> ## Documentation Index
> Fetch the complete documentation index at: https://moengage.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Setting Unique Id for SDK versions below 9.23.0

> Set a unique user ID for login and logout in MoEngage iOS SDK versions below 9.23.0.

# User Login / Logout

It is important that you handle user login and logout as mentioned below. There is a definite possibility that your data gets corrupted if this is not done properly.

* Make sure to get hold of a `unique id` for your app users and pass that information to our SDK using the [*setUniqueID(\_:)*](https://moengage.github.io/ios-api-reference/Classes/MoEngageSDKAnalytics.html#/c:@M@MoEngageAnalytics@objc\(cs\)MoEngageSDKAnalytics\(im\)setUniqueID:) \[Link]method. We use this `unique id` to identify a user and also to merge user profiles across installs and platforms.
* And also, once the user logs out of your app, it is necessary to call [*resetUser()*](https://moengage.github.io/ios-api-reference/Classes/MoEngageSDKAnalytics.html#/c:@M@MoEngageAnalytics@objc\(cs\)MoEngageSDKAnalytics\(im\)resetUser) \[Link]of SDK so that we create a new anonymous user and track the events following this to the new user's profile.

Kindly ensure you call the following methods on user login/logout.

## Login

<CodeGroup>
  ```swift Swift wrap theme={null}
  MoEngageSDKAnalytics.sharedInstance.setUniqueID(UNIQUE_ID) // UNIQUE_ID is used to uniquely identify a user.
  ```

  ```objective-c Objective C wrap theme={null}
  [[MoEngageSDKAnalytics sharedInstance] setUniqueID:UNIQUE_ID]; // UNIQUE_ID is used to uniquely identify a user.
  ```
</CodeGroup>

<Note>
  The following values are not allowed in the UniqueID field: "unknown", "guest", "null", "0", "1", "true", "false", "user\_attribute\_unique\_id", "(empty)", "na", "n/a", "", "dummy\_seller\_code", "user\_id", "id", "customer\_id", "uid", "userid", "none", "-2", "-1", "2"
</Note>

<Warning>
  **UNIQUE ID chaos**

  * When you go live with MoEngage iOS SDK for the first time, please ensure that you are setting the **Unique ID** of the already logged-in user along with other user attributes.
  * Kindly make sure that you are not using a single unique id for all the users, this can happen if you hard code the value, instead of fetching it from your servers.
  * If you pass 2 different unique id information without calling [*resetUser()*](https://moengage.github.io/ios-api-reference/Classes/MoEngageSDKAnalytics.html#/c:@M@MoEngageAnalytics@objc\(cs\)MoEngageSDKAnalytics\(im\)resetUser) method in between, the SDK will internally force logout the existing user.
</Warning>

## Logout

<CodeGroup>
  ```swift Swift wrap theme={null}
  MoEngageSDKAnalytics.sharedInstance.resetUser()
  ```

  ```objective-c Objective C wrap theme={null}
  [[MoEngageSDKAnalytics sharedInstance] resetUser];
  ```
</CodeGroup>

# Updating User Attribute Unique ID

<Warning>
  **Important**

  Please make sure that you use [*setAlias(\_:)*](https://moengage.github.io/ios-api-reference/Classes/MoEngageSDKAnalytics.html#/c:@M@MoEngageAnalytics@objc\(cs\)MoEngageSDKAnalytics\(im\)setAlias:)\[Link] for updating the User Attribute Unique ID and not [*setUniqueID(\_:)*](https://moengage.github.io/ios-api-reference/Classes/MoEngageSDKAnalytics.html#/c:@M@MoEngageAnalytics@objc\(cs\)MoEngageSDKAnalytics\(im\)setUniqueID:) \[Link]as calling [*setUniqueID(\_:)*](https://moengage.github.io/ios-api-reference/Classes/MoEngageSDKAnalytics.html#/c:@M@MoEngageAnalytics@objc\(cs\)MoEngageSDKAnalytics\(im\)setUniqueID:) \[Link]with a new value will reset the current user and lead to the creation of unintended users in our system.
</Warning>

In a scenario where you have to update the existing user's Unique ID value make use of [*setAlias(\_:)*](https://moengage.github.io/ios-api-reference/Classes/MoEngageSDKAnalytics.html#/c:@M@MoEngageAnalytics@objc\(cs\)MoEngageSDKAnalytics\(im\)setAlias:) \[Link]method as shown below with the updated Unique ID value:

<CodeGroup>
  ```swift Swift wrap theme={null}
  MoEngageSDKAnalytics.sharedInstance.setAlias(UPDATED_UNIQUE_ID)
  ```

  ```objective-c Objective C wrap theme={null}
  [[MoEngageSDKAnalytics sharedInstance] setAlias:UPDATED_UNIQUE_ID];
  ```
</CodeGroup>