What is memcached ?

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’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 requests and additions into RAM before falling back on a slower backing store, such as a database.

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 MB 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.

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

Converting 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) {
    data = db_select("SELECT * FROM users WHERE userid = ?", userid);
    return data;
 }

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

 function get_foo(int userid) {
    /* first try the cache */
    data = memcached_fetch("userrow:" + userid);
    if (!data) {
       /* not found : request database */
       data = db_select("SELECT * FROM users WHERE userid = ?", userid);
       /* then store in cache until next get */
       memcached_add("userrow:" + userid, data);
    }
    return data;
 }

The client 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.

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