System design interview question
"Design WhatsApp. One-to-one and group chat, messages delivered in order, delivery and read receipts, and it has to work when the recipient's phone is off. 2 billion users, 100 billion messages a day."
stateful connection management plus exactly-once-feeling delivery over an unreliable network. The hard parts are message ordering, the offline queue, and the fact that every phone is intermittently connected. It is the most commonly asked question in this entire list, so the bar is high — a generic answer will not pass.
On Practick another engineer interviews you live for 60 minutes on a question from this bank. They have the full interviewer guide beside the call: the answers to your clarifying questions, the numbers to hand out, a ladder of follow-ups and a scoring rubric. You get the model solution once the session settles. It is free: interview someone for an hour and you have earned your own.
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