> For the complete documentation index, see [llms.txt](https://docs.hackle.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.hackle.io/en/development-guide/sdk/user-identifier.md).

# User Identifier & Properties

## Identifier

{% hint style="info" %}
For the meaning and importance of User Identifiers and guidelines for choosing one, refer to the [Managing User Identifiers](/en/getting-started/user-identifier.md) document.
{% endhint %}

The User Identifier is used to uniquely identify a user.\
You can manage User Identifiers through the Hackle SDK.

<table><thead><tr><th width="199.671875">Category</th><th>Description</th></tr></thead><tbody><tr><td><a href="/pages/UdqQgqweP4X89oJwqpmf">Client</a></td><td>The client manages User Identifiers internally.<br>The client uses two main types of identifiers.<br>- The Device ID provided by the Hackle SDK<br>- Custom identifiers</td></tr><tr><td><a href="/pages/HfUDxx8Kl2fndaP3rIUj">Server</a></td><td>The server does not manage User Identifiers internally.<br>A User Identifier must always be passed for every server request.</td></tr></tbody></table>

## Property

The Hackle SDK supports adding properties to the User object.\
By sending user-specific information as User Properties, you can reduce repetitive coding while making rich use of them in A/B Tests and data analytics.

* Properties must be sent as key-value pairs (Property Key and Property Value).
* A maximum of 128 properties can be added.

<table><thead><tr><th width="133.04296875">Category</th><th width="127.60546875">Type</th><th>Constraints</th></tr></thead><tbody><tr><td>Property Key</td><td><code>string</code></td><td><ul><li>Character limit is 128 characters.</li><li>Case-insensitive.</li><li>For example, AGE and age are recognized as the same Property Key.</li></ul></td></tr><tr><td>Property Value</td><td><code>boolean</code>, <code>string</code>, <code>number</code>, <code>array</code></td><td><ul><li>For string type, the character limit is 1024 characters.</li><li>String type is case-sensitive.</li><li>For example, APPLE and apple are recognized as different Property Values.</li><li>For number type, up to 15 integer digits and up to 6 decimal places are supported.</li></ul></td></tr></tbody></table>

### Example

The User object is used as a parameter in variation distribution, Feature Flag decisions, and event tracking. The example below shows three properties (`age`, `grade`, `is_paying_user`) being added.

```java
import io.hackle.android.HackleApp
import io.hackle.sdk.common.User
import io.hackle.sdk.common.Variation

User user = User.builder()
  	.userId(userId)
    .property("age", 30)
    .property("grade", "GOLD")
    .property("is_paying_user", false)
    .build();

// Variation distribution
Variation variation = hackleApp.variation(experimentKey, user);

// Feature Flag decision
boolean featureOn = hackleApp.isFeatureOn(featureKey, user);

// Event tracking
hackleApp.track(event, user);
```

```kotlin
import io.hackle.android.Hackle
import io.hackle.sdk.common.User
import io.hackle.sdk.common.Variation

val user = Hackle.user() {
  	userId(userId)
    property("age", 30)
    property("grade", "GOLD")
    property("is_paying_user", false)
}

// Variation distribution
val variation = hackleApp.variation(experimentKey, user)

// Feature Flag decision
val featureOn = hackleApp.isFeatureOn(featureKey, user)

// Event tracking
hackleApp.track(event, user)
```


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