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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.
If all client libraries use the same hashing algorithm to determine servers, then clients can read each other’s cached data; this is obviously desirable.
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.
read the code there