# A/B Test

{% hint style="info" %}
The HTTP API can only be used in a server environment.
{% endhint %}

You can use the HTTP API to check the A/B test variation distribution result for a user.

Authentication is performed using the SDK Key.

* Add the SDK Key to the HTTP header as shown below.
* You can find the SDK Key in the Hackle Dashboard under [SDK Integration Info](https://dashboard.hackle.io/config/sdk-setting).

```http
X-HACKLE-API-KEY: {YOUR_SERVER_SDK_KEY}
```

### Request

```http
POST https://api.hackle.io/v1/variation
```

#### Request Body

| Field         | Type        | Required | Description                              |
| ------------- | ----------- | -------- | ---------------------------------------- |
| experimentKey | number      | O        | Experiment Key of the A/B test to assign |
| user          | User object | O        | User to distribute                       |

**User Object**

| Field       | Type   | Required | Description                                      |
| ----------- | ------ | -------- | ------------------------------------------------ |
| userId      | string | X        | User ID (Hackle ID can be used)                  |
| deviceId    | string | X        | Device ID (Hackle ID can be used)                |
| identifiers | object | X        | Custom identifier key(string)-value(string) pair |
| properties  | object | X        | User Property key(string)-value(string)          |

#### Example

```shell
curl -i -X POST \
  'https://api.hackle.io/v1/variation' \
  -H 'X-HACKLE-API-KEY: YOUR_SDK_KEY_HERE' \
  -H 'Content-Type: application/json' \
  -d '{
    "experimentKey": 42,
    "user": {
      "userId": "19285035",
      "deviceId": "ae2182e0",
      "identifiers": {
        "sessionId": "16930492542",
        "myCustomId": "ff01eae2"
      },
      "properties": {
        "grade": "GOLD",
        "age": 30,
        "isLogin": true
      }
    }
  }'
```

### Response

#### 200 OK

Returns a 200 response and the distribution result for a successful request.

**Response Body**

| Field      | Type   | Description                             |
| ---------- | ------ | --------------------------------------- |
| variation  | string | Assigned variation (A, B, C, D, ...)    |
| reason     | string | Decision reason                         |
| parameters | object | Parameters configured for the variation |

**Example**

```json
{
  "variation": "B",
  "reason": "TRAFFIC_ALLOCATED",
  "parameters": {
    "buttonText": "Purchase",
    "buttonSize": 16
  }
}
```

#### 400 Bad Request

Returns a 400 status for invalid request information.

#### 401 Unauthorized

Returns a 401 status when the `X-HACKLE-API-KEY` header value is missing or the SDK Key is invalid.


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.hackle.io/en/development-guide/http-api/api-variation.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.