Redis (data store)
|Stable release||2.2.2 / March 4, 2011 (2011-03-04)|
|Written in||ANSI C|
 Data model
The Redis data model is - in its outer layer - a dictionary where keys are mapped to values. One of the main differences between Redis and other structured storage systems is that values are not limited to strings. In addition to strings, the following abstract data types are supported:
- 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 are strings and values are either strings or integers
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 RAM, but can also be configured to use virtual memory. Persistence is 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. Since version 1.1 the safer alternative is 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.
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.
 See also
- 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.
- ^ VMware: the new Redis home
- ^ VMWare: The Console: VMware hires key developer for Redis
- ^ Redis documentation "Virtual Memory", redis.io, accessed January 18, 2011.
- ^ http://code.google.com/p/redis/wiki/ReplicationHowto
- ^ A. Charnock: "Redis Benchmarking on Amazon EC2, Flexiscale, and Slicehost"