RedisLab

Foundations 1 of 5 · Visual guide

Hello Redis: keys, values, first commands

Redis is one big dictionary that lives in RAM. You send it a command, it looks up a key, and it answers in well under a millisecond.

One dictionary in memory

Every key is a string. Every value has a type. The type decides which commands work on it.

The Redis keyspace Clients connect over TCP port 6379 to one Redis server. Inside, a table maps keys to typed values: a string, a hash, a list, a set, and a sorted set. redis-cli your app TCP :6379 Redis server · all data in RAM key value type greeting "hello" string user:42 name→Ada city→Pune hash queue:emails [ j3 | j2 | j1 ] list tags:post:9 { redis, cache } set board ada 90 · bea 70 sorted set
A key is a name. Its value is one of a few shapes. GET on a list fails with WRONGTYPE.

One command, start to finish

The life of SET and GET redis-cli sends SET greeting hello. Redis parses the command, executes it against the in-memory table, and replies OK. A later GET reads the same table. redis-cli SET greeting hello Redis parse SET · 2 args execute one thread RAM table greeting→"hello" OK no disk on this path · typically < 1 ms
  1. redis-cli sends SET greeting hello as plain text over TCP port 6379.
  2. Redis parses it: command SET, two arguments.
  3. One thread executes it: greeting → "hello" goes into the in-memory table. No disk write on this path.
  4. Redis replies OK. On one machine the round trip is far below a millisecond.
  5. GET greeting walks the same path and reads the same table: "hello".
Parse, execute, reply. Persistence to disk happens in the background, not on this path.

Run it yourself

Real Redis in 10 seconds 
docker run -d --name redis -p 6379:6379 redis:7
docker exec -it redis redis-cli
127.0.0.1:6379> PING
PONG
Or use the lab below

It is a small simulator of the common commands. Click a chip or type. The right side draws every key, its type, and its TTL.

Read the reply

Six reply shapes cover almost everything.

OKsimple status: it worked
(integer) 3a count or a number
"hello"a string value
(nil)no such key — not an error
1) "a" 2) "b"an array of replies
(error) WRONGTYPEwrong command for this type

Common operations

The string commands you will type daily
SET / GETWrite and read one value. SET k v EX 60 adds a TTL, NX only if missing.
MSET / MGETMany keys in one round trip. Missing keys come back as (nil).
INCR / INCRBY / DECRAtomic counters on a numeric string. A missing key counts as 0.
APPEND / STRLENGrow a string, measure it.
DEL / UNLINKRemove keys. UNLINK frees big values in the background.
EXISTS / TYPEIs it there? What shape is it?
KEYS / SCANFind keys by pattern. KEYS blocks on big databases — see chapter 2.

Quick check

GET user:99 returns (nil). Did the command fail?
Show answer

No. (nil) means “no such key”. Errors start with (error).

You run GET queue:emails on a list. What happens?
Show answer

WRONGTYPE. Lists are read with LRANGE, LPOP, and friends. Try it in the lab: RPUSH q a, then GET q.