Redis is a key-value database. It is similar to memcached but the dataset is not volatile, and values can be strings, exactly like in memcached, but also lists and sets with atomic operations to push/pop elements.
In order to be very fast but at the same time persistent the whole dataset is taken in memory and from time to time and/or when a number of changes to the dataset are performed it is written asynchronously on disk. You may lost the last few queries that is acceptable in many applications but it is as fast as an in memory DB (Redis supports non-blocking master-slave replication in order to solve this problem by redundancy).
Compression and other interesting features are a work in progress. Redis is written in ANSI C and works in most POSIX systems like Linux, *BSD, Mac OS X, and so on. Redis is free software released under the very liberal BSD license.
It is possible to think at Redis as a data structures server, it is not just another key-value DB, see all the commands supported by Redis to get the first feeling. Redis supports operations like atomic push and pop of elements on lists, taking ranges of elements from this lists at once, trimming of lists, server-side intersections of sets and even sorting data! To show our points we wrote a simple Twitter clone with PHP + Redis: it's a very simple but still a real world example of web application, the article explains step by step how to write scalable applications with Redis.
Supported languages: PHP, Java, Ruby, Python, Perl, Tcl, Erlang, Scala, Lua etc.
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