Build your own Redis

Goal

Build an in-memory key-value server that real Redis clients can talk to — RESP protocol in, cache semantics out — then make it correct under concurrent clients and bounded in memory. Proves you can carry a wire protocol, a concurrency model, and eviction from paper to a benchmarked, profiled server.

Subject: full brief & instructions

Practices

• Cache management — TTL expiry, eviction policies, and what actually determines the hit rate.
• Concurrency and Parallelism — event loop vs. thread-per-connection, and keeping shared state honest.
• Performance engineering — benchmark honestly, profile, fix the widest tower, measure again.
• System design fundamentals — one of the core building blocks, built instead of drawn.

Milestones

1. RESP parser — decode/encode the Redis serialization protocol as a standalone, unit-tested library: simple strings, errors, integers, bulk strings, arrays.
2. PING/ECHO server — a TCP server answering a single client; redis-cli PING against your port works unmodified.
3. Concurrent clients — choose a model (event loop, or thread-per-connection over a locked store) and write down why; parallel redis-cli sessions stay correct.
4. GET/SET with expiry — SET with EX/PX options, passive expiry on read, then an active expiry sweep so dead keys don't linger.
5. Eviction under a memory cap — a maxmemory limit with approximate LRU; fill the cache past the cap and verify hot keys survive.
6. Benchmark & profile — run redis-benchmark against it, flame-graph the hot path, fix the biggest cost, and report before/after numbers.

Stretch goals

• More commands: INCR, lists (LPUSH/RPUSH/LRANGE), hashes.
• Persistence: an append-only file replayed on startup.
• Pub/sub: SUBSCRIBE/PUBLISH — fan-out without persistence, the opposite corner from a durable log (message brokers & event streaming).
• Compare your throughput with real Redis on the same box and explain the gap.
• Continue with CodeCrafters' staged version (codecrafters.io) for guided extensions such as replication.

Related

• Build your own Redis — subject — the self-contained work statement: RESP2 wire format, command set, expiry/eviction semantics, benchmark protocol.
• Cache management — cites this exercise from its Practice list.
• Concurrency and Parallelism — likewise; the concurrency-model decision is milestone 3.
• Message brokers & event streaming — the pub/sub stretch goal crosses into its territory.
• Build a load balancer & rate limiter — kin exercise: another edge box built from scratch.