RevenueCat provides a source of truth for a subscriber's status across different platforms. To do this, each subscriber has an App User ID that uniquely identifies them within your application.

If you don't provide an App User ID when instantiating the Purchases SDK, RevenueCat will create a new random App User ID for you and cache it on the device.

In the event that the user deletes and reinstalls the app, this random App User ID will be unrecoverable and a new random App User ID will be generated. Restoring in-app purchases will recover any subscriptions and alias the new and old random App User IDs together.

Setting your own App User ID will allow you to reference users in the RevenueCat dashboard, via the API, as well as in the webhooks and other integrations.

Using an externally managed App User ID also provides a mechanism by which to restore purchases in a few scenarios:

1. When a user deletes and reinstalls your app - using the same App User ID will ensure they still haves access to subscriptions previously started.
2. When the user logs in on multiple devices - you can honor a subscription that was purchased on one device across any other platform.

Set App User ID on instantiation

If you have your own App User IDs, you can pass those on instantiation to Purchases.

let purchases = RCPurchases.configure(withAPIKey: "my_api_key", appUserID: "my_app_user_id")

Purchases.sharedInstance = Purchases.Builder(this, "my_api_key")
                    .appUserID("my_app_user_id")
                    .build()

Set App User ID on login

If you don't have an App User ID on instantiation, you can set it later through the identify() method. Using identify() will clear any caches and re-fetch info for that user. This is a common flow if your app implements a login screen.

purchases.identify("my_app_user_id")

purchases.identify("my_app_user_id")

Aliases are a way for RevenueCat to attach multiple App User IDs to a single subscriber. This is useful when a user has multiple devices and you are relying on random App User IDs or when a user makes a purchase before logging in. For example:

There are two ways that aliases are created:

1. Restoring purchases
2. Calling createAlias

1. Restoring Purchases

Restoring purchases is a mechanism by which your user can restore their in-app purchases, reactivating any content that had previously been purchased from the same store account (Apple or Google).

If two different App User IDs try to restore transactions from the same underlying store account (Apple or Google) RevenueCat will create an alias between the two App User IDs and count them as the same user going forward.

This is a common if your app does not have accounts and is relying on RevenueCat's random App User IDs.

2. Manually Creating Aliases

Sometimes, it is useful to manually create an alias. This is most often used when an app allows users to purchase before logging in.

To do this, you can call the createAlias() method on Purchases.

purchases.createAlias("my_app_user_id")

purchases.createAlias("my_app_user_id", object : Purchases.AliasHandler {
  override fun onSuccess() {
  }

  override fun onError(domain: Purchases.ErrorDomains, code: Int, message: String) {
  }
})

This is great if a user makes a purchase with a generated App User ID, that you later want to link to a provided App User ID. For example:

1. User opens the app, Purchases is instantiated without an App User ID therefore generating a random App User ID
2. User purchases a subscription, attaching the subscription to the generated ID
3. User creates an account
4. Developer calls createAlias(newAppUserID) to link the previous purchase to newAppUserID in RevenueCat

Now the subscriber can be referenced in the Purchases SDK or the REST API by using the developer specified App User ID.

When a user logs out you can call the reset() method on Purchases to reset the App User ID to a new generated App User ID. This sets you up with a new anonymous user for the logged out state.

• If you're moving to RevenueCat from another system, see our guide on migrating your existing subscriptions
• Once you're ready to test your integration, you can follow our guides on testing purchases