Question

This is a rather involved question, but I’ll try to explain it as simply & concisely as I can…

I am using Firebase to build a multi-user, web-based game. I am keeping a list of every round in the game. At the end of a round, every user is presented with a “Start” button that they click when they are ready to begin the next round. The round begins when at least 50% of users have clicked “Start”.

I have a Firebase reference gameRef for the game, a reference roundListRef that represents the list of rounds, and a reference roundRef that represents the current round.

I have attached a child_added callback to roundListRef so that when a new round is added, it becomes everyone’s current round:

roundListRef.on('child_added', function(childSnapshot, prevChildName) {
    roundRef = childSnapshot.ref();
});

I can track newRoundVotes and activePlayers, and calculate 50% easily from there. If 50% is reached, a new round is added, which triggers everyone’s child_added event and the new round will begin from there…

gameRef.child('newRoundVotes').on('value', function(snapshot) {
    var newRoundVotes = snapshot.val();

    gameRef.child('activePlayers').once('value', function(snapshot) {
        var activePlayers = snapshot.val();

        if (newDriveVotes / activePlayers >= 0.5)
            addNewRound();
    });
});

My questions is, how do I ensure that only one new round is added, and that everyone is on the same round?

For example, say there are 10 players and 4 have already voted to start the next round. If the 6th player votes before his child_added event is triggered from the 5th player, then a round will also be added for the 6th player.

The problem is similar to .set() vs .transaction(), but no quite the same (from my understanding).

Does anyone have a solution?

Answer 1

I think you can likely solve this with a transaction if the round names are known ahead of time. E.g. if you just use /round/0, /round/1, /round/2, etc.

Then you could have some code like:

function addNewRound() {
    var currentRound = Number(roundRef.name());
    var nextRound = currentRound + 1;

    // Use a transaction to try to create the next round.
    roundRefList.child(nextRound).transaction(function(newRoundValue) {
        if (newRoundValue == null) {
            // create new round.
            return { /* whatever should be stored for the round. */ };
        } else {
            // somebody else already created it.  Do nothing.
        }
    });
}

Does this work for your scenario?