System design interview question
"Design a 1:1 video calling app like Google Duo or FaceTime. Two people, high quality video, low latency, and it has to work when both are behind home routers on mobile networks. 100 million users."
real-time media over the public internet — a domain most backend candidates have never touched. The distinguishing content is NAT traversal (STUN/TURN), signalling vs media planes, codec/bitrate adaptation, and the fact that reliability is the enemy here: TCP retransmission makes video worse, not better.
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