How can I ensure atomic user creation between Firebase Auth and my custom backend in Flutter?

Below is one recommended solution: offload the backend user‐creation step to a trusted environment (such as a Cloud Function) that listens for new Firebase users and then creates the corresponding record in your custom database. This way, you remove the need for your client to make two separate calls and you make the process more atomic.

How to Do It

1. Use a Cloud Function Triggered on User Creation:
   Write a Cloud Function using the Firebase Admin SDK that triggers on the Firebase Authentication "user onCreate" event. In that function, call your backend API (or write directly to your custom database if possible) to create the user record using the Firebase UID.

2. Handle Failures in the Cloud Function:
   Because this code runs on the backend with administrative privileges, you can implement robust error handling (and even retry logic) if your backend API call fails. If desired, you could also remove the newly created Firebase user (or mark it for cleanup) when the backend call fails. This guarantees that if you cannot create the record on your custom DB, you won’t leave orphaned Firebase users.

3. Keep Your App Code Simple:
   With this approach the client only needs to perform Firebase login. When the Cloud Function creates the record in your custom database, your backend and security rules based on the Firebase UID remain in sync. Your app state (for example using your bloc) can simply check that the user is properly logged in via Firebase (and your API can be queried if its data is needed later).

Example Cloud Function

Below is a simplified example that mimics setting custom claims in the context but instead calls your API (or writes to your custom database):

JSCopied
const functions = require('firebase-functions');
const { initializeApp } = require('firebase-admin/app');
const { getAuth } = require('firebase-admin/auth');
const axios = require('axios');

initializeApp();

exports.syncUserToCustomDB = functions.auth.user().onCreate(async (user) => {
  const uid = user.uid;
  try {
    // Call your custom backend API endpoint.
    // For example, if you have an HTTP endpoint that accepts a POST request to create a user:
    await axios.post('https://your-backend.example.com/api/create-user', {
      uid: uid,
      email: user.email
      // add any additional info needed
    });
    console.log('User', uid, 'synced with custom DB.');
  } catch (error) {
    console.error('Failed to create custom DB user for:', uid, error);

    // Optionally, you can undo the Firebase user creation:
    // await getAuth().deleteUser(uid);

    // Depending on your requirements, you might even implement a retry mechanism.
  }
});

Benefits of This Approach

• Centralized Logic:
  Consolidate synchronization between Firebase Auth and your custom DB into one trusted, administrative piece of code. It is easier to maintain and monitor than spreading that logic on the client.

• Transactional Behavior:
  Even though it is not a true transaction, if the custom endpoint fails, you can choose to remove the Firebase authentication record or mark it for later reconciliation.

• Reduced Client Complexity:
  Your Flutter app needs only to complete standard Firebase sign-in. The synchronization happens automatically in the backend so that edge cases (like a successful Firebase login but missing API creation) are handled out-of-band.

Using Cloud Functions not only helps avoid double API calls being made from the client but also leverages Firebase’s native security and operational practices to keep your user data consistent.

Final Thoughts

While it is possible to check on the client side (for instance, in your bloc) whether the custom DB user exists, it is much safer and cleaner to handle this in a backend environment that can safely roll back or retry operations. This design avoids leaving orphaned Firebase users and creates a more robust, maintainable system.