# Redis, Memcached and Cache Clusters — Caching Strategies & CDN Design

Source: https://www.skillbyai.com/en/caching-strategies/d-distributed

> Compare Redis and Memcached and understand how cache clusters shard and replicate.

## Two popular distributed caches

**Memcached** is a simple, multi-threaded, in-memory key-value cache: strings in, strings out, LRU eviction, no persistence or replication built in. Clients usually shard keys across servers with **consistent hashing**, so adding a server remaps only a fraction of keys. It is fast and predictable for plain caching. **Redis** (and its open-source fork **Valkey**) is an in-memory data structure server: strings, hashes, lists, sets, sorted sets, streams, with atomic operations, Lua scripting, optional persistence, replication and pub/sub. Its command execution is essentially single-threaded per instance, which keeps operations atomic but means one slow command blocks others. **Redis Cluster** splits the keyspace into **16,384 hash slots** spread over primary nodes, each with replicas for failover; multi-key operations work only when keys share a slot, which you can force with **hash tags** such as `{user:42}:profile`. Managed services include Amazon ElastiCache, Azure Cache for Redis and Google Memorystore.

## Keeping related keys in one Redis Cluster slot

Only the part inside braces is hashed, so these keys land together and can be used in one transaction.

```bash
redis-cli -c SET "{user:42}:profile" '{"name":"Asha"}'
redis-cli -c SET "{user:42}:cart"    '{"items":3}'
redis-cli -c CLUSTER KEYSLOT "{user:42}:profile"   # same slot as {user:42}:cart

# without a hash tag these two keys may live on different nodes,
# and MGET or MULTI across them fails with a CROSSSLOT error
```

## Pigeonholes in a staff room

Redis Cluster has 16,384 numbered pigeonholes spread over a few cabinets. A key's name decides its pigeonhole. Writing `{user:42}` on several letters makes them all go into the same pigeonhole, so you can collect them together.

**Quiz:** What is a key difference between Memcached and Redis?

- [ ] Memcached supports sorted sets and streams
- [ ] Redis cannot be used as a cache
- [ ] Memcached uses 16,384 hash slots
- [x] Redis offers rich data structures, replication and optional persistence; Memcached is a simple key-value cache

*Answer:* Redis offers rich data structures, replication and optional persistence; Memcached is a simple key-value cache. Redis is a data-structure server with replication and persistence; Memcached focuses on simple caching.