Mastering Redis HSET: Basics, Commands, and Internal Encoding

What is Redis HSET?

Redis HSET is a hash table where both field and value are strings, stored in memory. Each hash can store up to 2^32‑1 key‑value pairs (over 40 billion).

Applicable Scenarios

Suitable for O(1) dictionary lookups of a field, e.g., task configuration where the task type is the field and the configuration parameters are the value.

Common Operations

We discuss creation, query, update, and deletion.

Creation

Use HSET or HSETNX to create a hash.

Query

HGET – retrieve a single element.

HGETALL – retrieve all data.

HLEN – get the number of elements.

HSCAN – cursor‑based iteration.

Update

HSET can add new elements; HDEL deletes elements.

Deletion

DEL removes an entire HSET object.

Write Commands

HSET

127.0.0.1:6379> HSET hsetniuniu f1 v1 f2 v2 f3 v3
(integer) 3

HSETNX

127.0.0.1:6379> HSETNX hsetniuniu f1 newv1
(integer) 0
127.0.0.1:6379> HSETNX hsetniuniu f4 v4
(integer) 1
127.0.0.1:6379> HSETNX hsetniuniu f5 v5
(integer) 1

Since f1 already exists, its value remains unchanged; f4 and f5 are added.

HDEL

127.0.0.1:6379> HDEL hsetniuniu f4 f5
(integer) 2

DEL

127.0.0.1:6379> DEL hsetniuniu
(integer) 1

HMSET

Syntax: HMSET key field value [field value ...] – sets multiple field‑value pairs. Historically HMSET was required for multi‑field sets, but since Redis 4.0.0 HSET also supports multiple fields. HMSET is deprecated.

As per Redis 4.0.0, HMSET is considered deprecated. Please use HSET in new code.

Read Commands

HGETALL

127.0.0.1:6379> HGETALL hsetniuniu
1) "f1"
2) "v1"
3) "f2"
4) "v2"
5) "f3"
6) "v3"

HGET

127.0.0.1:6379> HGET hsetniuniu f1
"v1"

HLEN

127.0.0.1:6379> HLEN hsetniuniu
(integer) 3

HSCAN

127.0.0.1:6379> HSCAN hsetniuniu 0 COUNT 10
1) "0"
2) "f1"
3) "v1"
4) "f2"
5) "v2"
6) "f3"
7) "v3"

Using MATCH to filter results:

127.0.0.1:6379> HSCAN hsetniuniu 0 MATCH *2
1) "0"
2) "f2"
3) "v2"

Underlying Encoding

Encoding Formats

HSET uses either a ziplist or a hashtable. Ziplist is used when all keys and values are shorter than 64 bytes and the number of entries is less than 512; otherwise a hashtable is used.

Ziplist stores entries compactly; hashtable stores entries similarly to the Set structure but with non‑null values.

Summary

HSET is a hash dictionary that can store multiple field‑value mappings, such as student scores or task configurations. Its internal encoding switches between ziplist and hashtable based on size thresholds, concepts also seen in List and Set chapters.