|Initial release||April 10, 2009|
|Stable release||2.8.1 / November 25, 2013|
|Written in||ANSI C|
Redis is an open-source, networked, in-memory, key-value data store with optional durability. It is written in ANSI C. The development of Redis has been sponsored by Pivotal since May 2013; until then, it was sponsored by VMWare. According to the monthly ranking by DB-Engines.com, Redis is the most popular key-value store.
In its outer layer, the Redis data model is a dictionary which maps keys to values. One of the main differences between Redis and other structured storage systems is that Redis supports not only strings, but also abstract data types:
- Lists of strings
- Sets of strings (collections of non-repeating unsorted elements)
- Sorted sets of strings (collections of non-repeating elements ordered by a floating-point number called score)
- Hashes where keys and values are strings
The type of a value determines what operations (called commands) are available for the value itself. Redis supports high level atomic server side operations like intersection, union, and difference between sets and sorting of lists, sets and sorted sets.
Redis typically holds the whole dataset in memory. Versions up to 2.4 could be configured to use what they refer to as virtual memory (not to be confused with the memory-management technique virtual memory), in which some of the dataset is stored on disk, but this feature is deprecated. Persistence is now reached in two different ways: one is called snapshotting, and is a semi-persistent durability mode where the dataset is asynchronously transferred from memory to disk from time to time, written in RDB dump format. Since version 1.1 the safer alternative is AOF, an append-only file (a journal) that is written as operations modifying the dataset in memory are processed. Redis is able to rewrite the append-only file in the background in order to avoid an indefinite growth of the journal.
Redis supports master-slave replication. Data from any Redis server can replicate to any number of slaves. A slave may be a master to another slave. This allows Redis to implement a single-rooted replication tree. Redis slaves are writable, permitting intentional and unintentional inconsistency between instances. The Publish/Subscribe feature is fully implemented, so a client of a slave may SUBSCRIBE to a channel and receive a full feed of messages PUBLISHed to the master, anywhere up the replication tree. Replication is useful for read (but not write) scalability or data redundancy.
When the durability of data is not needed, the in-memory nature of Redis allows it to perform extremely well compared to database systems that write every change to disk before considering a transaction committed. There is no notable speed difference between write and read operations. Redis operates as a single process and single threaded. Therefore a single Redis instance cannot utilize parallel execution of tasks e.g. stored procedures (Lua scripts).
- Aerospike database
- Apache Cassandra
- Berkeley DB
- Couchbase Server
- Garantia Data
- Kyoto Cabinet
- MySQL Cluster
- Oracle Coherence
- Tokyo Cabinet
- Redis Sponsors – Redis
- VMware: the new Redis home
- VMWare: The Console: VMware hires key developer for Redis
- DB-Engines Ranking of Key-value Stores
- Redis language bindings
- CRAN – Package rredis
- Redis documentation "Virtual Memory", redis.io, accessed January 18, 2011.
- ReplicationHowto – redis – A persistent key-value database with built-in net interface written in ANSI-C for Posix systems – Google Project Hosting
- Jeremy Zawodny, Redis: Lightweight key/value Store That Goes the Extra Mile, Linux Magazine, August 31, 2009
- Isabel Drost and Jan Lehnard (29 October 2009), Happenings: NoSQL Conference, Berlin, The H. Slides for the Redis presentation. Summary.
- Billy Newport (IBM): "Evolving the Key/Value Programming Model to a Higher Level" Qcon Conference 2009 San Francisco.