Firebase RTDB batched transactions (increment 2 values at the same time)

Question

I am aware that you can do batched, atomic all or nothing updates using update - but can you do the same thing with a transaction?

Currently I am trying to increment a users friend count (2 users) at the same time when the friend request is accepted.

Here is what I am doing which works, but if something goes wrong it will lead to bad data inconsistencies which came about a couple times.

  const upOneFriend = firebase
    .database()
    .ref("users")
    .child(friend.uid)
    .child("friendCount");
  const upOneCurrentUser = firebase
    .database()
    .ref("users")
    .child(userUid)
    .child("friendCount");
  upOneFriend
    .transaction(currentCount => {
      return currentCount + 1;
    })
    .then(() => {
      upOneCurrentUser.transaction(currentCount2 => {
        return currentCount2 + 1;
      });
    })
    .catch(() => {
      console.log("error increment");
    });

Like I said, is works, but I need to do this at the same time! I have looked around and have not found anything related to batch transactions for the Realtime Database.

Cheers.

Answer 1

Transactions in Firebase Realtime Database work on a single node. If you need to update multiple nodes in a transaction, you'll need to run the transaction on the first common node above the ones you're looking to update. In your scenario that'd mean you run the transaction across users, which would probably significantly reduce throughput.

An alternative would be to use a multi-location update. But since a multi-location update doesn't auto-protect against concurrent writes, you'd have to include the information to protect against that in the write itself.

For an example of this, see my answer here: Is the way the Firebase database quickstart handles counts secure?, and How to update multiple children under child node using a transaction?