Memcached

Memcached is a free and open-source high-performance distributed memory caching system. It provides an in-memory key-value store intended to speed up dynamic web applications by alleviating database load. Data are stored as opaque byte strings, with keys chosen by the application. Memcached is typically used to cache query results, session data, and rendered page fragments.

Memcached runs as a daemon on one or more servers and is accessed via a client library in the application language. It stores data in RAM and provides fast lookups. Memory management is handled internally using a slab allocator and an eviction policy, commonly least recently used, to reclaim space when memory fills.

The service is designed to be simple and stateless; there is no built-in persistence or replication. If a memcached process restarts, cached data is lost. Clients often distribute keys across multiple servers using hashing, with many deployments adopting a cache-aside pattern: check cache first, fall back to the data store, then populate cache.

Typical deployments involve clusters of memcached nodes behind firewalls or in private networks. Capacity planning focuses on cache hit rate and available memory. The software is widely supported by client libraries in languages such as Java, Python, PHP, C#, Ruby, and C.

Security considerations include limited authentication and no encryption by default, so memcached is usually restricted to trusted internal networks. Exposing it directly to the internet is discouraged; optional tunneling or proxies can mitigate risks.

Memcached was originally developed by Brad Fitzpatrick in 2003 for LiveJournal and released under a BSD-compatible license. It became a widely adopted caching layer and inspired similar systems, though it remains distinct from more feature-rich data stores like Redis.