Redis Interview Guide: Concepts, Caching, Persistence, Replication, and Sentinel

The interview begins with a brief definition of Redis as an open‑source, high‑performance, in‑memory key‑value store written in C, supporting use cases such as database, cache, and message broker.

Key Redis data types are introduced: String, Hash, List, Set, and Sorted Set, each with typical use cases and internal storage details.

Caching with Spring Boot is demonstrated using RedisTemplate and Spring Cache annotations (@Cacheable, @CachePut, @CacheEvict). Sample Maven dependencies, application.yml configuration, entity class User.java, custom cache configuration RedisCacheConfig.java, and controller code illustrate how to store and retrieve objects from Redis.

Common cache problems are discussed, including cache inconsistency, cache avalanche, cache penetration, and cache breakdown, along with mitigation strategies such as random expiration, hot‑key handling, Bloom filters, and mutex locks (code example provided).

The article explains why Redis is fast despite being single‑threaded: in‑memory operations (O(1) hash lookups), simple data structures, avoidance of context switches, and non‑blocking I/O multiplexing.

Differences between Redis and Memcached are highlighted: persistence, data type support, value size limits, and underlying architecture.

Redis eviction policies (volatile‑lru, allkeys‑lru, volatile‑ttl, volatile‑random, allkeys‑random, noeviction) and the newer LFU policies are listed.

Persistence mechanisms are covered: RDB snapshots (forking a child process to write a temporary dump file) and AOF (append‑only file with configurable fsync strategies). Their advantages, disadvantages, and typical usage scenarios are explained.

Master‑slave replication is described, including the replication handshake, full and partial synchronization using PSYNC, runId, offset, and replication backlog. Problems of single‑master architecture and the role of Sentinel for automatic failover, monitoring, and configuration provisioning are detailed with step‑by‑step workflow diagrams.

Finally, the article summarizes the covered topics, emphasizing the importance of thorough preparation for Redis‑related interview questions.