Redis is an advanced key-value cache and store. It is often referred to as a data structure server since keys can contain strings, hashes, lists, sets, sorted sets, bitmaps, and hyperloglogs. Redis is an open source (BSD licensed), in-memory data structure store, used as a database, cache and message broker.
It supports data structures such as strings, hashes, lists, sets, sorted sets with range queries, bitmaps, hyperloglogs and geospatial indexes with radius queries. Redis has built-in replication, Lua scripting, LRU eviction, transactions and different levels of on-disk persistence, and provides high availability via Redis Sentinel and automatic partitioning with Redis Cluster.
You can run atomic operations on these types, like appending to a string; incrementing the value in a hash; pushing an element to a list; computing set intersection, union, and difference; or getting the member with highest ranking in a sorted set.
In order to achieve its outstanding performance, Redis works with an in-memory dataset. Depending on your use case, you can persist it either by dumping the dataset to disk every once in a while or by appending each command to a log. Persistence can be optionally disabled if you just need a feature-rich, networked, in-memory cache.
Redis also supports trivial-to-setup master-slave asynchronous replication, with very fast non-blocking first synchronization, auto-reconnection with partial resynchronization on netsplit.
Transactions: MULTI, EXEC, DISCARD and WATCH are the foundation of transactions in Redis. They allow the execution of a group of commands in a single step.
Pub/Sub: SUBSCRIBE, UNSUBSCRIBE and PUBLISH implement the Publish/Subscribe messaging paradigm where (citing Wikipedia) senders (publishers) are not programmed to send their messages to specific receivers (subscribers).
Lua scripting: EVAL and EVALSHA are used to evaluate scripts using the Lua interpreter built into Redis starting from version 2.6.0.
The first argument of EVAL is a Lua 5.1 script. The script does not need to define a Lua function (and should not). It is just a Lua program that will run in the context of the Redis server.
Keys with a limited time-to-live: Set a timeout on key. After the timeout has expired, the key will automatically be deleted. A key with an associated timeout is often said to be volatile in Redis terminology.
LRU eviction of keys: LRU is actually only one of the supported eviction methods. This page covers the more general topic of the Redis max memory directive that is used in order to limit the memory usage to a fixed amount, and it also covers in depth the LRU algorithm used by Redis, that is actually an approximation of the exact LRU.
You can use Redis from most programming languages out there.
Redis is written in ANSI C and works in most POSIX systems like Linux, *BSD, OS X without external dependencies. Linux and OS X are the two operating systems where Redis is developed and more tested, and we recommend using Linux for deploying. Redis may work in Solaris-derived systems like SmartOS, but the support is the best effort. There is no official support for Windows builds, but Microsoft develops and maintains a Win-64 port of Redis.
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