Cloud Functions for Firebase: how to use a Transaction promise?

Question

I am trying to write a function in Cloud Functions that triggers every time a user gets created and which then saves that user into a list of users and finally increments a user counter. However I am not sure if I am using promises correctly.

exports.saveUser = functions.auth.user().onCreate(event => {
    
   const userId = event.data.uid
  
   const saveUserToListPromise = db.collection("users").doc(userId).set({
      "userId" : userId
   })
   
   var userCounterRef = db.collection("users").doc("userCounter");
   
   const transactionPromise = db.runTransaction(t => {
      return t.get(userCounterRef)
          .then(doc => {
              // Add one user to the userCounter
              var newUserCounter = doc.data().userCounter + 1;
              t.update(userCounterRef, { userCounter: newUserCounter });
          });
  })
  .then(result => {
      console.log('Transaction success!');
  })
  .catch(err => {
      console.log('Transaction failure:', err);
  });
  
  return Promise.all([saveUserToListPromise, transactionPromise])

})

I want to make sure that even if many users register at once that my userCounter is still correct and that the saveUser function won't be terminated before the transaction and the save to the list has happened.

So I tried this out and it works just fine however I don't know if this is the correct way of achieving the functionality that I want and I also don't know if this still works when there are actually many users triggering that function at once.

Hope you can help me. Thanks in advance.

Answer 1

The correct way to perform multiple writes atomically in a transaction is to perform all the writes with the Transaction object (t here) inside the transaction block. This ensures at all of the writes succeed, or none.

exports.saveUser = functions.auth.user().onCreate(event => {
    const userId = event.data.uid
    return db.runTransaction(t => {
        const userCounterRef = db.collection("users").doc("userCounter")
        return t.get(userCounterRef).then(doc => {
            // Add one user to the userCounter
            t.update(userCounterRef, { userCounter: FirebaseFirestore.FieldValue.increment(1) })
            // And update the user's own doc
            const userDoc = db.collection("users").doc(userId)
            t.set(userDoc, { "userId" : userId })
        })
    })
    .then(result => {
        console.info('Transaction success!')
    })
    .catch(err => {
        console.error('Transaction failure:', err)
    })
})