Job queue as SQL table with multiple consumers (PostgreSQL)

Question

I have a typical producer-consumer problem:

Multiple producer applications write job requests to a job-table on a PostgreSQL database.

The job requests have a state field that starts contains QUEUED on creation.

There are multiple consumer applications that are notified by a rule when a producer inserts a new record:

CREATE OR REPLACE RULE "jobrecord.added" AS   ON INSERT TO jobrecord DO    NOTIFY "jobrecordAdded"; 

They will try to reserve a new record by setting its state to RESERVED. Of course, only one consumer should succeed. All other consumers should not be able to reserve the same record. They should instead reserve other records with state=QUEUED.

Example: some producer added the following records to table jobrecord:

id state  owner  payload ------------------------ 1 QUEUED null   <data> 2 QUEUED null   <data> 3 QUEUED null   <data> 4 QUEUED null   <data> 

now, two consumers A, B want to process them. They start running at the same time. One should reserve id 1, the other one should reserve id 2, then the first one who finishes should reserve id 3 and so on..

In a pure multithreaded world, I would use a mutex to control access to the job queue, but the consumers are different processes that may run on different machines. They only access the same database, so all synchronization must happen through the database.

I read a lot of documentation about concurrent access and locking in PostgreSQL, e.g. http://www.postgresql.org/docs/9.0/interactive/explicit-locking.html Select unlocked row in Postgresql PostgreSQL and locking

From these topics, I learned, that the following SQL statement should do what I need:

UPDATE jobrecord   SET owner= :owner, state = :reserved    WHERE id = (       SELECT id from jobrecord WHERE state = :queued          ORDER BY id  LIMIT 1       )    RETURNING id;  // will only return an id when they reserved it successfully 

Unfortunately, when I run this in multiple consumer processes, in about 50% of the time, they still reserve the same record, both processing it and one overwriting the changes of the other.

What am I missing? How do I have to write the SQL statement so that multiple consumers will not reserve the same record?

Answer 1

I use postgres for a FIFO queue as well. I originally used ACCESS EXCLUSIVE, which yields correct results in high concurrency, but has the unfortunate effect of being mutually exclusive with pg_dump, which acquires a ACCESS SHARE lock during its execution. This causes my next() function to lock for a very long time (the duration of the pg_dump). This was not acceptable since we are a 24x7 shop and customers didn't like the dead time on the queue in the middle of the night.

I figured there must be a less-restrictive lock which would still be concurrent-safe and not lock while pg_dump is running. My search led me to this SO post.

Then I did some research.

The following modes are sufficient for a FIFO queue NEXT() function which will update the status of a job from queued to running without any concurrency fail, and also not block against pg_dump:

SHARE UPDATE EXCLUSIVE SHARE ROW EXCLUSIVE EXCLUSIVE 

Query:

begin; lock table tx_test_queue in exclusive mode; update      tx_test_queue set      status='running' where     job_id in (         select             job_id         from             tx_test_queue         where             status='queued'         order by              job_id asc         limit 1     ) returning job_id; commit; 

Result looks like:

UPDATE 1  job_id --------      98 (1 row) 

Here is a shell script which tests all of the different lock mode at high concurrency (30).

#!/bin/bash # RESULTS, feel free to repro yourself # # noLock                    FAIL # accessShare               FAIL # rowShare                  FAIL # rowExclusive              FAIL # shareUpdateExclusive      SUCCESS # share                     FAIL+DEADLOCKS # shareRowExclusive         SUCCESS # exclusive                 SUCCESS # accessExclusive           SUCCESS, but LOCKS against pg_dump  #config strategy="exclusive"  db=postgres dbuser=postgres queuecount=100 concurrency=30  # code psql84 -t -U $dbuser $db -c "create table tx_test_queue (job_id serial, status text);" # empty queue psql84 -t -U $dbuser $db -c "truncate tx_test_queue;"; echo "Simulating 10 second pg_dump with ACCESS SHARE" psql84 -t -U $dbuser $db -c "lock table tx_test_queue in ACCESS SHARE mode; select pg_sleep(10); select 'pg_dump finished...'" &  echo "Starting workers..." # queue $queuecount items seq $queuecount | xargs -n 1 -P $concurrency -I {} psql84 -q -U $dbuser $db -c "insert into tx_test_queue (status) values ('queued');" #psql84 -t -U $dbuser $db -c "select * from tx_test_queue order by job_id;" # process $queuecount w/concurrency of $concurrency case $strategy in     "noLock")               strategySql="update tx_test_queue set status='running{}' where job_id in (select job_id from tx_test_queue where status='queued' order by job_id asc limit 1);";;     "accessShare")          strategySql="lock table tx_test_queue in ACCESS SHARE mode; update tx_test_queue set status='running{}' where job_id in (select job_id from tx_test_queue where status='queued' order by job_id asc limit 1);";;     "rowShare")             strategySql="lock table tx_test_queue in ROW SHARE mode; update tx_test_queue set status='running{}' where job_id in (select job_id from tx_test_queue where status='queued' order by job_id asc limit 1);";;     "rowExclusive")         strategySql="lock table tx_test_queue in ROW EXCLUSIVE mode; update tx_test_queue set status='running{}' where job_id in (select job_id from tx_test_queue where status='queued' order by job_id asc limit 1);";;     "shareUpdateExclusive") strategySql="lock table tx_test_queue in SHARE UPDATE EXCLUSIVE mode; update tx_test_queue set status='running{}' where job_id in (select job_id from tx_test_queue where status='queued' order by job_id asc limit 1);";;     "share")                strategySql="lock table tx_test_queue in SHARE mode; update tx_test_queue set status='running{}' where job_id in (select job_id from tx_test_queue where status='queued' order by job_id asc limit 1);";;     "shareRowExclusive")    strategySql="lock table tx_test_queue in SHARE ROW EXCLUSIVE mode; update tx_test_queue set status='running{}' where job_id in (select job_id from tx_test_queue where status='queued' order by job_id asc limit 1);";;     "exclusive")            strategySql="lock table tx_test_queue in EXCLUSIVE mode; update tx_test_queue set status='running{}' where job_id in (select job_id from tx_test_queue where status='queued' order by job_id asc limit 1);";;     "accessExclusive")      strategySql="lock table tx_test_queue in ACCESS EXCLUSIVE mode; update tx_test_queue set status='running{}' where job_id in (select job_id from tx_test_queue where status='queued' order by job_id asc limit 1);";;     *) echo "Unknown strategy $strategy";; esac echo $strategySql seq $queuecount | xargs -n 1 -P $concurrency -I {} psql84 -U $dbuser $db -c "$strategySql" #psql84 -t -U $dbuser $db -c "select * from tx_test_queue order by job_id;" psql84 -U $dbuser $db -c "select count(distinct(status)) as should_output_100 from tx_test_queue;" psql84 -t -U $dbuser $db -c "drop table tx_test_queue;"; 

Code is here as well if you want to edit: https://gist.github.com/1083936

I am updating my application to use the EXCLUSIVE mode since it's the most restrictive mode that a) is correct and b) doesn't conflict with pg_dump. I chose the most restrictive since it seems the least risky in terms of changing the app from ACCESS EXCLUSIVE without being an uber-expert in postgres locking.

I feel pretty comfortable with my test rig and with the general ideas behind the answer. I hope that sharing this helps solve this problem for others.