A few engineers at Google and other developers are doing some good work towards getting Rails running on AppEngine both robustly and in a way that provides a good local development environment. One problem is simply that if your web app is not active, initializing JRuby + Rails + and all required gems can time out (30 second window for handling requests).
The Java and Python support for AppEngine is fantastic, but for two projects I want to do (my own projects, but may be revenue generating I want a more agile programming language that Java and while my Python skills are sort-of OK, my knowledge of Django is very light.
I should probably just bite the bullet and spin up on Django, but I would strongly prefer working in Ruby. I have been experimenting with the JRuby + Sinatra + ERB + datamapper combination and at least an inactive web application spins up well within the 30 second request timeout window. I very much like datamapper (object identity issues) and it should not be too difficult to be completely portable on two platforms (given data import/export utilities):
- JRuby + AppEngine
- Ruby (1.8.x or 1.9.x) on any server
I like Sinatra as a light weight framework, and this technology choice is OK with me, except for one worry: my reason for wanting to use AppEngine (rather that Amazon EC2+S3+EBS, which is what I have been using for most customer projects and my own stuff) is to minimize my hosting costs if one or both of my ideas works out – I worry that using JRuby on Java AppEngine will not provide the same high performance as Java or Python web apps. I did compare (using the Apache benchmark tool) request times for JRuby + Sinatra + AppEngine (about 400 milliseconds/request) with Java + JSP + AppEngine (about 600 milliseconds per request). The rendered Java page was more complex so the benchmark between JRuby + Sinatra vs. Java + JSP looks like a wash.
For future projects if I need lots of back end processing (map reduce, spidering sites, etc.) then I will stick with Amazon EC2. If I can get by with just a web interface and a data store, then I would prefer AppEngine (to save a little money). Two great platforms!