How Whatsapp, Facebook and Skype messaging works in the background?

Question

What I'm trying to ask is:

1. Whether these apps are relying on FCM/GCM (or any other type of) push notifications for syncing their data or are they using a socket connection in background through a service?
2. If they are relying on push notifications then why they don't have any loss in receiving notifications (in background) while I miss 10-15% of notifications in busy hours.
3. And if they are relying on a socket connection in background, then ain't this eat battery like a dinosaur?

What I have found is that if I force stop Messenger and Facebook I still receive messages but somewhat late than usual. Plus there is an option of 'Allow background activity' in Messenger, Facebook, Slack and Skype, which when I disable, I can't receive a notification at all. Interesting this is that WhatsApp don't have this kinda option in its app info and If I just force stop WhatsApp it simply stops showing me message notifications.

Answer 1

Whether these apps are relying on FCM/GCM (or any other type of) push notifications for syncing their data or are they using a socket connection in background through a service?

They must be relying on FCM.

If they are relying on push notifications then why they don't have any loss in receiving notifications (in background) while I miss 10-15% of notifications in busy hours.

They must be using FCM high priority which are reliably delivered in doze mode.

And if they are relying on a socket connection in background, then ain't this eat battery like a dinosaur?

They might not be running CPU intensive work.

Mobile applications like Whatsapp must be requesting permission to exempt them from Doze/battery saving and App standby mode. you can refer to my answer here for more details.

Answer 2

Adding on to Sagar answer.

1)Never show the notification from FCM notification payload unless required.

2)Once High priority FCM received you can do network operations for sometime and fetch data from server and then show up. If it fails whatsapp do puts ups with special msg. https://www.quora.com/Why-do-I-get-You-may-have-new-messages-notification-on-WhatsApp

3)use foreground service/ cpu lock tied with app icon momentarily till the data is fetched ,later close it.

PS:No whatsapp do not request for Battery Optimization. they probably reply on FCM high-priority message.