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Perl: So, I wrote a new CPAN module recently — IPC::DirQueue. It implements a nifty design pattern for slightly larger systems, ones where multiple processes, possibly on multiple machines, must collaborate to deal with incoming task submissions. To quote the POD:

This module implements a FIFO queueing infrastructure, using a directory as the communications and storage media. No daemon process is required to manage the queue; all communication takes place via the filesystem.

A common UNIX system design pattern is to use a tool like lpr as a task queueing system; for example, this article describes the use of lpr as an MP3 jukebox.

However, lpr isn’t as efficient as it could be. When used in this way, you have to restart each task processor for every new task. If you have a lot of startup overhead, this can be very inefficient. With IPC::DirQueue, a processing server can run persistently and cache data needed across multiple tasks efficiently; it will not be restarted unless you restart it.

Multiple enqueueing and dequeueing processes on multiple hosts (NFS-safe locking is used) can run simultaneously, and safely, on the same queue.

Since multiple dequeuers can run simultaneously, this provides a good way to process a variable level of incoming tasks using a pre-defined number of worker processes.

If you need more CPU power working on a queue, you can simply start another dequeuer to help out. If you need less, kill off a few dequeuers.

If you need to take down the server to perform some maintainance or upgrades, just kill the dequeuer processes, perform the work, and start up new ones. Since there’s no ‘socket’ or similar point of failure aside from the directory itself, the queue will just quietly fill with waiting jobs until the new dequeuer is ready.

Arbitrary ‘name = value’ metadata pairs can be transferred alongside data files. In fact, in some cases, you may find it easier to send unused and empty data files, and just use the ‘metadata’ fields to transfer the details of what will be worked on.

Sound interesting? Here’s the tarball.