Firebase - Multiple users simultaneously updating same object using its old value

Question

I'm keeping track of a count that users update on the Firebase database through an Android app. The way it works right now is that upon interaction the user's app looks up the current count on the database (using a addListenerForSingleValueEvent() and onDataChange() method defined within the new ValueEventListener) and adds one to it and then sets the count to this new value using mRef.setValue() where mRef is the reference to the database.

The issue I'm worried about is what would happen if a large number of users interacted with the database together at the same time; does Firebase take care of making sure that the value is read and incremented properly or is there a lot of overlap and potentially a loss of data because of that.

Answer 1

When working with complex data that could be corrupted by concurrent modifications, such as incremental counters, Firebase provides a transaction operation.

You give this operation two arguments: an update function and an optional completion callback. The update function takes the current state of the data as an argument and will return the new desired state you would like to write.

For example, if we wanted to increment the number of upvotes on a specific blog post, we would write a transaction like the following (Legacy code):

    Firebase upvotesRef = new Firebase("https://docs-examples.firebaseio.com/android/saving-data/fireblog/posts/-JRHTHaIs-jNPLXOQivY/upvotes");
upvotesRef.runTransaction(new Transaction.Handler() {
    @Override
    public Transaction.Result doTransaction(MutableData currentData) {
        if(currentData.getValue() == null) {
            currentData.setValue(1);
        } else {
            currentData.setValue((Long) currentData.getValue() + 1);
        }
        return Transaction.success(currentData); //we can also abort by calling Transaction.abort()
    }
    @Override
    public void onComplete(FirebaseError firebaseError, boolean committed, DataSnapshot currentData) {
        //This method will be called once with the results of the transaction.
    }
});

Legacy source

New firebase version source