.. _caching-pattern:

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
:class:`~flask.Flask` objects are created.  If you are using the
development server you can create a
:class:`~werkzeug.contrib.cache.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 <http://pypi.python.org/>`_) 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: :meth:`~werkzeug.contrib.cache.BaseCache.get` and 
:meth:`~werkzeug.contrib.cache.BaseCache.set`.  This is how to use them:

To get an item from the cache call
:meth:`~werkzeug.contrib.cache.BaseCache.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 :meth:`~werkzeug.contrib.cache.BaseCache.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
