Redis vs Memcached

Redis and Memcached are the two dominant in-memory data stores, but they serve different niches in modern system design.

Architecture Differences

Redis traditionally uses a single-threaded event loop for command processing, though recent versions added io_threads for network I/O parallelism. This single-threaded model simplifies atomicity guarantees for complex operations. Memcached is multi-threaded from the ground up, distributing connections across worker threads for parallel request processing.

Redis supports diverse data structures: strings, hashes, lists, sets, sorted sets, bitmaps, HyperLogLogs, and streams. Memcached stores only string key-value pairs with a maximum value size of 1MB by default.

Performance Characteristics

For simple get/set operations, both systems deliver sub-millisecond latency. Memcached can edge ahead under extreme concurrency for simple operations due to its multi-threaded design. Redis compensates with pipelining, scripting, and the ability to perform server-side computation that reduces round trips.

Redis's sorted sets enable real-time leaderboards with O(log N) insertions and range queries, something Memcached cannot do natively.

Trade-offs

Redis offers optional persistence (RDB snapshots and AOF logging) so data survives restarts. Memcached is purely volatile; a restart means a cold cache. For system design interviews, this distinction matters when discussing cache warming strategies.

Redis Cluster provides automatic sharding and failover. Memcached requires client-side consistent hashing, meaning the client library handles data distribution. This makes Memcached simpler but less resilient.

Memory Efficiency

Memcached uses a slab allocator that can be more memory-efficient for uniform-sized values. Redis has higher per-key overhead due to its richer data structures, but features like ziplist encoding optimize memory for small collections.

Real-World Usage

Redis is used by Twitter for timelines, GitHub for job queues, and Pinterest for real-time analytics. Memcached powers Facebook's caching layer (handling billions of requests per second) and is widely used at Wikipedia.

Learn more about caching strategies and cache invalidation patterns. Practice with our caching interview questions and see pricing for full platform access.