> For the complete documentation index, see [llms.txt](https://documentation.pushly.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://documentation.pushly.com/integration/implementation-steps/apple-ios/sdk-swift-obj-c/identity-synchronization.md).

# Identity Synchronization

## Setting the Subscriber's External ID

Providing a unique User ID for your subscriber allows you to later interact with that subscriber via the API.

Once a subscriber's external ID has been set you may work with our team to set up bi-directional automated data syncs. The following use cases are common ways that publishers use the external ID via server-to-server requests:

* Sending notifications to individual subscribers programmatically
* Processing unsubscribe requests
* Adding and removing profile attributes and events to subscribers

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

```swift
PushSDK.UserProfile.externalId = "external-id"
```

{% endtab %}

{% tab title="Objective-C" %}

```objectivec
PushSDKUserProfile.externalId = @"external-id";
```

{% endtab %}
{% endtabs %}

You can also check to see if the subscriber is already tagged with an External ID:

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

```swift
let currentExternalId = PushSDK.UserProfile.externalId
```

{% endtab %}

{% tab title="Objective-C" %}

```objectivec
NSString* currentExternalId = PushSDKUserProfile.externalId;
```

{% endtab %}
{% endtabs %}

### External ID Restrictions

To prevent subscribers from being accidentally merged via External ID and to ensure integrity in  the subscriber documents, Pushly enforces the following restrictions upon External IDs:

* No empty or strings that contain white-space
* No identifiers that contain placeholder text: `[test, testing, example, guest, anon, anonymous, user, guest, visitor, n/a]`
* No common JavaScript fall-through words or keywords: \[`[Object ... ], undefined, NaN, Infinity, true, false]`
* Cannot be only punctuation/symbols, must contain alphanumeric characters: `[---, ***, -*-, -._+/=:@~]`  
* Identifiers should be between 3 and 256 characters in length

A [regex playground](https://regex101.com/r/2wEVEc/1) has been set up for testing the validity of External ID schema.


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## 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, and the optional `goal` query parameter:

```
GET https://documentation.pushly.com/integration/implementation-steps/apple-ios/sdk-swift-obj-c/identity-synchronization.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

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.