They were mentioned in https://www.mediawiki.org/wiki/Requests_for_comment/Master-slave_datacenter_strategy_for_MediaWiki#Job_queuing though it was never set up (partly from people being busy with other things). In general jobs are enqueued on POST requests or from other jobs, all in the master datacenter. In some cases, jobs are enqueued on GET or possibly POST (if the api-promise-nonwrite thing is set up in vlc) in rare cases. This should work in a way where the cross-DC propagation is async, rather than having JobQueue::push() blocking on cross-DC traffic.

AFAIK, the new queue would be able to deal with that, though I'm not 100% sure on how writes initiated in different DC perform. Where are the best docs for this area?

Another use of enqueue job is when you want to enqueue multiple jobs that you *really* prefer to all be enqueued or not at all. Since enqueue job retries several times on failure (configurable too), it's more likely to get the all enqueued than MediaWiki pushing them in one try. These kind of jobs are idempotent, so extra runs don't break. This case is not related to multi-DC. I don't see any callers relying on this.

AFAIK, the new queue would be able to deal with that, though I'm not 100% sure on how writes initiated in different DC perform.

In the kafka-based queue jobs are always enqueued in the local DC. Then the jobs are asynchronously picked up and executed on the master DC, because they're posted to jobrunner.discovery.wmnet which in turn points to the job runner LVS in the master DC, so usages of the enqueue job for this use-case is unnecessary.

Change 393199 had a related patch set uploaded (by Ppchelko; owner: Ppchelko):
[mediawiki/extensions/FlaggedRevs@master] Remove usage of EnqueueJob

https://gerrit.wikimedia.org/r/393199

Change 393199 merged by jenkins-bot:
[mediawiki/extensions/FlaggedRevs@master] Remove usage of EnqueueJob

https://gerrit.wikimedia.org/r/393199

Grepping the current MW release code on tin, it looks like only FlaggedRevs and Wikibase use this job:

$ grep -iIREn enqueuejob .
./includes/DefaultSettings.php:7432:	'enqueue' => 'EnqueueJob', // local queue for multi-DC setups
./includes/jobqueue/jobs/EnqueueJob.php:36:final class EnqueueJob extends Job {
./includes/jobqueue/jobs/EnqueueJob.php:49:	 * @return EnqueueJob
./includes/jobqueue/jobs/EnqueueJob.php:59:	 * @return EnqueueJob
./autoload.php:434:	'EnqueueJob' => __DIR__ . '/includes/jobqueue/jobs/EnqueueJob.php',
./extensions/FlaggedRevs/backend/FRDependencyUpdate.php:69:				JobQueueGroup::singleton()->push( EnqueueJob::newFromLocalJobs(
./extensions/FlaggedRevs/backend/FlaggableWikiPage.php:345:		JobQueueGroup::singleton()->push( EnqueueJob::newFromLocalJobs(
./extensions/Wikidata/extensions/Wikibase/client/includes/Hooks/DataUpdateHookHandlers.php:7:use EnqueueJob;
./extensions/Wikidata/extensions/Wikibase/client/includes/Hooks/DataUpdateHookHandlers.php:168:		$enqueueJob = EnqueueJob::newFromLocalJobs( $addUsagesForPageJob );
./extensions/Wikidata/extensions/Wikibase/client/includes/Hooks/DataUpdateHookHandlers.php:170:		$this->jobScheduler->lazyPush( $enqueueJob );
./extensions/Wikibase/client/includes/Hooks/DataUpdateHookHandlers.php:7:use EnqueueJob;
./extensions/Wikibase/client/includes/Hooks/DataUpdateHookHandlers.php:168:		$enqueueJob = EnqueueJob::newFromLocalJobs( $addUsagesForPageJob );
./extensions/Wikibase/client/includes/Hooks/DataUpdateHookHandlers.php:170:		$this->jobScheduler->lazyPush( $enqueueJob );

Since FlaggedRevs has been taken care of in Gerrit 393199, that leaves only the Wikibase extension.

Change 393750 had a related patch set uploaded (by Ppchelko; owner: Ppchelko):
[mediawiki/extensions/Wikibase@master] Stop using EnqueueJob for AddUsagesForPageJob

https://gerrit.wikimedia.org/r/393750

Change 393750 merged by jenkins-bot:
[mediawiki/extensions/Wikibase@master] Stop using EnqueueJob for AddUsagesForPageJob

https://gerrit.wikimedia.org/r/393750

@thiemowmde Let's wait till this week's media wiki train to deploy all over to verify that we indeed don't have anything posting this job type.

Change 395847 had a related patch set uploaded (by Aaron Schulz; owner: Aaron Schulz):
[mediawiki/core@master] Change EnqueueJob docs to discourage obsolete use-cases

https://gerrit.wikimedia.org/r/395847

Change 395847 merged by jenkins-bot:
[mediawiki/core@master] Change EnqueueJob docs to discourage obsolete use-cases

https://gerrit.wikimedia.org/r/395847