Thread Safe web apps - why does it matter?

Question

Why does being thread safe matter in a web app? Pylons (Python web framework) uses a global application variable which is not thread safe. Does this matter? Is it only a problem if I intend on using multi-threading? Or, does it mean that one user might not have updated state if another user... I'm just confusing myself. What's so important with this?

Answer 1

Threading errors can lead to serious and subtle problems.

Say your system has 10 members. One more user signs up to your system and the application adds him to the roster and increments the count of members; "simultaneously", another user quits and the application removes him from the roster and decrements the count of members.

If you don't handling threading properly, your member count (which should be 10) could easily be nine, 10, or 11, and you'll never be able to reproduce the bug.

So be careful.

Answer 2

You should care about thread safety. E.g in java you write a servlet that provides some functionality. The container will deploy an instance of your servlet, and as HTTP requests arrive from clients, over different TCP connections, each request is handled by a separate thread which in turn will call your servlet. As a result, you will have your servlet being call from multiple threads. So if it is not thread-safe, then erroneous result will be returned to the user, due to data corruption of access to shared data by threads.