How would you design a Payment Gateway integration that supports multiple providers (PayPal, Stripe)?
Provider-Agnostic Payment Gateway Design
I would define an application-level payment abstraction and implement one adapter per provider. The business layer should work with our own PaymentRequest and PaymentResult models rather than provider-specific SDK types. A factory or dependency-injection strategy can select the appropriate provider. This isolates vendor-specific APIs and makes adding another provider much safer.
- 1
Use an abstraction owned by the application rather than exposing provider SDK types.
- 2
Use Adapter implementations to translate provider-specific APIs.
- 3
Select providers through dependency injection, configuration, routing policy, or a factory.
- 4
Normalize provider errors into meaningful application-level error categories.
- 5
Design for idempotency because payment requests may be retried.
- 6
Handle webhooks, asynchronous payment states, timeouts, retries, reconciliation, and duplicate notifications.
- 7
Never assume a successful API response alone means the entire payment lifecycle is complete.
Scenario Questions
0-2 years experience
- 1We have a simple checkout service that currently talks directly to PayPal. How would you refactor it so we can also process Stripe payments without rewriting the whole flow?
- 2If Stripe returns an error code you don’t recognize, what would your code do to surface that to the user?
2-5 years experience
- 1Explain why you would choose a Strategy pattern over a big if‑else switch when supporting multiple payment providers.
- 2During a release we notice that some PayPal transactions are duplicated after a retry. How would you modify the design to prevent duplicate charges?
5-8 years experience
- 1Our platform needs to handle thousands of payments per second and must fall back to a secondary provider if the primary is slow. How would you design the integration layer to meet latency and reliability goals?
- 2Discuss how you would implement idempotent payment requests and reconcile failures across PayPal and Stripe while keeping PCI‑DSS compliance.
8+ years experience
- 1We plan to migrate the payment integration to a micro‑service architecture that will be used by multiple product teams. What architectural decisions would you make to keep the provider implementations reusable and versioned?
- 2How would you set up a cross‑team governance model for adding, deprecating, or updating payment providers without causing regressions in existing services?
Follow-up Questions
- What would you change if a new provider required a webhook callback?
- How do you ensure that sensitive credentials are not hard‑coded?
- Can you walk me through how you would test the integration locally?