Ledger

Accomplishments

• Built REST APIs for charges, refunds, payment events, and webhook endpoint management.
• Implemented deterministic card simulation for successful payments, card declines, and insufficient funds.
• Designed database-backed idempotency with request hashing, replayed responses, conflict detection, and expiration cleanup.
• Protected mutating operations against duplicate concurrent requests using transactional database locking.
• Implemented full and partial refunds with safeguards against refunding failed charges or exceeding the original charge amount.
• Created an immutable append-only payment event ledger for charge and refund state transitions.
• Implemented signed webhook delivery using HMAC-SHA256 signatures.
• Added asynchronous webhook dispatching with bounded executor configuration.
• Implemented persisted webhook delivery states, attempt tracking, failure responses, and exponential retry scheduling.
• Added scheduled retry processing for failed webhook deliveries with a maximum of eight attempts.
• Added API key authentication with consistent authentication error responses.
• Implemented centralized exception handling with a documented JSON error envelope and consistent error codes.
• Added Flyway-managed PostgreSQL schema migrations for charges, refunds, events, idempotency, endpoints, and deliveries.
• Configured Hibernate schema validation to prevent accidental automatic database schema creation.
• Added OpenAPI and Swagger UI documentation with request and response schemas and examples.
• Built unit tests for domain transitions, services, authentication, idempotency, webhook signing, and retry behavior.
• Built integration tests with Testcontainers PostgreSQL and MockWebServer.
• Verified 57 unit tests and 23 integration tests with zero failures.
• Achieved 94.7% line coverage for service packages and 88.2% for idempotency packages.
• Added Checkstyle quality gates with zero reported violations.
• Documented architecture, local setup, API examples, simulation rules, idempotency design, webhook retries, and engineering tradeoffs.