herodb/instructions/redis_hset_functions.md

# 🔑 Redis `HSET` and Related Hash Commands

## 1. `HSET`

* **Purpose**: Set the value of one or more fields in a hash.
* **Syntax**:

  ```bash
  HSET key field value [field value ...]
  ```
* **Return**:

  * Integer: number of fields that were newly added.
* **RESP Protocol**:

  ```
  *4
  $4
  HSET
  $3
  key
  $5
  field
  $5
  value
  ```

  (If multiple field-value pairs: `*6`, `*8`, etc.)

---

## 2. `HSETNX`

* **Purpose**: Set the value of a hash field only if it does **not** exist.
* **Syntax**:

  ```bash
  HSETNX key field value
  ```
* **Return**:

  * `1` if field was set.
  * `0` if field already exists.
* **RESP Protocol**:

  ```
  *4
  $6
  HSETNX
  $3
  key
  $5
  field
  $5
  value
  ```

---

## 3. `HGET`

* **Purpose**: Get the value of a hash field.
* **Syntax**:

  ```bash
  HGET key field
  ```
* **Return**:

  * Bulk string (value) or `nil` if field does not exist.
* **RESP Protocol**:

  ```
  *3
  $4
  HGET
  $3
  key
  $5
  field
  ```

---

## 4. `HGETALL`

* **Purpose**: Get all fields and values in a hash.
* **Syntax**:

  ```bash
  HGETALL key
  ```
* **Return**:

  * Array of `[field1, value1, field2, value2, ...]`.
* **RESP Protocol**:

  ```
  *2
  $7
  HGETALL
  $3
  key
  ```

---

## 5. `HMSET` (⚠️ Deprecated, use `HSET`)

* **Purpose**: Set multiple field-value pairs.
* **Syntax**:

  ```bash
  HMSET key field value [field value ...]
  ```
* **Return**:

  * Always `OK`.
* **RESP Protocol**:

  ```
  *6
  $5
  HMSET
  $3
  key
  $5
  field
  $5
  value
  $5
  field2
  $5
  value2
  ```

---

## 6. `HMGET`

* **Purpose**: Get values of multiple fields.
* **Syntax**:

  ```bash
  HMGET key field [field ...]
  ```
* **Return**:

  * Array of values (bulk strings or nils).
* **RESP Protocol**:

  ```
  *4
  $5
  HMGET
  $3
  key
  $5
  field1
  $5
  field2
  ```

---

## 7. `HDEL`

* **Purpose**: Delete one or more fields from a hash.
* **Syntax**:

  ```bash
  HDEL key field [field ...]
  ```
* **Return**:

  * Integer: number of fields removed.
* **RESP Protocol**:

  ```
  *3
  $4
  HDEL
  $3
  key
  $5
  field
  ```

---

## 8. `HEXISTS`

* **Purpose**: Check if a field exists.
* **Syntax**:

  ```bash
  HEXISTS key field
  ```
* **Return**:

  * `1` if exists, `0` if not.
* **RESP Protocol**:

  ```
  *3
  $7
  HEXISTS
  $3
  key
  $5
  field
  ```

---

## 9. `HKEYS`

* **Purpose**: Get all field names in a hash.
* **Syntax**:

  ```bash
  HKEYS key
  ```
* **Return**:

  * Array of field names.
* **RESP Protocol**:

  ```
  *2
  $5
  HKEYS
  $3
  key
  ```

---

## 10. `HVALS`

* **Purpose**: Get all values in a hash.
* **Syntax**:

  ```bash
  HVALS key
  ```
* **Return**:

  * Array of values.
* **RESP Protocol**:

  ```
  *2
  $5
  HVALS
  $3
  key
  ```

---

## 11. `HLEN`

* **Purpose**: Get number of fields in a hash.
* **Syntax**:

  ```bash
  HLEN key
  ```
* **Return**:

  * Integer: number of fields.
* **RESP Protocol**:

  ```
  *2
  $4
  HLEN
  $3
  key
  ```



## 12. `HSCAN`

* **Purpose**: Iterate fields/values of a hash (cursor-based scan).
* **Syntax**:

  ```bash
  HSCAN key cursor [MATCH pattern] [COUNT count]
  ```
* **Return**:

  * Array: `[new-cursor, [field1, value1, ...]]`
* **RESP Protocol**:

  ```
  *3
  $5
  HSCAN
  $3
  key
  $1
  0
  ```