I

idempotency

Small PHP service for receiving, storing and delivering webhooks with idempotency, retries and background processing.

Kotlin/Java 21 trading wallet on Spring Boot (Web MVC, Security JWT, Data JPA/Hibernate, Bean Validation). Reserves funds under a row lock, rejects stale order edits, and publishes order events through a transactional outbox. Duplicate retries stay idempotent. PostgreSQL, Flyway, Redis, Kafka. Tests: JUnit 5, MockK, Google Truth, Testcontainers.

Cross-platform TypeScript contract layer: Zod API schemas, deterministic conflict resolution, cursor-based sync configuration, SHA-256 payload hashing, and health metric definitions for offline-first distributed systems