[Pdx-pm] queueing, avoiding race conditions

Tom Phoenix tom.phoenix at gmail.com
Wed Aug 10 13:03:48 PDT 2005

Maybe there's a good reason not to do it this way, but it's simple and

1. Every request gets put into a queue.

The queue could be kept in a file (locked with flock), or in a
database, or on a filesystem. (One simple implementation on a
unix-like filesystem would be to write the request as a complete file
in directory X, then hard-link the file into directory Y, which is the
actual queue directory. By using the hard link, the complete request
"appears" in the queue at once, avoiding race conditions.)

2. A simple process drains the queue, one request at a time.

This process is started automatically just after an item is queued,
unless there's already a process running.

3. When the queue is empty, the process simply quits. Or it sleeps and
awaits SIGCONT to be sent after an item is queued, perhaps.

> the bugbear of this problem seems to be race conditions.

If a single process drains the queue, there are relatively few race
conditions to deal with.

Maybe you don't want the overhead of queueing for every item when
there's usually no need for it. I can't blame you, but the simpler
algorithm has fewer places for race conditions to hide.

Or have I missed something important? Hope this helps!

--Tom Phoenix

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