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

1. Introduction to Redis

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 belongs to the NoSQL family.

2. Key Characteristics

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

Single‑threaded but thread‑safe, using non‑blocking I/O multiplexing.

Can serve as a distributed lock.

Supports five primary data structures.

Provides persistence to disk.

Offers publish/subscribe messaging capabilities.

3. Data Types

Redis supports five data types—strings, hashes, lists, sets, and sorted sets—each suited to different usage scenarios.

4. Caching in Spring Boot

Two typical integration methods are direct usage of RedisTemplate and Spring Cache annotations.

5. Common Caching Problems

(1) Data Consistency

In distributed environments, cache‑database consistency is hard to guarantee; strategies such as cache‑update after DB writes or retry mechanisms are used to mitigate inconsistency.

(2) Cache Avalanche

If the cache crashes, all traffic falls back to the database, potentially overwhelming it; solutions include high‑availability Redis clusters, local Ehcache with rate limiting, and graceful degradation.

(3) Cache Penetration

When requests query keys that do not exist in both cache and DB, attackers can overload the database; mitigations involve request validation, Bloom filters, and storing negative results in Redis.

(4) Cache Breakdown

Hot keys that expire simultaneously cause a surge of DB queries; solutions depend on data change frequency, ranging from never‑expiring hot keys to distributed locks or proactive cache refresh.

6. Why Redis Is Fast

Hash‑map‑like O(1) operations performed entirely in memory.

Simple KV data structures.

Single‑threaded model eliminates lock contention and context switches.

Non‑blocking I/O with multiplexing.

7. Eviction Policies

volatile‑* policies evict only expired keys.

allkeys‑* policies consider all keys.

LRU (Least Recently Used) and LFU (Least Frequently Used) are supported.

Policies trigger when Redis memory reaches the configured threshold.

8. Persistence Mechanisms

RDB : Periodic snapshots saved to a dump file.

AOF : Append‑only file that logs every write command.

Both can be enabled; on restart, AOF is preferred for minimal data loss.

9. Master‑Slave Replication

Slaves execute SLAVEOF to record master info, establish socket connections, exchange PING/PONG, and receive data synchronization; thereafter, masters stream write commands to slaves to keep data consistent.

10. Sentinel Mode

Sentinel monitors master and slave health, sends notifications, performs automatic failover by promoting a slave to master, and acts as a configuration provider for clients.