On Mon, Jan 9, 2012 at 2:58 PM, Chris Angelico <rosuav@gmail.com> wrote:
> In this project, we need to have a disk-based priority queue, with a
> few snazzy features (eg limited retries and scheduling) but nothing
> particularly exotic. Requests are added to the queue by any number of
> different processes, and it's not a problem if a request "misses the
> check" and is delayed by a few seconds. Requests are processed by a
> pool of workers which are unaware of each other (and may be running on
> different computers); the sole arbiter is the database itself.
Since posting this, I've researched advisory locks and ended up going
with a quite different model. In case anyone else wants to build a
system of this nature, here's what we're currently using.
The system is optimized, at any given time, for a particular number of
worker processes. For discussion purposes, let's say 8. (This number
is stored in the database.) Each worker queries the pg_locks table to
find an available slot, then claims it using pg_try_advisory_lock() -
the query is to avoid spamming the lock function unnecessarily (a
sequential scan would also work). The slots are numbered 0-7, and
represent partitions of the available work, using the sequential ID
modulo 8.
This is the biggest difference. Instead of marking individual calls as
'claimed', then processing them, then de-claiming them at the end, the
semaphoring is done on the entire partition and using advisory locks -
cutting MVCC spam 50% in the process. This technique does have a
vulnerability, however; if a worker process terminates (or the whole
computer it's running on dies), one eighth of the work will simply
never happen. This is dealt with out-of-band by having another process
"keep an eye on" the pg_locks table, checking that there are always 8
advisory locks in the appropriate section.
Performance is excellent, due in part to an index involving (id%8) -
this index is rebuilt by script any time the partition count is
changed, which is a significant administrative change involving a
shutdown of all workers.
To anyone who has need of unusual locking techniques, may I commend
advisory locks to your notice. They're high performance and come with
automatic unlocking on disconnect (which was an issue with the
original "update set claimed=N" technique - some process would have to
detect that a worker had gone, and reset the claimed marker for that
row). An excellent feature.
Chris Angelico
