zlacker

>>hyzyla+(OP)
You really don't need anything fancy to implement a queue using SQL. You need a table with a primary id and a "status" field. An "expired" field can be used instead of the "status". We used the latter because it allows easy retries.

1. SELECT item_id WHERE expire = 0. If this is empty, no items are available.

2. UPDATE SET expire = some_future_time WHERE item_id = $selected_item_id AND expire = 0. Then check whether UPDATE affected any rows. If it did, item_id is yours. If not, loop. If the database has a sane optimizer it'll note at most one document needs locking as the primary id is given.

All this needs is a very weak property: document level atomic UPDATE which can return whether it changed anything. (How weak? MongoDB could do that in 2009.)

Source code at https://git.drupalcode.org/project/drupal/-/blob/9.2.x/core/... (We cooked this up for Drupal in 2009 but I am reasonably sure we didn't invent anything new.)

Of course, this is not the fastest job queue there is but it is quite often good enough.

>>_ugfj+z2
What happens if the process that performed 2. crashes before it was able to complete whatever processing it was supposed to do?

>>soroko+Lo
The idea, I think, is you wouldn't delete the job from the queue until the processing was done.

Of course, this relies on the jobs being something that can be retried.

>>jffry+ps
Looking at the code, some steps are missing in the description:

3. When processing is complete the matching table entry is deleted.

4. There is a "reaper" process that checks for stale jobs ( based on time in the future ) and resets them to 0.

This of ouffcourse raises more questions.