32 Redis interview questions with answers

All Redis interview questions

• What is Redis and why is it extremely fast?

  Redis is an in-memory key-value store, commonly used for caching, sessions, counters, rate limiting and simple queues. Beyond strings it has built-in data structures such as lists, hashes, sets and sorted sets. Why it is fast: - Data lives in RAM. Reads and writes never…

• Redis data types: when should you use String, Hash, List, Set, and Sorted Set?

  Redis is a key-value store, but values come in several types, and each type comes with its own commands, so picking the right type saves you from doing the work in application code. The five core types: - String: the most basic type. Used for…

• Redis persistence: what is the difference between RDB and AOF, and when should you use each?

  RDB (snapshot) is a point-in-time image of the whole dataset, saved as an .rdb file. AOF (Append-Only File) is a log of every write command; on restart, Redis replays those commands to rebuild the data. The save line reads: snapshot if at least 1 change…

• What is Redis Pub/Sub? How does it work and what are its limitations compared to Kafka?

  Redis Pub/Sub lets a publisher push a message to a channel, and every subscriber currently listening on it receives the message immediately: It is fire-and-forget by nature: messages are never stored. If no subscriber is online at publish time the message is gone, and a…

• Transactions in Redis: how do MULTI/EXEC and pipeline differ?

  A transaction in Redis is a group of commands wrapped between MULTI and EXEC. After MULTI, the server only queues commands (replying QUEUED); at EXEC it runs the whole group back to back, with no other client's command in between. A pipeline is just a…

• Lua scripting in Redis: why use it and what are the typical use cases?

• Key expiration and eviction policies in Redis: what are the policies and when should you use each?

  These are two different mechanisms that people often mix up: expiration removes a key because its TTL ran out, while eviction removes keys because Redis hit the maxmemory limit, even keys that have not expired. Expiration. Set a TTL with EXPIRE key 60, PEXPIRE (ms)…

• Compare Redis and Memcached: when should you use each?

  Memcached is a pure cache: exactly one data type (string), multi-threaded, simple to operate. Redis brings a whole toolbox of data types (String, Hash, List, Set, Sorted Set, Stream, HyperLogLog, Geospatial) plus persistence, pub/sub, transactions, Lua scripting and native clustering. Choose Memcached when: you only…

• What are the most common production use cases for Redis?

  Redis is a multi-purpose in-memory store widely used for caching, sessions, rate limiting, leaderboards, and distributed locking in most modern production stacks. Most common production use cases for Redis: - Caching: storing database query results, API responses, and rendered HTML to reduce latency and database…

• Key naming conventions in Redis: best practices for organizing namespaces?

  The common convention is to join parts with colons in the order object type → id → attribute, e.g. user:1001:profile, order:5521:status. Redis does not interpret :, it is only a convention, but it lets tools such as RedisInsight show keys as a tree and makes…

• How does Redis Cluster work? Explain sharding, slot allocation, and resharding.

• What are Redis Streams? How do they compare to Pub/Sub and Kafka, and when should you use them?

• Memory optimization in Redis: what techniques reduce memory footprint?

• What is a cache stampede (thundering herd)? How do you prevent it in Redis?

• Redis in production: what are the most important best practices to follow?

• What is Redis HyperLogLog? When should you use it instead of a Set?

  HyperLogLog (HLL) is a probabilistic data structure for counting distinct elements (cardinality). It does not store the elements themselves, only a summary of at most 12KB; in exchange the result is an estimate with a 0.81% standard error. PFADD records elements, PFCOUNT returns the estimate,…

• What are Redis Bitmaps? What real-world applications are they best suited for?

  A Redis Bitmap is not a separate data type; it is a String manipulated bit by bit. A String can be up to 512MB, about 4.29 billion bits, and each bit is addressed by an offset. Bitmaps fit best when you need a yes/no answer…

• Redis Sorted Set: how do you implement a real-time leaderboard and a sliding window rate limiter?

  A Sorted Set (ZSET) is a set of unique members, each tagged with a numeric score. Redis always keeps them ordered by score, so inserts and updates cost O(log N) and reading by rank is cheap. Both problems rely on this property. Leaderboard: the member…

• When should you use Redis Hash vs String to store an object? What are the memory and performance trade-offs?

  There are two ways to store an object such as user:1: - String: serialize the whole object as JSON into one key. Reading the whole object takes a single GET. - Hash: each attribute is a field inside the key, so fields can be read…

• Caching strategies: Cache-Aside, Write-Through, Write-Behind — when should you use each?

  Cache-Aside (lazy loading) means the application manages the cache itself: read the cache first, and only on a miss read the DB and write the result into the cache. On a write, the application updates the DB and then deletes (or updates) the key in…

• Distributed locking with Redis: how do you implement it? What problem does the Redlock algorithm solve?

• Session storage with Redis: why is Redis a better choice than a database for sessions?

  Sessions have three requirements: very fast reads (every authenticated request has to look up the session), automatic expiry (log out after too long idle) and no need for long-term storage. Redis covers all three out of the box: data lives in RAM, GET/SET by key…

• Cache invalidation strategies: how do you handle data freshness in Redis?

• What is Redis Sentinel? How does it provide High Availability?

  Redis Sentinel is a set of separate monitoring processes running alongside a Redis setup of one master and its replicas. When the master dies, Sentinel promotes a replica to be the new master and tells clients the new address, with no human involved. Sentinel does…

• Redis Cluster: hash slots, MOVED/ASK redirects, and how sharding works?

• Redis eviction policies: LRU, LFU, allkeys vs volatile — when should you use each policy?

  When data reaches the maxmemory limit, the eviction policy decides which key Redis deletes to make room for a new write. You set it with maxmemory-policy, and it combines two choices: by what criterion to delete, and from which set of keys. Deletion criterion: -…

• What is Redis Pipelining? When does it significantly improve performance?

• What is a cache stampede (thundering herd) and how do you handle it?

• When should you NOT use Redis?

  Redis is powerful because it runs in-memory and executes commands single-threaded — those same traits are its limits: - Memory-bound: the whole (hot) dataset must fit in RAM, which is far more expensive than disk. A poor fit as a primary store for data larger…

• What security features does Redis provide?

  Redis is designed by default to run on a trusted internal network, so the protections below must be configured yourself once it is exposed beyond that. - AUTH (requirepass): set a password; clients must AUTH <password before issuing commands. A basic mechanism, one password for…

• Redis lives in RAM — is data lost on a power cut? Should Redis be the primary database?

  Redis does persist to disk, but its durability is weaker than an RDBMS and you must understand it before relying on it. Two mechanisms: - RDB: periodic point-in-time snapshots of the dataset. Small files, fast restart, but loses writes made after the last snapshot (often…

• What is a hot key in Redis Cluster? How do you detect and fix it?

  A hot key is one key receiving disproportionate traffic — a global config entry, a flash-sale product, the counter of a viral post. In Redis Cluster a key maps to a hash slot, and each slot lives on exactly one node, so all traffic for…