After you add the App Check SDK to your app, but before you enable App Check enforcement, you should make sure that doing so won't disrupt your existing legitimate users.
An important tool you can use to make this decision for Realtime Database, Cloud Firestore, Cloud Storage, and Authentication (beta) is the App Check request metrics screen.
To view the App Check request metrics for a product, open the App Check section of the Firebase console. For example:
The request metrics for each product are broken down into four categories:
Verified requests are those that have a valid App Check token. After you enable App Check enforcement, only requests in this category will succeed.
Outdated client requests are those that are missing an App Check token. These requests might be from an older version of the Firebase SDK before App Check was included in the app.
Unknown origin requests are those that are missing an App Check token, and don't look like they come from the Firebase SDK. These might be from requests made with stolen API keys or forged requests made without the Firebase SDK.
Invalid requests are those that have an invalid App Check token, which might be from an inauthentic client attempting to impersonate your app, or from emulated environments.
The distribution of these categories for your app should inform when you decide to enable enforcement. Here are some guidelines:
If almost all of the recent requests are from verified clients, consider enabling enforcement to start protecting your backend resources.
If a significant portion of the recent requests are from likely-outdated clients, to avoid disrupting users, consider waiting for more users to update your app before enabling enforcement. Enforcing App Check on a released app will break prior app versions that are not integrated with the App Check SDK.
If your app hasn't launched yet, you should enable App Check enforcement immediately, since there aren't any outdated clients in use.
When you understand how App Check will affect your users and you're ready to proceed, you can enable App Check enforcement for Realtime Database, Cloud Firestore, Cloud Storage, and Authentication (beta).