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

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

Features

Excellent performance with data stored in memory, supporting up to 100K QPS.

Single‑threaded process that is thread‑safe, using I/O multiplexing.

Can be used as a distributed lock.

Supports five primary data types.

Provides data persistence to disk.

Can serve as a message broker with publish/subscribe capabilities.

Data Types

Redis supports five main data structures—strings, lists, sets, sorted sets, and hashes—each suited to different use cases.

Caching

In Spring Boot, Redis is commonly used either directly via RedisTemplate or through Spring Cache annotations.

Cache Issues

Data Consistency

In distributed environments, cache‑DB consistency is difficult; typical strategies include updating the cache immediately after a DB write and implementing retry mechanisms for cache failures.

Cache Avalanche

If the cache crashes, all traffic falls back to the database, potentially causing overload; mitigation includes high‑availability setups (master‑slave + Sentinel or clustering), local Ehcache with rate limiting, and enabling persistence for quick recovery.

Cache Penetration

Requests for non‑existent keys can bypass the cache and hammer the DB; solutions involve request validation, Bloom filters, and caching negative results.

Cache Breakdown

Hot keys that expire can cause a sudden surge of DB requests; solutions include never‑expiring static data, using distributed locks for cache refresh, or proactive cache rebuilding before expiration.

Performance

Redis achieves high throughput by using O(1) hash‑map operations, a simple KV data model, single‑threaded execution that avoids lock contention, and non‑blocking I/O multiplexing.

Eviction Policies

volatile‑* (evicts expired keys)

allkeys‑* (evicts any key)

LRU (least recently used)

LFU (least frequently used)

Persistence

Redis offers two persistence mechanisms: RDB snapshots (periodic dumps of memory to disk) and AOF (append‑only file logging every write command). They can be used together for greater durability.

Replication

Master‑slave replication synchronizes data from the master to slaves and propagates write commands to keep slaves up‑to‑date.

Sentinel

Redis Sentinel provides monitoring, notification, automatic failover, and configuration services to ensure high availability when the master node fails.