An user X(Has Whatsapp in Android) tries to send a message to user Y(Has Whatsapp) in iOS when the user Y is offline. The user Y receives a push notification that he has a message from user X. As soon as the push notification is received, user X is able to see the double tick mark in his WhatsApp client. How WhatsApp is able to figure out that the push notification has been successfully delivered to the iOS client ?
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3 Answers
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It's not really possible to know what's going on without seeing at least a protocol dump of WhatsApp as this occurs, so on with the rampant speculation:
I'd guess that WhatsApp is using Message Delivery Receipts, which rely on the client responding to the message, rather than the server. Since push notifications talk to an intermediate service (e.g., APNS), which itself wouldn't send said receipt acknowledgements, the client running on the device, upon receiving the push notification, wakes up, analyzes the message, and sends the delivery acknowledgement to the initiator of the dialog.
At least, that's how I'd do it, if I were trying to leverage existing standards, which isn't necessarily what WhatsApp is doing.
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Whatsapp uses XMPP, which is TCP based. Thus, if the Server is able to get a connection to Y this should ensure that the message has been delivered. So if the whatsapp server is able to send the message to Y, the message is pushed out of the server. If Y has enabled the extension to send message states, Y automatically sends a repsone to X with the extension "delivered".
