Redis is a database application that keeps the entire database in RAM for fast performance. Though the entire dataset is kept in memory, it's also backed up on disk periodically, so it can serve as a persistent database. Redis supports complex data structures with a large number of list and set operations handled quickly on the server side.

The downside of the RAM storage is that speed falls off a cliff if the data size expands beyond available memory and the operating system starts paging virtual memory to handle accesses. This won't be a problem for small or predictably sized storage needs, but it does require some forward planning for application development. One way to deal with large data sets is by clustering multiple machines together, but the sharding is currently handled at the client level. There is currently active code development to make Redis support clustering at the server level.