python threadsafe object cache

Question

Hi,

I have implemented a python webserver. Each http request spawns a new thread. I have a requirement of caching objects in memory and since its a webserver, I want the cache to be thread safe. Is there a standard implementatin of a thread safe object cache in python? I found the following

http://freshmeat.net/projects/lrucache/

This does not look to be thread safe. Can anybody point me to a good implementation of thread safe cache in python?

Thanks!

Answer 1

You probably want to use memcached instead. It's very fast, very stable, very popular, has good python libraries, and will allow you to grow to a distributed cache should you need to:

http://www.danga.com/memcached/

Answer 2

Thread per request is often a bad idea. If your server experiences huge spikes in load it will take the box to its knees. Consider using a thread pool that can grow to a limited size during peak usage and shrink to a smaller size when load is light.

Answer 3

Well a lot of operations in Python are thread-safe by default, so a standard dictionary should be ok (at least in certain respects). This is mostly due to the GIL, which will help avoid some of the more serious threading issues.

There's a list here: http://coreygoldberg.blogspot.com/2008/09/python-thread-synchronization-and.html that might be useful.

Though atomic nature of those operation just means that you won't have an entirely inconsistent state if you have two threads accessing a dictionary at the same time. So you wouldn't have a corrupted value. However you would (as with most multi-threading programming) not be able to rely on the specific order of those atomic operations.

So to cut a long story short...

If you have fairly simple requirements and aren't to bothered about the ordering of what get written into the cache then you can use a dictionary and know that you'll always get a consistent/not-corrupted value (it just might be out of date).

If you want to ensure that things are a bit more consistent with regard to reading and writing then you might want to look at Django's local memory cache:

http://code.djangoproject.com/browser/django/trunk/django/core/cache/backends/locmem.py

Which uses a read/write lock for locking.