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**Hash** in Redis is a **"key → value" structure inside a single Redis key**.
Essentially, it's a **mini-object** or a **dictionary**, where several `field:value` pairs can be stored under one Redis key.

### Main properties

- It's an associative array (a map, a dictionary).
- Every element is a `field:value` pair.
- It fits storing **objects, user profiles, settings**.
- Values are strings (Redis String).

### Example

```bash
HSET user:1 name "Anna" age "27" city "Moscow"
HGETALL user:1
```

Result:

```javascript
1) "name"
2) "Anna"
3) "age"
4) "27"
5) "city"
6) "Moscow"
```

The `user:1` key holds three fields: `name`, `age`, `city`.

### Main commands

| Command | Description |
|---|---|
| `HSET key field value [field value ...]` | set one or several fields |
| `HGET key field` | get a field's value |
| `HGETALL key` | get every field and value |
| `HDEL key field [field ...]` | remove a field |
| `HEXISTS key field` | check whether a field exists |
| `HKEYS key` | get the list of every field |
| `HVALS key` | get the list of every value |
| `HLEN key` | the number of fields |
| `HINCRBY key field increment` | increment a field's numeric value |
| `HSETNX key field value` | set a field only if it doesn't already exist |

### An update-and-count example

```bash
HSET user:2 name "Ivan" points "100"
HINCRBY user:2 points 50
HGET user:2 points
```

Result:

```javascript
"150"
```

### Internal structure

Redis stores a Hash in two formats:

1. **Ziplist**, if there are few fields (up to ~512) and they're short; it saves memory.
2. **Hashtable**, if there's a lot of data or the fields are long; it provides fast access.

Redis automatically picks the format based on size.

### Uses

- Storing user data, a profile, a session.
- Caching objects from a database.
- Counters and state inside microservices.
- Fast access to individual fields without loading the whole object.

### An example used as a cache

```bash
HSET product:42 name "Phone" price "899" stock "24"
HGET product:42 price
```

Result:

```javascript
"899"
```

This is how a hash can represent a row of SQL data inside Redis's memory.

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