If Redis is single-threaded, how does it handle thousands of clients at once?

Through I/O multiplexing. An event loop monitors all client sockets and processes commands one at a time as they become ready, and since each command takes microseconds, one thread interleaves thousands of clients without them noticing. Newer versions also offload network reads and writes to I/O threads, but commands still execute serially.

Can I use Redis as my primary database?

You can, and some teams do for specific datasets, but you are accepting that everything must fit in RAM, that default persistence can lose about a second of writes, and that replication is asynchronous. For data you cannot regenerate or afford to lose, a disk-based database with synchronous durability is the safer primary, with Redis as the speed layer in front.

When should I move from a single Redis instance to Redis Cluster?

When the dataset no longer fits in one machine's memory after eviction tuning, or write throughput exceeds what one primary sustains. If you need availability rather than capacity, replicas with Sentinel failover are much simpler. Cluster adds multi-key command restrictions and operational overhead, so treat it as the second step, not the first.

Cachingchapter 6 of 8 · 4 lessons

Redis Deep Dive

In-memory data structure store powering caches, queues, leaderboards, and pub/sub.

← All chapters View in cheatsheet

You have been saying Redis for four chapters without looking inside it. It has been the box in the diagram that holds the copy.

Open it, because the answers to several interview questions are in there. Why a single-threaded program is faster than one using all your cores. What happens to your data when the process restarts, and why most caches deliberately choose to lose it. And what you do on the day one Redis is no longer enough, which is a different question from what you do when one database is no longer enough.

Lessons

4 in this chapter

Why Redis Is FastMemory-only data, one thread, no locks, and an event loop that never waits.3 min
Data Structures Beyond StringsLists, sets, hashes, and sorted sets replace application code with one server-side command.3 min
Persistence: RDB and AOFSnapshots are compact but lossy, the append-only file is durable but heavier, and many caches rightly use neither.3 min
Replication, Sentinel, and ClusterReplicas add read capacity and failover; Cluster shards the keyspace when one box is not enough.3 min