Redis hashes

Introduction to Redis hashes

Redis hashes are record types structured as collections of field-value pairs. You can use hashes to represent basic objects and to store groupings of counters, among other things.

Examples

  • Represent a basic user profile as a hash:
> HSET user:123 username martina firstName Martina lastName Elisa country GB
(integer) 4
> HGET user:123 username
"martina"
> HGETALL user:123
1) "username"
2) "martina"
3) "firstName"
4) "Martina"
5) "lastName"
6) "Elisa"
7) "country"
8) "GB"
  • Store counters for the number of times device 777 had pinged the server, issued a request, or sent an error:
> HINCRBY device:777:stats pings 1
(integer) 1
> HINCRBY device:777:stats pings 1
(integer) 2
> HINCRBY device:777:stats pings 1
(integer) 3
> HINCRBY device:777:stats errors 1
(integer) 1
> HINCRBY device:777:stats requests 1
(integer) 1
> HGET device:777:stats pings
"3"
> HMGET device:777:stats requests errors
1) "1"
2) "1"

Basic commands

  • HSET sets the value of one or more fields on a hash.
  • HGET returns the value at a given field.
  • HMGET returns the values at one or more given fields.
  • HINCRBY increments the value at a given field by the integer provided.

See the complete list of hash commands.

Performance

Most Redis hash commands are O(1).

A few commands - such as HKEYS, HVALS, and HGETALL - are O(n), where n is the number of field-value pairs.

Limits

Every hash can store up to 4,294,967,295 (2^32 - 1) field-value pairs. In practice, your hashes are limited only by the overall memory on the VMs hosting your Redis deployment.

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