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Wikipedia:

In computing, Memcached is a general-purpose distributed memory caching system. It is often used to speed up dynamic database-driven websites by caching data and objects in RAM to reduce the number of times an external data source (such as a database or API) must be read.

Memcached runs on Unix, Linux, Windows and Mac OS X and is distributed under a permissive free software license.[3]

Memcached’s APIs provide a giant hash table distributed across multiple machines. When the table is full, subsequent inserts cause older data to be purged in least recently used (LRU) order.[4][5] Applications using Memcached typically layer requests and additions into RAM before falling back on a slower backing store, such as a database.

Memcached was originally developed by Danga Interactive for LiveJournal, but is now used by many other systems, including MocoSpace,[6] YouTube,[7] Reddit,[8] Zynga,[9] Facebook,[10][11][12] Orange,[13] Twitter,[14] Tumblr[15] and Wikipedia.[16] Engine Yard and Jelastic are using Memcached as part of their platform as a service technology stack[17][18] and Heroku offers several Memcached services[19] as part of their platform as a service. Google App Engine, AppScale, Microsoft Azure and Amazon Web Services also offer a Memcached service through an API.[20][21][22][23]

The system uses a client–server architecture. The servers maintain a key–value associative array; the clients populate this array and query it. Keys are up to 250 bytes long and values can be at most 1 megabyte in size.

Clients use client-side libraries to contact the servers which, by default, expose their service at port 11211. Each client knows all servers; the servers do not communicate with each other. If a client wishes to set or read the value corresponding to a certain key, the client’s library first computes a hash of the key to determine the server to use. Then it contacts that server. The server will compute a second hash of the key to determine where to store or read the corresponding value.

The servers keep the values in RAM; if a server runs out of RAM, it discards the oldest values. Therefore, clients must treat Memcached as a transitory cache; they cannot assume that data stored in Memcached is still there when they need it. MemcacheDB, Couchbase Server, Tarantool and other database servers provide persistent storage while maintaining Memcached protocol compatibility.

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If all client libraries use the same hashing algorithm to determine servers, then clients can read each other’s cached data.

A typical deployment will have several servers and many clients. However, it is possible to use Memcached on a single computer, acting simultaneously as client and server.

Most deployments of Memcached exist within trusted networks where clients may freely connect to any server. There are cases, however, where Memcached is deployed in untrusted networks or where administrators would like to exercise control over the clients that are connecting. For this purpose Memcached can be compiled with optional SASL authentication support. The SASL support requires the binary protocol.

A presentation at BlackHat USA 2010 revealed that a number of large public websites had left Memcached open to inspection, analysis, retrieval, and modification of data.[28]

memcached.org:

memcached is a high-performance, distributed memory object caching system, generic in nature, but originally intended for use in speeding up dynamic web applications by alleviating database load.

You can think of it as a short-term memory for your applications.

php docs: Memcached   |   Memcache

Download: Memcached

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