Background workers only cost you money (5¢/hour) when they're running. It's prorated to the second. You can avoid running one constantly by using pedro's "autoscaling" branch, meaning you can run a few hours of background processing--which should equate to hundreds or thousands of jobs--every month for a few nickels.
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Background job processing is so essential to any app that I've ever built, that the lack of suitable free background processing (whether via cron or DJ) effectively makes Heroku not have a free option for me. Even if I'm writing a little toy app, it will probably involve DJ and cron, so it will need to be hosted somewhere else than Heroku.
(as for bigger apps, let's not get started... Heroku is pretty expensive at scale, not sure what they have that justifies it)
Ease of use, Heroku is really REALLY easy to use compared to GAE. I was going to go for GAE for a project a few months ago, and decided on Heroku because I can just get straight to work instead of fapping about configuring things.
On another, slightly related note you could use Mirah on GAE, a ruby like language. It's pretty neat.
I couldn't think of a reason to go with it with projects like Celery around, but I guess if my host was trying to charge a lot for background processing, this would do it.
This is a nice little hack that I'll stick into my toolbox. The comment on the post about turning the workers on and off when needed is interesting as well. The commenter uses less than an hour on average per month using the method. I wonder if it's time consuming to do this though?
Background processing and regular processing are ultimately the same thing. It doesn't make sense to me why Heroku would try to price discriminate around that feature.
They don't. They both cost 5¢/hr per process. The only difference is, you get 1 'foreground' worker for free (per application), but not any 'background' workers.
BTW, don't get me wrong: Heroku is a standout service, and I use it as an example of how dramatically easier it is to start a technology business compared to even 10 years ago. I just have this little nitpicky complaint, that they ought to include one background process for free. The lack of it is actually blocking me from using Heroku for a startup idea at the moment. (I could always pay, sure, but when you're at the proof-of-concept stage, you're reluctant to put out even that much money.)
http://github.com/pedro/delayed_job/tree/autoscaling
Pedro is a Heroku employee, so this use is totally fine by them, although the branch is not officially supported software.
[I'm friends with a few guys there, but I don't work there.]