Redis Overview: Introduction, Data Types, Commands, Persistence, Replication, and Transactions

Redis Overview

Redis is a completely free, open‑source, high‑performance key‑value database that follows the BSD license.

Key Features

Supports data persistence (RDB snapshots and AOF) so data survives restarts.

Provides multiple native data structures: strings, hashes, lists, sets, and sorted sets.

Offers master‑slave replication, Sentinel for automatic failover, and optional sharding via Redis Cluster.

Includes Pub/Sub messaging, Lua scripting, and transaction support (MULTI/EXEC).

Data Types

String – binary‑safe value up to 512 MB; analogous to a map entry where a key maps to a single value.

Hash – a map of field‑value pairs stored under one key, ideal for representing objects.

List – an ordered collection of strings that can be pushed or popped from both ends.

Set – an unordered collection of unique strings; operations are O(1) thanks to an underlying hash table.

Sorted Set (Zset) – similar to a set but each member carries a double‑precision score used for automatic sorting; scores may repeat while members are unique.

Basic Commands

127.0.0.1:6379> SET hello world
OK
127.0.0.1:6379> GET hello
"world"

Hash Commands

127.0.0.1:6379> HSET user:1 name "Alice"
(integer) 1
127.0.0.1:6379> HGETALL user:1
1) "name"
2) "Alice"

List Commands

127.0.0.1:6379> RPUSH mylist a b a
(integer) 3
127.0.0.1:6379> LRANGE mylist 0 -1
1) "a"
2) "b"
3) "a"
127.0.0.1:6379> LPOP mylist
"a"

Set Commands

127.0.0.1:6379> SADD colors red green blue
(integer) 3
127.0.0.1:6379> SMEMBERS colors
1) "red"
2) "green"
3) "blue"

Sorted Set Commands

127.0.0.1:6379> ZADD scores 100 alice 200 bob
(integer) 2
127.0.0.1:6379> ZRANGE scores 0 -1 WITHSCORES
1) "alice"
2) "100"
3) "bob"
4) "200"

Pub/Sub

Publish/Subscribe enables message broadcasting: a publisher sends a message to a channel, and all subscribed clients receive it.

127.0.0.1:6379> SUBSCRIBE chat
Reading messages... (press Ctrl‑C to quit)
1) "subscribe"
2) "chat"

127.0.0.1:6379> PUBLISH chat "hello world"
(integer) 1

-- subscriber output --
1) "message"
2) "chat"
3) "hello world"

Transactions

Redis transactions group multiple commands between MULTI and EXEC. Commands are queued and executed atomically as a batch, but individual command failures do not roll back earlier commands.

127.0.0.1:6379> MULTI
OK
127.0.0.1:6379> SET book "Redis Guide"
QUEUED
127.0.0.1:6379> SADD tags "database" "cache"
QUEUED
127.0.0.1:6379> EXEC
1) OK
2) (integer) 2

Persistence

Redis offers two persistence strategies:

RDB – periodic snapshots of the dataset saved to disk.

AOF – appends every write command to a log file; configurable sync policies (always, everysec, no).

Replication

Using SLAVEOF host port, a server becomes a replica of a master. The master first creates an RDB snapshot, sends it to the replica, then streams subsequent write commands.

Sentinel

Sentinel monitors a set of Redis instances, detects master failures, and automatically promotes a replica to master, ensuring high availability.

Sharding (Partitioning)

Data can be partitioned across multiple nodes to achieve linear scalability. Common approaches include:

Range‑based sharding (e.g., user IDs 0‑1000 on node R0).

Hash‑based sharding using CRC32 modulo the number of nodes.

Client‑side sharding, proxy sharding, or server‑side sharding via Redis Cluster.

These mechanisms allow Redis to handle large datasets and high request volumes efficiently.