Memcached¶

By far the fastest, most efficient type of cache available to Django, Memcached is an entirely memory-based cache framework originally developed to handle high loads at LiveJournal.com and subsequently open-sourced by Danga Interactive. It's used by sites such as Facebook and Wikipedia to reduce database access and dramatically increase site performance.

Memcached is available for free at http://danga.com/memcached/ . It runs as a daemon and is allotted a specified amount of RAM. All it does is provide an fast interface for adding, retrieving and deleting arbitrary data in the cache. All data is stored directly in memory, so there's no overhead of database or filesystem usage.

After installing Memcached itself, you'll need to install the Memcached Python bindings, which are not bundled with Django directly. Two versions of this are available. Choose and install one of the following modules:

• The fastest available option is a module called cmemcache, available at http://gijsbert.org/cmemcache/ .
• If you can't install cmemcache, you can install python-memcached, available at ftp://ftp.tummy.com/pub/python-memcached/ . If that URL is no longer valid, just go to the Memcached Web site (http://www.danga.com/memcached/) and get the Python bindings from the "Client APIs" section.

New in Django 1.0: The cmemcache option is new in 1.0. Previously, only python-memcached was supported.

To use Memcached with Django, set CACHE_BACKEND to memcached://ip:port/, where ip is the IP address of the Memcached daemon and port is the port on which Memcached is running.

In this example, Memcached is running on localhost (127.0.0.1) port 11211:

CACHE_BACKEND = 'memcached://127.0.0.1:11211/'

One excellent feature of Memcached is its ability to share cache over multiple servers. This means you can run Memcached daemons on multiple machines, and the program will treat the group of machines as a single cache, without the need to duplicate cache values on each machine. To take advantage of this feature, include all server addresses in CACHE_BACKEND, separated by semicolons.

In this example, the cache is shared over Memcached instances running on IP address 172.19.26.240 and 172.19.26.242, both on port 11211:

CACHE_BACKEND = 'memcached://172.19.26.240:11211;172.19.26.242:11211/'

In the following example, the cache is shared over Memcached instances running on the IP addresses 172.19.26.240 (port 11211), 172.19.26.242 (port 11212), and 172.19.26.244 (port 11213):

CACHE_BACKEND = 'memcached://172.19.26.240:11211;172.19.26.242:11212;172.19.26.244:11213/'

A final point about Memcached is that memory-based caching has one disadvantage: Because the cached data is stored in memory, the data will be lost if your server crashes. Clearly, memory isn't intended for permanent data storage, so don't rely on memory-based caching as your only data storage. Without a doubt, none of the Django caching backends should be used for permanent storage -- they're all intended to be solutions for caching, not storage -- but we point this out here because memory-based caching is particularly temporary.

Django も Memcached サポート。