Queuing systems - what is a good way to start up multiple workers?

Question

• How have you set-up one or more worker scripts for queue-oriented systems?
• How do you arrange to startup - and restart if necessary - worker scripts as required? (I'm thinking about such tools as init.d/, Ruby-based 'god', DJB's Daemontools, etc, etc)

I'm developing an asynchronous queue/worker system, in this case using PHP & BeanstalkdD (though the actual language and daemon isn't important). The tasks themselves are not too hard - encoding an array with the commands and parameters into JSON for transport through the Beanstalkd daemon, picking them up in a worker script to action them as required.

There are a number of other similar queue/worker setups out there, such as Starling, Gearman, Amazon's SQS and other more 'enterprise' oriented systems like IBM's MQ and RabbitMQ. If you run something like Gearman, or SQS - how do you start and control the worker pool? The questions is on the initial worker startup, and then being able to add additional extra workers, shutting them down at will (though I can send a message through the queue to shut them down - as long as some 'watcher' won't automatically restart them). This is not a PHP problem, it's about straight Unix processes of setting up one or more processes to run on startup, or adding more workers to the pool.

A bash script to loop a script is already in place - this calls the PHP script which then collects and runs tasks from the queue, occasionally exiting to be able to clean itself up (it can also pause a few seconds on failure, or via a planned event). This works fine, and building the worker processes on top of that won't be very hard at all.

Getting a good worker controller system is about flexibility, starting one or two automatically on a machine start, and being able to add a couple more from the command line when the queue is busy, shutting down the extras when no longer required.

Answer 1

Real message queuing middleware like WebSphere MQ or MSMQ offer "triggers" where a service that is part of the MQM will start a worker when new messages are placed into a queue.

AFAIK, no "web service" queuing system can do that, by the nature of the beast. However I have only looked hard at SQS. There you have to poll the queue, and in Amazon's case overly eager polling is going to cost you some real $$.

Answer 2

I've been helping a friend who's working on a project that involves a Gearman-based queue that will dispatch various asynchronous jobs to various PHP and C daemons on a pool of several servers.

The workers have been designed to behave just like classic unix/linux daemons, thanks to simple shell scripts in /etc/init.d/, and commands like :

invoke-rc.d myWorker start|stop|restart|reload

This mechanism is simple and efficient. And as it relies on standard linux features, even people with a limited knowledge of your app can launch a daemon or stop one, if they know how it's called system-wise (aka "myWorker" in the above example).

Another advantage of this mechanism is it makes your workers pool management easy as well. You could have 10 daemons on your machine (myWorker1, myWorker2, ...) and have a "worker manager" start or stop them depending on the queue length. And as these commands can be run through ssh, you can easily manage several servers.

This solution may sound cheap, but if you build it with well-coded daemons and reliable management scripts, I don't see why it would be less efficient than big-bucks solutions, for any average (as in "non critical") project.