Caching

When your application runs slow, throw some caches in. Well, at least it’s the easiest way to speed up things. What does a cache do? Say you have a function that takes some time to complete but the results would still be good enough if they were 5 minutes old. So then the idea is that you actually put the result of that calculation into a cache for some time.

Flask itself does not provide caching for you, but Werkzeug, one of the libraries it is based on, has some very basic cache support. It supports multiple cache backends, normally you want to use a memcached server.

Setting up a Cache

You create a cache object once and keep it around, similar to how Flask objects are created. If you are using the development server you can create a SimpleCache object, that one is a simple cache that keeps the item stored in the memory of the Python interpreter:

from werkzeug.contrib.cache import SimpleCache
cache = SimpleCache()

If you want to use memcached, make sure to have one of the memcache modules supported (you get them from PyPI) and a memcached server running somewhere. This is how you connect to such an memcached server then:

from werkzeug.contrib.cache import MemcachedCache
cache = MemcachedCache(['127.0.0.1:11211'])

If you are using App Engine, you can connect to the App Engine memcache server easily:

from werkzeug.contrib.cache import GAEMemcachedCache
cache = GAEMemcachedCache()

Using a Cache

Now how can one use such a cache? There are two very important operations: get() and set(). This is how to use them:

To get an item from the cache call get() with a string as key name. If something is in the cache, it is returned. Otherwise that function will return None:

rv = cache.get('my-item')

To add items to the cache, use the set() method instead. The first argument is the key and the second the value that should be set. Also a timeout can be provided after which the cache will automatically remove item.

Here a full example how this looks like normally:

def get_my_item():
    rv = cache.get('my-item')
    if rv is None:
        rv = calculate_value()
        cache.set('my-item', rv, timeout=5 * 60)
    return rv

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