Redis

Redis (short for Remote Dictionary Server) is an in-memory key-value data store prized for extremely low latency — individual operations complete in microseconds. Although it is called a database, in practice it usually acts as a universal layer between your application and a slower persistent store such as MySQL or PostgreSQL.

How Redis works

Redis holds all data in RAM, which makes reads and writes orders of magnitude faster than disk-backed storage. It is more than a plain cache, though: Redis understands the structure of the values it stores and ships ready-made operations for several data types:

• strings — simple values, counters, flags,
• lists — job queues (push and pop from either end),
• sets and sorted sets — unique collections and weighted leaderboards,
• hashes — multi-field objects such as a user session,
• pub/sub and streams — real-time event delivery.

So data is not lost on restart, Redis provides durability through RDB snapshots and the AOF log; it also supports master-replica replication and clustering for high availability.

Redis in practice

The most common role is offloading a relational database by acting as a caching buffer: query results or whole page fragments live in memory and are served in a fraction of a millisecond. Beyond caching, Redis serves as a web-session store, a broker for background task queues (sending mail, generating reports), a request-limit counter for rate limiting, and an event bus for real-time features. On hosting platforms and VPS servers Redis is frequently the default cache and session engine for PHP applications, measurably cutting response times under load.