# User Identifier & Properties

{% hint style="info" %}
User Identifier Management

User Identifiers are used to uniquely identify users.\
For the meaning and importance of User Identifiers and guidelines on how to choose them, refer to the [Managing User Identifiers](/en/getting-started/user-identifier.md) documentation.
{% endhint %}

### Using User Identifiers You Manage

Because server-side SDKs cannot identify users on their own, you must **always pass the User Identifier directly as a parameter**.\
The identifier you pass can be a primary key you manage, a device identifier, a member ID, an email, a hash value, etc.

### Example

The following is an example of distributing a Variation and then sending a user event.

{% tabs %}
{% tab title="Kotlin" %}

```kotlin
import io.hackle.sdk.HackleClient
import io.hackle.sdk.Hackle
import io.hackle.sdk.common.Variation
import io.hackle.sdk.common.Event
import io.hackle.sdk.common.User

val user: User = Hackle.user("ae2182e0")

// Variation Distribution
val variation: Variation = hackleClient.variation(experimentKey, user)

// Send user event
hackleClient.track(event, user)
```

{% endtab %}

{% tab title="Java" %}

```java
import io.hackle.sdk.HackleClient
import io.hackle.sdk.common.Variation
import io.hackle.sdk.common.Event
import io.hackle.sdk.common.User

User user = User.of("ae2182e0");

// Variation Distribution
Variation variation = hackleClient.variation(experimentKey, user);

// Send user event
hackleClient.track(event, user);
```

{% endtab %}
{% endtabs %}

## Additional Identifiers

Additional identifiers beyond the default identifiers can be sent using the example code below.

{% tabs %}
{% tab title="Kotlin" %}

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


val user = Hackle.user("ae2182e0") { // Default identifier
    userId("143")                    // User ID (Hackle ID compatible)
    deviceId("ae2182e0")             // Device ID (Hackle ID compatible)
    identifier("myCustomId", "42")   // Custom ID
}
```

{% endtab %}

{% tab title="Java" %}

```java
import io.hackle.sdk.common.User

User user = User.builder("ae2182e0") // Default identifier
    .userId("143")                   // User ID (Hackle ID compatible)
    .deviceId("ae2182e0")            // Device ID (Hackle ID compatible)
    .identifier("myCustomId", "42")  // Custom ID
    .build()
```

{% endtab %}
{% endtabs %}

## Property

The Hackle SDK supports adding properties to a User object.

* A property must be sent as a key-value pair consisting of a Property Key and a Property Value.
* You can add a maximum of 128 properties per user object.

<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 Decision, and user event tracking.\
In the example below, three properties (`age`, `grade`, `is_paying_user`) have been added.

{% tabs %}
{% tab title="Kotlin" %}

```kotlin
import io.hackle.sdk.HackleClient
import io.hackle.sdk.common.Variation
import io.hackle.sdk.common.Event
import io.hackle.sdk.common.User

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

// Variation Distribution
val variation: Variation = hackleClient.variation(experimentKey, user)

// Feature Flag Decision
val featureOn: Boolean = hackleClient.isFeatureOn(featureKey, user)

// Send user event
hackleClient.track(event, user)
```

{% endtab %}

{% tab title="Java" %}

```java
import io.hackle.sdk.HackleClient
import io.hackle.sdk.common.Variation
import io.hackle.sdk.common.Event
import io.hackle.sdk.common.User

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

// Variation Distribution
Variation variation = hackleClient.variation(experimentKey, user);

// Feature Flag Decision
boolean featureOn = hackleClient.isFeatureOn(featureKey, user);

// Send user event
hackleClient.track(event, user);
```

{% endtab %}
{% endtabs %}


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