At the Geek Night last night there was a lot of questions about how Heroku compared with Google App Engine (and I guess implicit in that how it compares to GAE/J with the JRuby gem). Since it came up several times I thought it would be interesting to capture some of the answers.
Firstly there are the similarities, both deal with the application as the base unit of cloud scaling. Heroku offers several services that are similar to GAE, e.g. mailing, scheduled and background jobs. Some aspects like Memcache are in beta with Heroku but fundamentally both services seem to concur.
They are both free to start developing and then the pricing for the system varies a bit between the two, GAE might be cheaper but also has a more complex set of variables that go into the pricing, Heroku prices on: data size, web processes required and then options, so it’s easier to project your costs.
Differences then: well a big one is that Heroku uses a standard SQL database and also has excellent import/export facilities. GAE uses Google BigTable which is great for scaling and availability but data export and interactions are very limited. Heroku has an amazing deployment mechanism that involves simply pushing to a remote branch. GAE is slightly more involved but it is still relatively easy compared with deploying your own applications.
GAE provides access to a variety of Google services that are handy such as Google Account integration and image manipulation. GAE also has XMPP integration which is pretty unique if you have the requirement. However these do tie you to the platform and if you wanted to migrate you’d have to avoid using the Google webapp framework and the API. If flexibility is an issue you are probably going to prefer Heroku’s use of standard Ruby libraries and frameworks.
Heroku has few restrictions on what can and can’t be done in a web process, you also have quite powerful access to the user space your application runs in. GAE and particularly GAE/J has a lot of restrictions which can be quite painful particularly as there can be a discrepency between development and deployment environments.
However running a JVM in the cloud provides a lot of flexibility in terms of languages that can be used and deployed. Heroku is focussing on Ruby for now.
From my point of view, choose Heroku if you need: uncomplicated Rails/Rack application deployment, a conventional datastore that you want to have good access to, a simple pricing model. Choose GAE if you want to have out of the box access to Google’s services and a complete scaling solution.