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Redis Introduction

Redis is an in-memory data store — it keeps data in RAM, so reads and writes take *microseconds*. It's one of the most widely used pieces of infrastructure in modern backends.

Not just a cache

Redis is famous as a cache, but it does far more: session store, rate limiter, message broker, job queue, leaderboard, and real-time analytics. It stores rich data types, not just strings.

Use	How Redis helps
Caching	serve hot data without hitting the database
Sessions	fast, shared session storage with expiry
Rate limiting	atomic counters per user/IP
Queues / Pub-Sub	background jobs and real-time messaging
Leaderboards	sorted sets ranked by score

Why it's fast

Data lives in memory, not on disk.
It's effectively single-threaded for commands — so each command is atomic, with no locking needed.
Simple, efficient data structures purpose-built for speed.

The trade-off

Note: Because it's in-memory, Redis is limited by RAM and data can be lost on crash — but it can persist to disk for durability (covered later). It complements your main database rather than replacing it.

Tip: Think of Redis as a fast layer *alongside* your primary store (PostgreSQL, MongoDB): keep the source of truth there, and use Redis for the speed-critical, ephemeral, or high-throughput parts.

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