I've independently built a high-concurrency flash sale engine using Java 21, Spring Boot, PostgreSQL, Redis, Kafka, and microservices. I designed atomic admission, idempotency, transactional outbox workflows, and Testcontainers coverage to prevent overselling and duplicate effects.
As the sole backend owner for a library management system, I delivered REST APIs for authentication, catalog, circulation, holds, payments, and async imports. I implemented JWT and RBAC security, Redis-backed refresh-token rotation, pessimistic locking, background processing, and CI/CD with GitHub Actions.
