Memcached vs. Redis: The First Choice

When you create an ElastiCache cluster, the very first decision is the engine. This choice is permanent — you cannot switch engines without rebuilding the cluster. Understanding the trade-offs is critical.

Memcached: The Simple, Multithreaded Cache

Memcached was designed in 2003 with a single purpose: cache objects as fast as possible. It does that one thing extremely well.

• Architecture: Truly multithreaded. A single Memcached node can saturate all CPU cores simultaneously, making it highly efficient on large multi-core instance types (e.g., r6g.16xlarge with 64 vCPUs).
• Data Model: Flat key-value only. Keys are strings; values are opaque blobs (strings, serialized JSON, binary). No structure beyond that.
• Persistence: None. If the node restarts, all data is gone. This is by design — Memcached treats itself as a disposable cache, not a data store.
• Replication: None. There is no built-in mechanism to copy data to a standby node.
• Horizontal Scaling: Client-side sharding only. The application (or a client library) decides which node holds which key. There is no cluster coordination.
• Use Case: Caching large volumes of simple, homogeneous objects — HTML fragments, serialized API responses, database query results — where data loss on node failure is acceptable and you need maximum throughput on multi-core hardware.

Redis: The Advanced Data Structure Store

Redis (Remote Dictionary Server) was designed as a data structure server, not just a cache. It happens to be extraordinarily fast, which makes it excellent for caching, but its capabilities extend far beyond that.

• Architecture: Single-threaded command processing (I/O is handled by multiple threads in Redis 6+, but command execution remains serialized). This eliminates lock contention and makes Redis's behavior highly predictable.
• Data Structures: Strings, Lists (queues/stacks), Sets (unique membership), Sorted Sets (ranked leaderboards), Hashes (field-value maps), Bitmaps, HyperLogLog (cardinality estimation), Streams (append-only log), Geo-spatial indexes.
• Persistence: Optional. Redis can write snapshots (RDB files) to disk on a schedule, or log every write operation (AOF — Append Only File) for near-zero data loss on restart.
• Replication: Built-in primary-replica replication with automatic failover (Redis Sentinel / ElastiCache's managed failover).
• Advanced Features: Pub/Sub messaging, Lua scripting, transactions (MULTI/EXEC), distributed locks (via SET NX PX).
• Use Case: Session stores, real-time leaderboards, rate limiting, distributed locks, message queues, geospatial queries, and complex caching patterns. Redis is the right choice for 95%+ of modern workloads.

Decision Matrix