Redis is the most popular in-memory data store, used for caching, sessions, rate limiting, and queues. Its interviews test its rich data types, caching patterns, persistence options, and — importantly — when Redis is the right tool. Here are the Redis interview questions that actually get asked. (See also our backend and system design guides.)
Fundamentals & data types
- What is Redis, and why is it fast (in-memory, single-threaded)?
- Data types — strings, lists, sets, sorted sets, hashes — and a use case for each.
- When would you use a sorted set (e.g. leaderboards)?
- How does single-threaded Redis handle so many requests?
Caching & expiry
- Common caching patterns — cache-aside (lazy loading), write-through.
- TTL and expiration; eviction policies (LRU, LFU).
- How do you handle cache invalidation?
- Cache stampede / thundering herd — how do you prevent it?
Persistence & scaling
- Persistence — RDB snapshots vs AOF (append-only file).
- Is Redis a cache or a database — can you rely on it for durability?
- Replication, Sentinel, and Redis Cluster.
- Pub/sub and using Redis as a simple queue.
How to prepare
Redis rounds probe data types, caching, and persistence verbally. Practise explaining caching patterns and RDB vs AOF out loud. Greenroom runs spoken technical interviews that follow up on your reasoning. Pair it with our backend and DBMS guides.
Frequently asked questions
What are the most common Redis interview questions?
Common Redis questions cover what Redis is and why it's fast (in-memory, single-threaded), the data types (strings, lists, sets, sorted sets, hashes) and their use cases, caching patterns (cache-aside, write-through), TTL and eviction policies (LRU, LFU), cache invalidation and stampede prevention, persistence (RDB vs AOF), whether Redis can be a database, and replication, Sentinel and Cluster.
What data types does Redis support?
Redis supports strings (simple values, counters), lists (ordered, good for queues), sets (unique unordered members), sorted sets (members ordered by score, ideal for leaderboards and rate limiting), and hashes (field-value maps, good for objects). It also has bitmaps, HyperLogLog and streams. Choosing the right data type for the access pattern — like a sorted set for a leaderboard — is a common interview signal.
What is the difference between RDB and AOF persistence in Redis?
RDB takes point-in-time snapshots of the dataset at intervals, which is compact and fast to restore but can lose data written since the last snapshot. AOF (append-only file) logs every write operation, offering better durability with minimal data loss but larger files and slower restarts. Many production setups use both, choosing the balance of durability versus performance they need.
How should I prepare for a Redis interview?
Focus on the data types and when to use each, caching patterns like cache-aside, eviction policies, cache invalidation and stampede prevention, and persistence trade-offs (RDB vs AOF). Practise explaining caching patterns and persistence out loud with a voice-based mock interview that follows up, since Redis rounds probe practical usage and trade-offs rather than commands.