The follow-up round

Your first answer is only the start of a mobile system design round. The interviewer then asks a twist, such as "the reply got lost" or "the app was killed". These rounds train that moment.

Each round has the question, a weak reply, a strong reply you can say in about 30 seconds, and a second twist to be ready for. Each one takes about 3 minutes to read.

All rounds

1. The reply gets lost
   17 Oct 2026. Chat app. The server saved the message but the phone never heard back.

Coming next: page numbers vs cursor, a token that expires in the middle of an upload, and updates that arrive out of order.

How to use a round

1. Read only the question. Answer out loud for one minute.
2. Read the weak and strong replies. Note what you missed.
3. Score yourself on the score sheet.