs completely in the background, via the command line. Domyślnie zadania są uruchamiane na końcu żądania internetowego. Wyłącz to domyślne zachowanie, ustawiając na.

Cron
You could use cron to run the jobs every hour. Add the following to your crontab file:

Using Cron makes it easy to get started, but can make email notifications and cascading template feel slow (to wait up to an hour). Consider using one of the below approaches to set up a continuous job runner instead.

Continuous service
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 :

Create script
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.

Create service
If using systemd, create a new service unit by creating the file. Change the  parameter to the user that runs PHP on your web server:

Enable it and start it with those commands:

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 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:

Abandoned jobs
A job can fail for some reasons. To understand why, you have to inspect the related log file.

In any case, if a job fails 3 times (so if the system has done that number of attempts), the job is then considered "abandoned" and it's not executed again.

Relevant source code:

https://doc.wikimedia.org/mediawiki-core/master/php/JobQueue_8php_source.html#l00085

An abandoned job is:


 * not executed anymore from
 * not counted from
 * not automatically removed from the database
 * but are included in the count of Special:Statistics

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 fall back 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 allows the execution of code to be scheduled for the end of the request, after all content has been sent to the browser. This is similar to queuing a job, except that it runs immediately instead of upto several minutes/hours in the future.

DeferredUpdates was introduced in MediaWiki 1.23 and received major changes during MediaWiki 1.27 and 1.28. The goal of this mechanism is speed up the web responses by doing less work, as well as to prioritise some work that would previously be a job to run as soon as possible after the end of the response.

A deferrable update can implement  in order to be queueable as a Job as well. This is used by RefreshSecondaryDataUpdate in core, for example, which means if the update fails for any reason, MediaWiki will fallback to queuing as a job and try again later as to fulfil the contract in question.

Changes in MediaWiki 1.22
In, the job queue execution on each page request was changed 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  in the background. It will only work if 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 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  is disallowed (, fixed in 1.23).
 * Performance: even if the job queue is empty, the new PHP process is started anyway (, 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 (, 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 (, fixed in 1.23)
 * Normal shell limits like, and  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 to , for example, which may cause other problems. It can be disabled completely by setting, but jobs will no longer run on page requests, and you must explicitly run runJobs.php to periodically run pending jobs.

Changes 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 but later changed to be the unlisted special page Special:RunJobs.

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

Changes in MediaWiki 1.28
Between MediaWiki 1.23 and MediaWiki 1.27, use of 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 in MediaWiki 1.29
In MediaWiki 1.27.0 to 1.27.3 and 1.28.0 to 1.28.2, when 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 to clear the job queue. It has been reported as a bug and was solved in 1.27.4 and 1.28.3.

Job examples


Aktualizacja tabeli linków po zmianach w szablonie
MediaWiki 1.6 dodaje zadanie do kolejki zadań dla każdego artykułu, który zawiera w sobie jakiś szablon. Każde zadanie to polecenie, aby przejrzeć artykuł, rozwinąć szablony i odpowiednio zaktualizować tabelę linków. Wcześniej artykuły hosta pozostawały nieaktualne do czasu wygaśnięcia ich pamięci podręcznej parsera lub do czasu edycji artykułu przez użytkownika.



Inwalidacja pamięci podręcznej HTML
Szeroka klasa operacji może spowodować potrzebę inwalidacji (czyli unieważnienia/aktualizacji) pamięci podręcznej HTML dla dużej liczby stron; są to operacje typu:


 * zmiana grafiki (wszystkie miniaturki muszą być ponownie zrenderowane a ich rozmiary od nowa przeliczone)
 * usunięcie strony (we wszystkie linkach prowadzących do niej na innych stronach trzeba zmienić kolor z niebieskiego na czerwony)
 * utworzenie lub przywrócenie strony (jak wyżej ale kolor należy zmienić z czerwonego na niebieski)
 * zmiana zawartości szablonu (wszystkie strony, które zawierają dany szablon potrzebują aktualizacji)

Za wyjątkiem zmian w szablonie, działania te nie wymagają inwalidacji tabeli linków, ale inwalidują pamięć podręczną HTML wszystkich stron zawierających linki do tej strony lub wykorzystujących daną grafikę. Inwalidacja pamięci podręcznej strony jest operacją krótkotrwałą; wymaga tylko aktualizacji pojedynczego pola bazy danych i wysyłanie pakietu danych typu multicast, w celu wyczyszczenia pamięci podręcznej. Ale jeśli jest ich więcej niż około 1000, zajmuje to dużo czasu. Domyślnie jedno zadanie jest dodawane na operacje 300 (patrz )

Note, however, that even if purging the cache of a page is a short operation, reparsing a complex page that is not in the cache may be expensive, especially if a highly used template is edited and causes lots of pages to be purged in a short period of time and your wiki has lots of concurrent visitors loading a wide spread of pages. This can be mitigated by reducing the number of pages purged in a short period of time, by reducing to a small number (20, for example) and also set  for   to a low number (5, for example).

Audio and video transcoding
When using 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.



Typowe wartości
W okresie niskiego obciążenia, długość kolejki zadań może wynosić zero. W praktyce, w projektach Wikimedii, długość kolejki zadań rzadko kiedy wynosi zero. Poza godzinami szczytu, może to być wartość od kilkuset do tysiąca zadań. Podczas pracowitego dnia, może to być nawet kilka milionów, ale wartość ta zwykle szybko fluktuuje w granicach ±10% i powyżej.

Special:Statistics
Do wersji 1.16 MediaWiki, długość kolejki zadań była pokazywana na stronie specjalnej Specjalna:Statystyka; od wersji 1,17 ( 75272) została stamtąd usunięta ale może być obserwowana poprzez 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.

Code stewardship
