Preventing multiple users accessing same Cloud Function simultaneously - firebase

Question

I'm creating a simple 2d-multiplayer game with Unity and I chose to use Firebase as the backend. I'm facing some issues when trying to fill up rooms using Firebase Cloud functions. How I planned this to work is following:
Player clicks "Join Room"-button
Unique device ID is send to Realtime database under "Player Searching For Room" and event listener is added to that ID
Cloud function will be triggered when the 'onWrite'-event happens. Function will then check if the room array is empty. If the room array is empty the Cloud function will then push new room to the realtime database.
Cloud function pushes the room ID under the player ID in the "Players Searching For Room"
Because player is already listening to his own ID under the "Players Searching For Room", a function will be run when room ID is pushed under his own ID. This will tell the player that the Cloud function successfully found a room for him.
Below is the Cloud function:
const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);
// Create and Deploy Your First Cloud Functions
// https://firebase.google.com/docs/functions/write-firebase-functions
var room = [];
// ID for the room we are filling at the moment
var room_currentID;
// This functions triggers each time something is added or deleted to "Players Searching For Room"
exports.findRoom = functions.database
.ref('/Players Searching For Room/{pushId}')
.onWrite(event => {
// Check if data exists (if not, this was triggered by delete -> return)
if(!event.data.exists())
{
return;
}
// If this player already has a room, we want to return
if(event.data.val().inRoom != "none")
return;
// Store data under changed pushId (if player was added to waiting players then data is -> "size" : 4)
const data = event.data.val();
// Name of the player. We get this from the pushId of the item we pushed ("container" for data pushed).
var name = event.params.pushId;
// Size of the room the player wants to join
var size = data.size;
// With IF-statements check which room_size_n object array we want to loop through
console.log("Player called " + name + " is waiting for room that has maxium of " + size + " players")
// We can push the user to the room array since it can never be full
// (we clear the array before allowing next guy to join)
room.push(name);
// If this was the first guy
// we need to create new room
if(room.length == 1)
{
admin.database().ref('/Rooms').push({
onGoing: false // We need to set something, might aswell set something usefull
}).then(snapshot => {
// because this function is triggered by changes in firebase realtime database
// we can't return anything to the player. BUT we can inform player about the room
// he's been attached to by adding roomID to the playername in the "Players Searching For Room"
// then players device will handle removing
// Store ID of the room so that we can send it to later joiners in this room
room_currentID = snapshot.key;
data.inRoom = room_currentID;
return event.data.ref.set(data);
});
}
// If there already exists a suitable room with space on it
else if(room.length > 1)
{
// We can attach the stored roomID to the player so he knows which rooms onGoing flag to watch for.
data.inRoom = room_currentID;
// Attach roomID to the player and then check if room is full
// waiting roomID to attach to player before setting onGoing TRUE
// prevents other player to get a head start
event.data.ref.set(data).then(snapshot => {
// If roomId was attached to the player we can check the room size
if(room.length == size)
{
// ...and if the room became full we need to set onGoing to true
admin.database().ref('/Rooms/'+room_currentID).set({
onGoing: true
}).then(snapshot => {
room = [];
});
}
});
}
});
Problem is that if multiple users click the Join Game-button in a short period of time it messes up the system. Adding player ID under "Players Searching For Room" seems to work everytime, but sometimes Cloud function never attach room ID to the player ID and sometimes Cloud function creates more rooms than it should. I tested this simply by having a button that attached random ID under "Players Searching For Room" each time it was clicked. Then I rapidly clicked that button for ´10 times. That should have attached 5 different room IDs to those 10 random IDs and also generate 5 new rooms. Instead it generated 7 rooms and added room IDs to only 8 random IDs not 10.
Problem is, i think:
Abe clicks Join Game-button at 12.00.00
Cloud function starts (execution takes 5 seconds)
Rob clicks Join Game-button at 12.00.02
Cloud function triggers again before finishing Abe's request
Eveything gets messed
Is it possible with firebase to change this so that if Rob triggers the Cloud function before Abe's request is done, Rob will be put on hold while Abe finishes. When Abe is finished than it's Rob's turn. Ugh, awfully long explanation hopefully somebody will read this :)

Answer 1

At Google I/O 2017, I gave a talk on building a multiplayer game using only Firebase on the backend. Cloud Functions implements pretty much all the logic of the game. It also has a simple matching feature, and you could extend that scheme to do more complicated things. You can watch the talk here, and source code for the project will be coming soon.