README.md

nodejs-fcm-notification-server

This is a Node.js template which communicates with FCM to send push notifications to several devices at the same time. Meant to be used with a Firebase database and deployed on Heroku.

How it works

The solution presented here is as follows:

1. The client adds a child to notifications/ in the Firebase database using childByAutoId. This child has the following structure:

["attempts": 0,
"key": notificationKey,
"message": "Hello world!",
"read": false,
"sent": false,
"senderUID": abc...xyz,
"recipientUID": 123...789,
"time": timeStamp]

Here notificationKey is the key created using childByAutoId. Any other information that is app specific can also be added to this dictionary. For example, and app with posts may add a postID key value pair.
2. Once the client adds a new child child to notifications/ this change it detected by the Node.js backend in the child_added callback. 3. The new notification dictionary, snapshot, is passed to handleNotificationSnapshot(snapshot).
4. handleNotificationSnapshot retrieves the notifications ids (tokens) of the recipient using their UID. The notification ids are stored in notification_ids/recipientUID/ as an array of tokens like so: [token1, token2, token 3] FCM can handle up to 1000 tokens in one push notification request. (Number may need to be double checked because I've also read a load limit of 2K)
5. The notification ids, message, notification key, and number of attempts are then passed into sendNotification(notificationID, message, key, attempts).
6. sendNotification uses an HTTP POST request to communicate with FCM to send the push notification.
7. If the POST request return a 200 success code, the notification dictionary's sent value is changed to true, and attempts is incremented by one. Otherwise, attempts is incremented by one, and the change is detected in child_changed which calls handleNotificationSnapshot. The notification will not be sent after 10 failed attempts.

Deploying the Node.js backend

This project is made to be deployed on Heroku, although it can be changed to work on any Node.js server.

In addition to hosting the code, there are to Firebase credentials you need for this to work. First is a service account. Make sure to add the json service account file to the root directory of your project. Note that the service account grants administrative access to anyone who has it. Also, consider using a databaseAuthVariableOverride to ensure any code mistakes do not affect your entire database. The other thing you need is your Firebase project's Server Key which can be found in Settings, -> Cloud Messaging.

When testing locally, make sure to run npm install to install all of the dependencies.

Saving notification ids (tokens)

Follow the Firebase documentation to set up the client side.

The below swift code adds the devices token to the user's notification ids array.

func connectToFcm() {   // Connect to the FCM platform and save the instance token in the Firebase database for the backend Node.js app server to send notifications when childs are updated
        FIRMessaging.messaging().connectWithCompletion { (error) in
            if (error != nil) {
                NSLog("Unable to connect with FCM. \(error)")
            } else if FIRInstanceID.instanceID().token() != nil {
                NSLog("Connected to FCM.")

                if let user = FIRAuth.auth()?.currentUser {     // Check that the user is logged in
                    // User is signed in.

                    let ref = FIRDatabase.database().reference()    // Create the Firebase database reference

                    ref.child("notification_ids/\((user.uid))").observeSingleEventOfType(.Value, withBlock: { (snapshot) in

                        if snapshot.exists() {  // Ensure that snapshot exists to avoid option unwrapping errors

                            let notificationIDsArray = snapshot.value as! NSMutableArray    // All of all user notification tokens. Having an array allowa the Node.js server to send push notifications to multiple devices simultaneously

                            if notificationIDsArray.indexOfObject(FIRInstanceID.instanceID().token()!) == NSNotFound {  // Check is the token is alrrady saved. If not, add the token to the notificationIDsArray and save it to the Firebase database
                                notificationIDsArray.addObject(FIRInstanceID.instanceID().token()!)

                                ref.child("notification_ids/\(user.uid)").setValue(notificationIDsArray)
                            }
                        }
                        else {  // If the snapshot does not exist, there is no record of notification tokens for the logged in user. In this case, create a database child to store the user's notifications tokens and save it

                            let saveArray: NSArray = NSArray(object: FIRInstanceID.instanceID().token()!)

                            ref.child("notification_ids/\(user.uid)").setValue(saveArray)
                        }

                    }) { (error) in
                        NSLog("user notification load error")
                        NSLog(error.localizedDescription)
                    }
                } else {
                    // No user is signed in.
                    NSLog("not signed in")
                }
            }
        }