PHP + MySQL Queue

Question

I need a simple table that acts as a Queue. My MySQL server restriction is I can\'t use InnoDB tables, only MyISAM.

Clients/workers will work at the same time and they w

Answer 1

Just for information, there's another option that is using Gearman instead of a table to make the queue: Rasmus Lerdorf wrote a very nice article about it.

Answer 2

You need to turn your ordering around so there is no timing window.

Consumer POP (each consumer has a unique $consumer_id)

Update queue 
set last_pop = '$consumer_id' 
where last_pop is null 
order by id limit 1;

$job = 
  Select * from queue 
  where last_pop = '$consumer_id' 
  order by id desc 
  limit 1;

Supplier PUSH

insert into queue 
  (id, last_pop, ...) 
values 
  (NULL, NULL, ...);

The queue is ordered in time by the id column and assigned upon POP by to the consumer_id.

Answer 3

Oleg,

The solution is correct. $consumer_id must be a unique identifier for the processor. If you had a couple cron jobs on one machine, for example, you could use their pid as the consumer ID .

The UPDATE is atomic, so it marks exactly one row in the queue as being consumed by your ID.

For some applications I also have a status field for finished, so that if last_pop's consumer_id is set, but the finished flag is not set and the job is older than X, it can be marked to be restarted.