Payment system

How this question breaks down

1. 1FlowCore25 minTake a buyer's card from checkout to money in the bank. Authorize, capture and refund, and learn the final outcome of every payment, even when it arrives days later by webhook.
2. 2Exactly onceCore24 minMake every charge safe to retry with one idempotency key per intent, a recorded attempt before every PSP call, and a recovery path for the call whose outcome we never heard.
3. 3LedgersCore27 minRecord every cent as balanced, immutable double-entry postings, then prove each day that our ledger, the PSP's settlement report and the bank statement tell the same story.

Work through the parts in order and you’ve covered the full design. Each part is one topic.