Redis is an open-source, in-memory data structure store that can be used as a database, cache, message broker, and streaming engine. It stores data in RAM rather than on disk, which gives it sub-millisecond response times that disk-based databases cannot match for read-heavy workloads.
The data structure support is what makes Redis more than just a cache. Beyond simple key-value pairs, it supports strings, hashes, lists, sets, sorted sets, bitmaps, and streams. Sorted sets in particular are useful for leaderboards, rate limiting, and time-series data.
For web applications, Redis is most commonly used as a caching layer in front of a primary database. Frequently accessed data user sessions, API responses, computed results is stored in Redis so that repeated requests are served from memory rather than hitting the database.
For real-time features like live notifications, chat, and collaborative editing, Redis pub/sub provides a lightweight message passing system. The sorted set data structure is a natural fit for leaderboards and rate limiting, where you need to rank items by score and query ranges efficiently.
Redis is free and open source. Redis Cloud offers managed hosting with a free tier. Paid plans start at 7 dollars per month for production use with persistence and backups.