memcached

memcached (pronunciation: mem-cash-dee) is a general-purpose distributed memory caching system that was originally developed by Danga Interactive for LiveJournal, but is now used by many other sites. 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 is distributed under a permissive free software license.^[1]

By default, memcached uses port 11211. Among other technologies, it uses libevent.

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. Applications using memcached typically layer memcached requests and additions into core before falling back on a slower backing store, such as a database.

The system is used by several very large, well-known sites including YouTube^[2], LiveJournal, Wikipedia/Wikimedia, Amazon.com, Wikia, SourceForge, Metacafe, Facebook^{[3] [4]}, Digg, Twitter^[5], Fotolog, The Pirate Bay^[6] and Netlog.^[7]

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 complied with optional SASL authentication support. The SASL support requires the binary protocol.

Note that all functions described on this page are pseudocode only. Memcached calls and programming languages may vary based on the used API.

Converting a database or object creation queries to use memcached is simple. Typically, when using straight database queries, example code would be as follows:

 function get_foo(int userid) {
    result = db_select("SELECT * FROM users WHERE userid = ?", userid);
    return result;
 }

After conversion to memcached, the same call might look like the following

 function get_foo(int userid) {
     result = memcached_fetch("userrow:" + userid);
     if (!result) {
         result = db_select("SELECT * FROM users WHERE userid = ?", userid);
         memcached_add("userrow:" + userid,  result);
     }
     return result;
 }

The server would first check whether a memcached value with the unique key "userrow:userid" exists, where userid is some number. If the result does not exist, it would select from the database as usual, and set the unique key using the memcached API add function call.

However, if only this API call were modified, the server would end up fetching incorrect data following any database update actions: the memcached "view" of the data would become out of date. Therefore, in addition to creating an "add" call, an update call would be also needed, using the memcached set function.

 function update_foo(int userid, string dbUpdateString) {
     result = db_execute(dbUpdateString);
     if (result) {
         data = createUserDataFromDBString(dbUpdateString);
         memcached_set("userrow:" + userid, data);
     }
 }

This call would update the currently cached data to match the new data in the database, assuming the database query succeeds. An alternative approach would be to invalidate the cache with the memcached delete function, so that subsequent fetches result in a cache miss. Similar action would need to be taken when database records were deleted, to maintain either a correct or incomplete cache.

• Memcachedb
• Redis

• Official memcached site
• memcached wiki and faq
• PHP Memcached Manager with Tag Support