What is Redis?

What is Redis?

Redis (Remote Dictionary Server) is an in-memory data structure store. Unlike traditional databases that store data on disk, Redis keeps all data in RAM — making it orders of magnitude faster than disk-based databases for read and write operations.

Why Redis?

• Blazing fast — sub-millisecond response times, hundreds of thousands of operations per second
• Versatile — stores strings, lists, sets, hashes, sorted sets, streams, and more
• Persistent — optional disk persistence via RDB snapshots and AOF logging
• Atomic — all operations are atomic; no race conditions for single commands
• Pub/Sub — built-in messaging between clients

Where Redis is Used

Redis is most commonly used for:

• Caching — store expensive query results in memory; retrieve them in microseconds
• Session storage — web sessions that expire automatically
• Rate limiting — count requests per user per time window
• Leaderboards — sorted sets make ranking trivial
• Message queues — lists as FIFO queues; streams as durable logs
• Real-time analytics — HyperLogLog for approximate counts

Companies using Redis in production: Twitter, GitHub, Stack Overflow, Pinterest, Snapchat, Craigslist, and many others.

The Redis CLI Format

In this course, you write Redis commands one per line — the same syntax as the redis-cli tool:

SET name "Alice"
GET name

Commands are case-insensitive (SET = set = Set). Keys and values are case-sensitive.

Core Commands: SET, GET, DEL, EXISTS

SET key value        -- store a string value
GET key              -- retrieve it ("value" or (nil))
DEL key [key ...]   -- delete one or more keys, returns count deleted
EXISTS key [key ...] -- 1 if exists, 0 if not (sums for multiple keys)

Example:

SET greeting "Hello, Redis!"
GET greeting
DEL greeting
GET greeting

Output:

OK
"Hello, Redis!"
(integer) 1
(nil)

Your Task

Set a key city to "Tokyo", then retrieve it with GET.