Core concept
Redis: single node, cluster mode (sharded), or Serverless. HA via replica + auto-failover. Data structures + persistence.
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How it works
Memcached: simple. Multi-thread. No persistence, no HA, no replication. Rare new choice.
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Trade-offs + gotchas
Session store. Query cache. Rate limiting. Leaderboards (Redis sorted sets). Message queues (Redis Streams).