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PayPal's "HTTP API Design" interview question focuses on designing a RESTful HTTP API for a specific backend scenario, often involving resource management like user accounts, transactions, or payments, with emphasis on best practices in API design.
Candidates are typically asked to design an API for a payments or transfers system (e.g., "Design the HTTP API for a peer-to-peer money transfer service"). Key requirements include handling CRUD operations for resources like /users, /accounts, and /transfers, supporting idempotency, pagination, versioning, authentication (e.g., OAuth/JWT), rate limiting, and error handling. The design must follow REST principles: use appropriate HTTP methods (GET, POST, PUT, DELETE, PATCH), status codes (200, 201, 400, 401, 429, 5xx), HATEOAS links, and JSON payloads.[3]
POST /v1/transfers: Initiate a transfer between accounts.GET /v1/transfers/{transferId}: Retrieve transfer details.GET /v1/accounts/{accountId}/transfers: List paginated transfers for an account.PATCH /v1/transfers/{transferId}/cancel: Cancel a pending transfer.Example 1: Create Transfer (POST /v1/transfers)
Input (JSON body):
{ "fromAccountId": "acc_123", "toAccountId": "acc_456", "amount": 100.50, "currency": "USD", "idempotencyKey": "uuid-1234" }
Output (201 Created):
{ "transferId": "trans_789", "status": "pending", "fromAccountId": "acc_123", "toAccountId": "acc_456", "amount": 100.50, "currency": "USD", "createdAt": "2026-02-02T04:41:00Z", "_links": { "self": {"href": "/v1/transfers/trans_789"}, "cancel": {"href": "/v1/transfers/trans_789/cancel"} } }
Example 2: Retrieve Transfer (GET /v1/transfers/{transferId})
Output (200 OK):
{ "transferId": "trans_789", "status": "completed", "fromAccountId": "acc_123", "toAccountId": "acc_456", "amount": 100.50, "currency": "USD", "completedAt": "2026-02-02T04:42:00Z" }
Example 3: List Transfers (GET /v1/accounts/{accountId}/transfers?limit=10&status=completed)
Output:
{ "transfers": [...], "nextPageToken": "token123", "totalCount": 50 }