Realtime system architecture

Long-lived connections, presence and what to do when the network goes.

• Ride hailing app, like UberMatching riders and drivers: live locations in a geo index, dispatch, surge, trips and payments. A classic system design interview.
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• Video calls, like ZoomPeer to peer for two, an SFU for a meeting, signaling and what happens on a bad network.
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• Food delivery, like iFood or DoorDashThree sides at once: customer, restaurant and courier, with one order moving through a state machine.
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• Chat app, like WhatsAppOne-to-one and group messages: long-lived connections, delivery receipts, offline inboxes and end-to-end encryption.
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