Requests for comment/Job queue redesign
| Job queue redesign | |
|---|---|
| Component | General |
| Creation date | |
| Author(s) | Brion Vibber |
| Document status | accepted |
Current model
editEach wiki carries a "job" table with a queue like this, used to record actions for future background processing:
-- Jobs performed by parallel apache threads or a command-line daemon
CREATE TABLE /*$wgDBprefix*/job (
job_id int unsigned NOT NULL auto_increment,
-- Command name
-- Limited to 60 to prevent key length overflow
job_cmd varbinary(60) NOT NULL default '',
-- Namespace and title to act on
-- Should be 0 and '' if the command does not operate on a title
job_namespace int NOT NULL,
job_title varchar(255) binary NOT NULL,
-- Any other parameters to the command
-- Presently unused, format undefined
job_params blob NOT NULL,
PRIMARY KEY job_id (job_id),
KEY (job_cmd, job_namespace, job_title)
) /*$wgDBTableOptions*/;
New jobs are pushed in on the end at runtime; multiple duplicates of a single job may be pushed.
A later queue-run operation (either from running runJobs.php on the command line/cron, or a random "background operation" during a web hit) pulls an item from the front, loads up the associated class, executes it, and when completed deletes it and any duplicates of it in the queue.
Example jobs
edit- RefreshLinksJob
- Re-renders pages to update their link tables after a template they used was edited
- RefreshLinksJob2
- Same thing, but optimized for high-use templates (pulls the list of pages to update at job-run time instead of job-insert time)
- HTMLCacheUpdateJob
- Updates page_touched timestamps and clears pages from the HTTP proxy or HTML file caches, if in use. Again, used to hit pages for indirect updates via templates.
- EnotifNotifyJob
- Sends e-mails of page change notifications, if configured. This is relatively time-critical.
- DoubleRedirectJob
- Automatically tidies up double-redirects that might have been created by a page rename to point to the final target.
- RenameUserJob
- For the RenameUser extension -- updates *_user_text fields in various tables with a users' new name. Delays in this are highly noticed.
Problems
edit- Lack of a timestamp in the queue entry means we can't easily get a sense of how lagged our operations are.
- Duplicate-entry model allows the queue to grow disproportionately large when things happen like multiple edits to a widely-used template; it becomes difficult to tell how many actual jobs there are to do.
- This model was used over avoiding duplicate inserts to maximize insert speed; a unique index is not needed, and we don't have to manually check for dupes.
- There's not a good way to get a summary of what's currently in the table; if there are a million entries, querying them to get a breakdown is pretty slow.
- There's no prioritization, making it wildly unsuitable for things like e-mail notifications which should happen within a couple minutes -- if we spend six hours updating link tables due to a template change, we shouldn't be pushing everything else back.
Execution model
editOn the Wikimedia setup, we have a number of app servers which additionally serve as "job queue runners", cycling through the various wikis running runJobs.php. There are some issues here as well:
- It can take a while to cycle through 700+ separate sites. While you're running through empty queues you're not processing something with a giant queue.
- Wikia, with a much larger number of smaller wikis, have been experimenting with a single shared job queue table to make this easier to handle.
- runJobs.php has been known to hang sometimes when database servers get rearranged. Needs better self-healing?
Monitoring model
editJob queue runners log activity to files in /home/wikipedia/logs/jobqueue/. While sometimes useful, it'd be nice to have more of a "top"-style view that's more openly accessible:
- Web-accessible list of which jobs are being run on which wikis on which queue runners, with count, rate & lag info
- Processing rate per machine in Ganglia