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It must be incredibly difficult to turn the whatsapp service back on after an outage, since most active users have probably sent a message that is waiting in the queue. Literally hundreds of millions of messages would hit the server once it goes up. Does anyone know how they handle this?

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Actually this is the one of the things that annoyed me about WhatsApp - when it is down or you don't have a connection you can't queue messages to be sent. I frequent places without phone coverage (e.g. the Tube in London) and this was one of my big gripes.

On iOS Facebook, iMessage and SMS allow you to queue messages and "Retry" sending them when you have a connection (often in the wrong order). Surprisingly the best app for this is Skype which automatically resends them.

I'm pretty sure whatsapp does that - just tried sending a message while in flight mode, it appeared with a little clock to it, indicating it was going to be sent once back online.

Random. It just says 'Connecting' for me.

Perhaps they process them sequentially as opposed to concurrently.

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