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69 lines
2.5 KiB
69 lines
2.5 KiB
.. _caching-pattern: |
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Caching |
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======= |
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When your application runs slow, throw some caches in. Well, at least |
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it's the easiest way to speed up things. What does a cache do? Say you |
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have a function that takes some time to complete but the results would |
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still be good enough if they were 5 minutes old. So then the idea is that |
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you actually put the result of that calculation into a cache for some |
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time. |
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Flask itself does not provide caching for you, but Werkzeug, one of the |
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libraries it is based on, has some very basic cache support. It supports |
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multiple cache backends, normally you want to use a memcached server. |
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Setting up a Cache |
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------------------ |
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You create a cache object once and keep it around, similar to how |
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:class:`~flask.Flask` objects are created. If you are using the |
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development server you can create a |
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:class:`~werkzeug.contrib.cache.SimpleCache` object, that one is a simple |
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cache that keeps the item stored in the memory of the Python interpreter:: |
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from werkzeug.contrib.cache import SimpleCache |
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cache = SimpleCache() |
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If you want to use memcached, make sure to have one of the memcache modules |
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supported (you get them from `PyPI <https://pypi.python.org/pypi>`_) and a |
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memcached server running somewhere. This is how you connect to such an |
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memcached server then:: |
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from werkzeug.contrib.cache import MemcachedCache |
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cache = MemcachedCache(['127.0.0.1:11211']) |
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If you are using App Engine, you can connect to the App Engine memcache |
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server easily:: |
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from werkzeug.contrib.cache import GAEMemcachedCache |
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cache = GAEMemcachedCache() |
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Using a Cache |
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------------- |
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Now how can one use such a cache? There are two very important |
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operations: :meth:`~werkzeug.contrib.cache.BaseCache.get` and |
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:meth:`~werkzeug.contrib.cache.BaseCache.set`. This is how to use them: |
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To get an item from the cache call |
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:meth:`~werkzeug.contrib.cache.BaseCache.get` with a string as key name. |
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If something is in the cache, it is returned. Otherwise that function |
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will return ``None``:: |
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rv = cache.get('my-item') |
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To add items to the cache, use the :meth:`~werkzeug.contrib.cache.BaseCache.set` |
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method instead. The first argument is the key and the second the value |
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that should be set. Also a timeout can be provided after which the cache |
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will automatically remove item. |
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Here a full example how this looks like normally:: |
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def get_my_item(): |
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rv = cache.get('my-item') |
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if rv is None: |
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rv = calculate_value() |
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cache.set('my-item', rv, timeout=5 * 60) |
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return rv
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