How I Built a FinTech Backend That Actually Works | Java Spring Boot Project Showcase

I built CoreLedgerEngine — a production-grade financial ledger backend using the same accounting and engineering principles that power real-world banking systems. In this video I walk through five core engineering features with live demos: → Double-Entry Bookkeeping — every transfer creates two balanced ledger entries. The net across debit and credit is always zero. Money moves, never gets created. → Blockchain-Style Audit Trail — each ledger entry is SHA-256 hashed and chained to the previous entry. Tamper with any historical record and the audit endpoint detects exactly where the chain breaks. → Optimistic Locking with Spring Retry — concurrent transfers from the same account are handled safely using Hibernate's @Version field. I demonstrate this with a script that fires 9 simultaneous transfer requests and show only one goes through. → Idempotency via Reference ID — duplicate transfer requests are rejected with HTTP 409 regardless of how many times they arrive. Enforced at application level and via a UNIQUE database constraint. → Distributed Rate Limiting + Redis Caching — Bucket4j token buckets stored in Redis (not app memory) so limits hold correctly across both app instances behind the Nginx load balancer. Balance responses cached with owner verification to prevent cache poisoning. ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ TECH STACK ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Java 17 · Spring Boot 3.3.5 · Spring Security · PostgreSQL 15 Redis · Bucket4j · Docker Compose · Nginx · GitHub Actions CI/CD JUnit 5 · Mockito · SpringDoc OpenAPI (Swagger) ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ LINKS ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ GitHub Repository → https://github.com/mayank1008-tech/Co... LinkedIn →   / mayank-jain-78a6bb321   ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ #Java #SpringBoot #BackendDevelopment #SystemDesign #Redis #Docker #FinTech #DatabaseDesign #Concurrency #InternshipProject #PostgreSQL