Gyaan

What is Redis & Common Use Cases

beginner redis basics use-cases

Redis stands for REmote DIctionary Server. In simple language, it’s a giant key-value store that lives entirely in RAM. We talk to it over TCP, throw keys at it, and get values back in microseconds.

Think of it like a Map<String, Value> running on a separate server, except the values can be strings, lists, hashes, sets, sorted sets, streams, and a few more exotic types. That’s what makes Redis special — it’s not “just” a cache, it’s a data-structure server.

Why we use Redis

Plain databases like PostgreSQL or MySQL live on disk. Disk is slow. Redis lives in memory. Memory is fast — we’re talking sub-millisecond reads. When an app needs to respond in under 50ms and the database takes 200ms, Redis sits in front and absorbs the heat.

The classic use cases

Cache
Store DB query results with TTL. The #1 reason teams adopt Redis.
Session Store
Stateless app servers share login sessions through Redis.
Rate Limiter
INCR + EXPIRE to count requests per user per minute.
Queue
Lists with LPUSH/BRPOP give us a simple job queue.
Leaderboard
Sorted sets give us top-N by score in O(log N).
Real-time Analytics
HyperLogLog and bitmaps count uniques cheaply.

Cache

The classic flow: check Redis first, fall back to the DB on miss, then write back into Redis.

GET product:42
# (nil) — miss, query DB
SET product:42 '{"name":"Shoe","price":99}' EX 3600
GET product:42
# "{\"name\":\"Shoe\",\"price\":99}"

Session store

When we run 10 app servers behind a load balancer, we can’t keep sessions in local memory — a user might hit a different server next request. Redis fixes that.

SET session:abc123 '{"userId":42,"role":"admin"}' EX 1800

Rate limiter

A fixed-window limiter in two commands:

INCR rate:user:42:2026-05-26T10:15
EXPIRE rate:user:42:2026-05-26T10:15 60

If INCR returns > 100, we reject the request.

Queue

LPUSH queue:emails "send:welcome:user42"
BRPOP queue:emails 0    # worker blocks until a job arrives

Leaderboard

ZADD game:leaderboard 4500 "alice" 3200 "bob" 5100 "carol"
ZREVRANGE game:leaderboard 0 9 WITHSCORES   # top 10

What Redis is not

Redis is not a primary database for everything. Anything you absolutely cannot lose still belongs in Postgres. Redis is a speed layer and a purpose-built data structure server — that’s the mental model to walk into the interview with.

References