I'm a backend engineer with 12+ years building distributed systems and payment platforms — across Amazon, Goldman Sachs, PayPal, and a handful of fintech and SaaS companies in between. Most of that time has been spent on Java, Spring Boot, and Kafka, solving the unglamorous problems every serious backend eventually runs into: idempotency, rate limiting, multi-tenancy, distributed transactions, auth. I put these together as ready-to-use source code and case studies — not toy CRUD apps, but the same patterns used in real production systems — so you can study, extend, or ship faster with them. No calls, no live sessions here — just code and documentation you can dig into on your own time.