One to One Android Chat using Firebase

Question

I need some clarifications.

I'm using Firebase realtime database.

Now I've to implement a realtime chat between two android devices. I need also push notifications.

So, the solution based on realtime database is to drop because if the app isn't running, it will not handle the onChildAdded event.

I read on the documentation that I can achieve my aim, using the upstream message from the device to the server.

Now, it's written also that we need an app server.

I just want to be clarified the need of the app server (XMPP or HTTP), and which is the entire flow of message m1 sent from the device A, and the notification on the device B.

In my opinion the app server should act as a man in the middle, so:

A ---> XMPP SERVER ---> FCM

FCM ---> XMPP SERVER ---> B

Is this the flow? On the documentation I can't understand if I'm right or not. If so, how do I send a message from the FCM back to the XMPP Server? The notification message is sent from FCM to the client o from XMPP Server?

Please someone clarify me.

Other solution thought by me: Since I find really powerful firebase realtime database, I thought these solution to walk around the problem:

1. Create an app server the is listening for onChildAdded and when it changes send a notification to client device.
2. Creating a background service on the client app listening to onChildAdded and create notification when necessary.

Even all, I don't like these solutions. Just want to understand the standard and correct way to achieve my aim.

Answer 1

Firebaser and author of the article debated in the question comments here

To send messages to a device with Firebase Cloud Messaging, you need to specify your project's FCM Server Key. As its name suggests, this key should only be used in code that runs in a trusted environment: i.e. a server that you control.

To send device-to-device messages (such as in a 1:1 chat application where the receiving user is not necessarily online) you need two steps:

1. one user send an upstream message
2. the other user receives the downstream message

Using Firebase Cloud Messaging and your custom app server code, you can handle step 2. But that still leaves step 1: the user needs to send a message that somehow triggers your code on the app server. There are quite a few ways to do this.

For example you could implement an HTTP endpoint on your app server and have the chat application send the messages there too (in addition to sending to the database).

You could also implement an XMPP endpoint and have the chat application send the message there (again in addition to sending it to the database).

My article suggest yet another way, one that doesn't require implementing an endpoint at all. It uses a node.js script that runs on your app server and is essentially just a client to the database. Whenever this script detects a relevant chat message, it calls the FCM API and sends the downstream message.

This pattern of using the Firebase Database as your endpoint and then using server-side scripts is quite common when using Firebase. We documented it in our classic blog post Where does Firebase fit in your app? (as option 2) and in this article in the Google Cloud documentation.