Redis

Redis is an open-source in-memory data structure store, used as database, cache, and message broker. It supports rich data structures: strings, lists, sets, sorted sets, hashes, bitmaps, hyperLogLogs, streams, and geospatial indexes. It is designed for low latency and high throughput. Redis is implemented in C and is distributed under the BSD license.

Durability and persistence: Redis stores data in memory for speed, with optional durability via snapshotting (RDB) and an append-only file (AOF). RDB provides point-in-time snapshots; AOF logs each write operation for more durable recovery, with configurable rewrite policies. Redis can use both or neither depending on configuration.

Replication and high availability: Redis supports master-slave replication, asynchronous by default, with options for read scaling and eventual consistency. Redis Sentinel provides monitoring, failover, and configuration management to achieve high availability. Redis Cluster adds built-in sharding across 16,384 hash slots, enabling horizontal scaling with automatic reconfiguration after rebalancing.

Use cases, performance, and limits: Common uses include caching, session stores, real-time analytics, leaderboards, pub/sub messaging, and queueing. Performance is typically measured in sub-millisecond latency; memory usage is a key consideration because data is stored in RAM. When memory limits are reached, eviction policies can remove entries based on various strategies; persistence tradeoffs apply.

Ecosystem and extensions: Redis has a rich ecosystem of client libraries for many languages and multiple modules that extend functionality with search, time series, graphs, and JSON support. Cloud offerings and commercial editions exist, including managed services. Licensing remains BSD-licensed, enabling wide adoption.