In light of the new billing changes on App Engine, I saw that you can enable concurrent requests in Java App Engine. What specifically do you have to be careful of when enabling concurrent requests?
The docs say:
By default, App Engine sends requests serially to a given web server.
You can configure App Engine to send multiple requests in parallel by
adding the <threadsafe> element to appengine-web.xml:
<threadsafe>true</threadsafe>
Note: If you wish to use concurrent
requests, your application code needs to use proper thread
synchronization before you enable <threadsafe>.
What exactly does this entail? I’ve developed against App Engine assuming requests are served in parallel by any number of machines. What actually changes when you assume concurrent requests on a single machine. I run no separate threads and have no global static variables. Is there anything else I need to worry about?
Adding the element to appengine-web.xml is all that should be required. The only reason you’d have trouble is if you have mutable globals; these could be modified in parallel by multiple threads, so you need to protect them with concurrency primitives.