Redis

About

Redis, which stands for Remote Dictionary Server, is a fast, open source, in-memory, key-value data store.

Main features:

  • in-memory (everything is stored on RAM for faster access)
  • stores data as key-value so its non-relational

Over the time it has added more features and data structures and now it has grown to be a full fledged data store that supports:

  • other data structures such as strings, hashes, lists, sets, etc..
  • built in replication, scripting (stored procedures), transactions, etc..
  • on-disk persistence

We can use Redis as a cache, broker, queue, stream, and even as a primary database!

Reference: https://redis.io/docs/about

Data Structures

Redis has 5 data structures: STRING, LIST, SET, HASH, and ZSET (sorted set).

String

Although they are stored as string literal (""), we can perform:

  • operations on substring
  • increment/decrement if it has an integer or float value inside the string
key1: value1
key2: value2
...

SET name Abhishek 	# or "Abhishek"

GET name
> "Abhishek"

KEYS *				# specify pattern
> "name"

SET marks 56		# overwrites
GET marks
> "56"

GET foo 			# fetching non-existing key
> (nil)

DEL name			# deletion

--------------------------------------------------
# increment/decrement on integer and float
SET marks 99		# integer string
INCR marks
GET marks
> "100"

SET marks 99
INCRBY marks 3
GET marks
> "102"

SET marks 99.8		# float string
INCRBYFLOAT marks 1	# no INCR command for float
GET marks
> "100.8"

SET marks 99.8
INCRBYFLOAT marks 0.2
GET marks
> "100"

Hash

Contains key-value pairs but inside another hashkey.

Terminology is a bit confusing here. hashkey (key), key(field), and value (value).

hash-key1{
	field1: value1
	field2: value2
	...
}

hash-key2{
	field1: value1
	...
}

HSET student name Abhishek
HSET student country India

HGET student name
> "Abhishek"

HLEN student		# no. of fields in a hash
> (integer) 2

HKEYS student		# list all keys
> 1) "name"
> 2) "country"

----------------------------------------------------------
# numeric related commands works the same as with String:
# HINCR - for integer only
# HINCRBY - for integer only
# HINCRBYFLOAT - for float only

List & Set

list-key1{ 
	item1
	item2
	...
}

list-key2{ 
	item1
	...
}

SET are same as a LIST but duplicate items aren’t allowed. On addition, Set returns 1 if element is present already, 0 otherwise.

Sorted Set

ZSET store key-value pairs but in a sorted order. Key (member) and value (score). Score can only be of type float.

zset-key1{
	member1: score1
	member2: score2
	...
}

zset-key2{
	member1: score1
	...
}

The scores are sorted by numeric value in ascending order and correpsonding members are also sorted by virtue of that.

marks{
	dinesh: 13
	richard: 50.5
	gilfoyle: 98
}

Commands

Quick Searchable Reference: https://redis.io/commands

Disk Persistence

Redis can also write data to secondary storage (disk).

Two strategies can be employed for this:

  1. point-in-time dump: either when certain conditions are met (a number of writes in a given period) or when one of the two dump-to-disk commands is called
  2. append-only file: it writes every command that alters data in Redis to disk as it happens (aka Write-Ahead Logging)

We can always disable this persistence to use Redis as a pure in-memory cache.

References