I am trying to figure out how Firebase connect to specific devices and send message to it. I now how to use Firebase and send notification to registed devices from my backend server but i don't know how is it working internally. Is firebase severs using persisted connection between device and server? What kind of tehnology is it using?
For now i am mostly interested in Android devices and how firebase wake up device even with background tasks restrictions.
FCM is able to wake a device because the message is actually received by software components that come with Play services, which operates with elevated privileges. Play services can choose to wake the device and deliver the message to the target app.
The FCM software in Play services keeps a persistent socket connection open to its backend under normal circumstances. If it gets connection is dropped, it has logic to reestablish the connection without taxing the battery too much.
I tried to explain how GCM works in my answer here. The internal working principle of Firebase is similar in my opinion.
When you are installing an application which has the Firebase SDK set up in it, it gets a push registration ID from Google server. Usually, you save the push registration ID in your server as well. Hence, when there is a situation for sending a push notification, your server uses this push registration ID to generate a request to the Firebase server for sending a push notification to your device.
Now let us think of the receiver part as well (i.e. your device). In my opinion, in newer versions of Android allows a JobScheduler to check repeatedly for push messages from the Firebase server and if there is one, the device notifies the corresponding application and the application then generates the push notification.
Hope that helps!
