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Android path · Beyond the screen · Topic 11 of 12

Push with Firebase Cloud Messaging

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Your server can't open a connection to a phone. Firebase Cloud Messaging keeps one shared, battery-friendly connection per device (through Google Play services), and your server sends to it through the FCM HTTP v1 API, addressed to a **registration token** that identifies one install of your app. FCM delivers the message, and the device decides whether your code runs, the system shows a notification, or both.

Why it matters: Push is how a server reaches an app that isn't running. What arrives, whether your code runs, and whether the user sees anything depends on the message type, its priority and the app's state.

  1. 1Tokens identify an install

    Each app install gets a registration token. Send it to your server, update it in onNewToken, and drop tokens that go stale.

  2. 2Notification vs data messages

    Notification messages are shown by the system when the app is in the background; data messages always go to your onMessageReceived.

  3. 3High priority is for the user

    High priority can wake a dozing device, but messages that don't lead to a visible notification get deprioritised.

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How it works

The explanation, with examples you can run.

Tokens

class PushService : FirebaseMessagingService() {    override fun onNewToken(token: String) {        // A new install, a restored backup, or a token rotation: tell your server.        enqueueTokenUpload(token)            // via WorkManager, so it survives being offline    }     override fun onMessageReceived(message: RemoteMessage) {        val orderId = message.data["orderId"] ?: return        if (message.data["kind"] == "sync") {            enqueueSync(orderId)             // anything slow goes to WorkManager        } else {            showOrderNotification(orderId, message.data["title"].orEmpty())        }    }}

Tokens change: on reinstall, when app data is cleared, when restored to a new device, and occasionally on their own. Store them with a timestamp, refresh from the app now and then (FirebaseMessaging.getInstance().token), and remove tokens your sends report as unregistered. Firebase treats devices that haven't connected for over a month as stale; sending to them wastes quota and distorts delivery numbers.

Two message types

  • Notification messages have a notification block (title, body, image). When the app is in the background, the FCM SDK posts the notification itself, and tapping it opens your launcher activity. When the app is in the foreground, onMessageReceived() gets it and you decide what to show.
  • Data messages carry only your key-value pairs and always go to onMessageReceived() (subject to delivery and priority rules). They give you full control, and full responsibility: you build the notification.

Payloads are limited to 4096 bytes. Send ids, not documents, and fetch the rest.

App stateNotificationDataBoth
ForegroundonMessageReceivedonMessageReceivedonMessageReceived
BackgroundSystem trayonMessageReceivedTray, data in the launch intent's extras

Priority and Doze

Normal priority (the default) is delivered straight away when the device is awake, and may wait for a maintenance window in Doze. High priority wakes the device, gives the app a short window of processing with limited network access, and allows it to start a foreground service from the background. FCM watches the last 7 days of behaviour per app instance: if high-priority messages don't result in user-visible notifications, later ones may be downgraded to normal. Use high priority for chats and calls, normal for "sync your inbox".

A force-stopped app (from Settings) receives nothing until the user opens it again.

The user's side

On Android 13+, notifications need the POST_NOTIFICATIONS runtime permission; a push that arrives without it is received but not shown. Create notification channels for each kind of message (orders, chat, marketing) so users can mute one without losing the others, and include an idempotency key in the data: messages can be delivered more than once.

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Check yourself

3 questions. Get them all right to finish this topic.

  1. 1.The app is in the background and receives a message with both notification and data payloads. What happens?
  2. 2.Your server sends high-priority data messages that the app processes silently, without showing a notification. What does FCM do over time?
  3. 3.How long should onMessageReceived() take?

0 of 3 right so far. Get all three to finish this topic.

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