Why does Google App Engine support a single thread of execution only?

App Engine uses a request-based execution model - that is, all work is scoped to a request, be it a user-facing one or one initated by another system such as the task queue. While it would be possible to use threads in this environment, most of the use-cases where multi-threading is useful involve long-running processes, which App Engine is not designed for, or offline work, in which case you're better off using the scalable facilities App Engine provides such as the task queue.

Put another way, threads are a specific solution to a general problem. App Engine provides an alternative for most use cases in the form of the Task Queue.

I don't know for sure, but I believe it is probably for security reasons. If they allow multiple threads, they are opening themselves up for a fork() bomb (or threading equivalent). Also, it greatly simplifies resource management - Google only needs to manage a single thread worth of resources per application.

There is a limited alternative to spawning threads in Google App Engine called task queues: http://code.google.com/appengine/docs/python/taskqueue/

EDIT

From http://code.google.com/appengine/docs/java/runtime.html#The_Sandbox:

To allow App Engine to distribute requests for applications across multiple web servers, and to prevent one application from interfering with another, the application runs in a restricted "sandbox" environment. In this environment, the application can execute code, store and query data in the App Engine datastore, use the App Engine mail, URL fetch and users services, and examine the user's web request and prepare the response.

Like other people have pointed out, threads are not supported for securities reason to sandbox applications.

There are many other restrictions within Google App Engine that forces developers to create scalable apps. I believe task queues are just another one of these restrictions because, as opposed to creating a thread on the current machine handling the HTTP request, a task is put into a queue which can then be schedule on and executed by other machines. Tasks queues allow work to shared and distributed amongst machines in a scalable manner.

I think this is a misleading question. The App Engine does not allow your application to spawn threads, but the app engine may launch multiple instances of your application or use some sort of threaded or multiprocess request handler. I don't know the specific details but without some sort of parallelism the app engine would be a pretty useless platform.

EDIT

My original answer incorrectly implied that threads are not a useful feature, they have many uses, but the majority of web developers do not manage threads within their applications. Threads are usually managed at lower levels of the application stack.