This felt manageable at first and then quietly became a lot.
Start by clarifying the requirements and constraints, then propose an idempotent API design with a unique transaction ID generated by the POS. Walk through the authorize-and-capture flow, emphasizing how idempotency keys and a durable transaction log prevent double charges, and discuss trade-offs around consistency, latency, and failure handling.
Pro tip: Emphasize that idempotency must be enforced at the server side with a persistent store, and that the POS should generate a unique idempotency key per transaction attempt. Also, mention that you would handle partial failures by making the capture operation idempotent as well, and use a state machine to track transaction status.
Ask about expected throughput, latency requirements, consistency guarantees, and failure scenarios. Confirm that the POS can generate unique transaction IDs and that the downstream banking system supports idempotent operations.
Define an API where each request includes a client-generated idempotency key (e.g., transaction ID). The server stores the key and the response, so repeated requests with the same key return the same result without re-executing the operation.
Describe the two-phase flow: first authorize (reserve funds), then capture (transfer funds). Ensure both phases are idempotent and that the transaction state is persisted atomically to avoid inconsistencies.
Explain how the system handles network drops and retries: the POS retries with the same idempotency key, the server checks the key, and if the transaction is already processed, returns the stored response. Use timeouts and retries with exponential backoff.
Address trade-offs: strong consistency vs. availability, latency of synchronous calls, and storage overhead for idempotency keys. Mention scaling strategies like sharding by transaction ID and using a distributed cache for idempotency keys.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.