Redis Overview: Features, Data Types, Caching Strategies, Persistence, Replication, and Sentinel

Redis is an open‑source, high‑performance, in‑memory key‑value store written in C, commonly used as a database, cache, or message broker and classified as a NoSQL system.

Key characteristics include a single‑threaded, thread‑safe design, support for distributed locks, five native data types, disk persistence, and publish/subscribe messaging.

The five data types—strings, hashes, lists, sets, and sorted sets—serve various scenarios such as simple value storage, object mapping, ordered collections, and ranking.

In Spring Boot, Redis can be accessed directly via RedisTemplate or integrated through Spring Cache annotations for declarative caching.

Typical cache issues include data consistency, cache avalanche, cache penetration, and cache breakdown; mitigation strategies involve local caches, rate limiting, random expiration, Bloom filters, and mutex locks.

Redis achieves high speed through O(1) hash‑map lookups, a simple KV structure, single‑threaded execution that avoids lock contention, and non‑blocking I/O multiplexing.

Eviction policies such as volatile‑*, allkeys‑*, LRU, and LFU are triggered when memory usage reaches configured thresholds.

Persistence is offered via RDB snapshots and AOF logs, each with trade‑offs; both can be enabled simultaneously for maximum durability.

Master‑slave replication follows a sequence of slave registration, socket connection, ping/pong health checks, data synchronization, and continuous command propagation.

Redis Sentinel provides monitoring, notification, automatic failover, and configuration provisioning to ensure high availability without manual intervention.