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

Question

Does anybody have an idea why Google App Engine permits only a single thread of execution for a deployed application?

I personally believe that it has something to do with the predictability of an application so that Google can size its performance more reliably. There does not appear to be any rationale posted on Google's site regarding single threaded execution hence my question.

Having an application that is already multi-threaded and presently deployed on a VM means that it is difficult for me to move to the cloud given this restriction.

EDIT: I've marked the answer below as it sounds quite plausible that threads are not permitted due to horizontal scaling requirements. Naturally threads all execute within the same process space and, as GAE can run many processes for your application, it would be difficult to share threads. That said, I still think that a small thread pool per process would be useful and might help migrate apps to the cloud. I shall request this as a feature. Thanks for the discussion!

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.