digital payments and checkout
PayPal system design practice is one slice of a 50-prompt set. Narrate the user and the API before you draw boxes. Call out the bottleneck you are willing to leave for a later version. Sequence the risk check relative to the capture. Order is the whole design.
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System design
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How to approach. Users and API first. For a checkout button flow, say the one request you must make fast and the one you can delay.
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How to approach. Draw one failure domain and what still works. PayPal interviewers listen for what you explicitly will not build yet.
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How to approach. Put the capacity math on the board with units. Round numbers are fine if every assumption is labeled.
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How to approach. Retries need an idempotency story. Say what the client sends the second time and what the user sees.
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How to approach. Split the hot path from analytics. Name the log or queue in one sentence and move on.
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How to approach. If a cache can be stale, say who notices and how the product tells the truth. Do not hide staleness.
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How to approach. List readers, writers, and auditors before you pick a database. Authorization is part of the design, not a footer.
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How to approach. Describe the migration in two phases and the rollback. A long freeze is a smell; say so.
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How to approach. Name the load you shed first and the promise you keep. Tie that promise to checkout.
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How to approach. Noisy-neighbor control is a product decision. Say the fairness unit — tenant, user, or key — and the limit.
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