Firestore Security Rules Mastery Hub: The Industry Foundatio

In Firestore Security Rules, what is the primary purpose of the `allow` statement in conjunction with a request method (e.g., `get`, `list`, `create`, `update`, `delete`)?

Consider a Firestore Security Rule designed to protect a `users/{userId}/privateData/{docId}` collection, where only the authenticated user whose `userId` matches the document path segment can read or write. Which of the following rules best implements this requirement, assuming `request.auth.uid` is available?

When using the `exists()` function in Firestore Security Rules to check for the presence of a document before allowing a write operation, what is a crucial consideration regarding performance and potential race conditions, especially in complex scenarios?