How can I effectively manage the amount of observers/listeners on a document in Firebase? - swift

Question

Let's say I have a collection of documents in the Cloud Firestore. I want people to observe these documents, but I don't want more than 100 people observing the same document at any point in time. How do I effectively track this?
I was initially thinking to track the amount of observers/listeners by managing an array of user IDs in each document. Effectively, users would add their ID to this array before observing/listening (or be turned away if this array was too large) and remove their IDs from this array when they stop. The problem with this is that I can't just stop the user from leaving if the call to delete this ID from the array fails. What if they terminate the app in some way and the call to remove their ID doesn't go through?
Is there a more reasonable solution to this problem? Could this be solved with the Realtime Database or Cloud Functions? Thanks!

Answer 1

I'm not sure, but try to google the Firebase access rules. I think that you can manage the access in this scope.
I talk about this rules.
rules_version = '2';
service cloud.firestore {
match /databases/{database}/documents {
match /{document=**} {
allow read, write: if true;
}
}
}
Sorry for answer in that style, i do not have the needed reputation to create comments

Answer 2

SOLVED! Big thanks to Renaud Tarnec for his original comment referencing documentation I had not yet seen. I was missing the keyword 'presence' in my Google searches. Please upvote his comment if you find this answer helpful!
SOLUTION: Using https://firebase.google.com/docs/firestore/solutions/presence
SWIFT:
var connectionReference: DatabaseReference = Database.database().reference(withPath: ".info/connected")
var personalReference: DatabaseReference?
var connectionIssued: Bool = false
func connect(room: String) {
// Remove any existing observations to ensure only one exists.
connectionReference.removeAllObservers()
// Start observing connection.
connectionReference.observe(.value) { (isConnected) in
if isConnected.value as! Bool {
// Connected!
// Use Bool connectionIssued to ensure this is run only once.
if !self.connectionIssued {
self.connectionIssued = true
self.personalReference = Database.database().reference(withPath: "OnlineUsers/\(userID)")
// Set onDisconnect before setting value.
self.personalReference!.onDisconnectRemoveValue()
self.personalReference!.setValue(room)
// Now the user is "online" and can proceed.
// Allow user to "enter" room.
} else {
// Connection already issued.
}
} else {
// The user has either disconnected from an active connection or they were already disconnected before connect() was called.
// Stop observing connection.
self.connectionReference.removeAllObservers()
// If the user disconnects after they have entered a room, kick them out.
if self.connectionIssued {
// User lost connection.
kickUserOutOfRoom()
self.connectionIssued = false
} else {
// User cannot log in.
}
}
}
}
// Call when users leave a room when still connected.
func leaveRoomManually() {
// Remove connection observation.
connectionReference.removeAllObservers()
// Attempt to remove "online" marker in database.
personalReference?.removeValue(completionBlock: { (error, reference) in
if error != nil {
// Removal failed, but that should be okay!
// onDisconnect will still be called later!
// This failure might result in ghost users if the user proceeds to join another room before disconnecting.
// Consider writing an onUpdate() cloud function in conjunction with the following onCreate() and onDelete() cloud functions to take care of that case.
} else {
// "Online" marker removed from database!
// We can now cancel the onDisconnect()
self.personalReference?.cancelDisconnectOperations()
}
})
leaveRoom()
}
CLOUD FUNCTIONS (javascript):
The following cloud functions update the respective room's guest count in Cloud Firestore.
const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();
const firestore = admin.firestore();
exports.userCameOnline = functions.database.ref('/OnlineUsers/{userID}').onCreate(
async (snapshot, context) => {
const room = snapshot.val();
const guestCountRef = firestore.doc(`Rooms/${room}`);
return guestCountRef.update({
Guests: admin.firestore.FieldValue.increment(1)
});
});
exports.userWentOffline = functions.database.ref('/OnlineUsers/{userID}').onDelete(
async (snapshot, context) => {
const room = snapshot.val();
const guestCountRef = firestore.doc(`Rooms/${room}`);
return guestCountRef.update({
Guests: admin.firestore.FieldValue.increment(-1)
});
});