Code Review: Partial Failure in a Java Payment Method That Charges Then Records

The interviewer shows the following Java service, which charges a user through an external payment provider and then records the payment in a database. The question: what happens if the client fails in the middle of the call, and how would you change the code? There is no coding in this round; explain the problems and your fix.

import java.util.Map;

public class PaymentService {
 private final PaymentProvider provider;
 private final Database db;

 public Map<String, String> createPayment(String userId, int amountCents) {
 // calls external provider (e.g., Stripe-like)
 String chargeId = provider.charge(userId, amountCents);
 db.insert("payments", Map.of(
 "user_id", userId,
 "amount", amountCents,
 "charge_id", chargeId
 ));
 return Map.of("status", "ok", "charge_id", chargeId);
 }
}

Assume provider.charge makes a network call that charges the user and returns the provider's charge ID, throwing an exception on failure, and that db.insert writes one row.

Clarifying Questions

• Who is "the client" here: the caller of createPayment (an app or another service that retries on timeout), or this service's own connection to the provider?
• Does the provider accept an idempotency key, or let us look up a charge by a reference we supply?
• What should the caller be told when the outcome of a charge is unknown?
• Can the caller send a unique ID for each purchase attempt?