Hi, my name is

Longin Koziolkiewicz. I build backend systems that scale.

Senior Java Engineer with 9 years of experience designing and shipping production microservices. I specialise in the Java ecosystem — Spring Boot, distributed systems, and clean architecture. I write about what I learn along the way.

Open to new opportunities
Read my writing See my work
01.

About Me

I'm a backend engineer based in Poland with nearly a decade of experience building distributed, high-throughput systems in Java. I've worked across logistics, finance, automotive, and retail — writing the kind of code that runs quietly in production while nobody notices, which is exactly how it should be.

My day-to-day revolves around Spring Boot, microservice design, event-driven architectures, and the never-ending pursuit of writing Java that future-me won't be embarrassed by.

Outside of work I enjoy dissecting tricky LeetCode problems, thinking through system design tradeoffs, and writing about the Java ecosystem for engineers who want to go deeper than the tutorial layer.

Here are a few technologies I work with regularly:

  • Java 17 / 21
  • Spring Boot / Spring Security
  • Apache Kafka / Camel
  • PostgreSQL / MongoDB / Redis
  • Docker / Kubernetes
  • REST & GraphQL APIs
  • Maven / Gradle
  • AWS / Azure
Longin Koziolkiewicz
02.

Experience

Software Engineer @ EPAM Systems

Jul 2019 — Present

Working on a money market & treasury operations system for UBS — a mission-critical platform handling internal lending, borrowing, and settlement for a global investment bank. The system processes high volumes of financial transactions under strict SLA requirements, built on Spring, MyBatis, Kafka, REST, and SOAP.

Software Engineer @ Capgemini

Oct 2017 — Jun 2019

Worked on a large-scale sea freight platform for Kuehne+Nagel — an enterprise logistics system managing global shipment workflows. Involved in feature development, production support, and CI/CD tooling. Stack: Java, Spring, Camunda BPMN, Jenkins.

Senior Java Software Engineer (Self-employed)

Jan 2022 — Present
  • Volvo — Backend maintenance and migration of a checkout page from Java to Next.js. Interesting mostly because of the cross-stack nature — keeping the Spring Boot backend stable while it moved to a completely different stack. Stack: Spring Boot, GraphQL, REST.
  • Bolero — Full-stack development on a financial brokerage platform. Worked across the whole feature lifecycle, from backend services in Spring Boot through to Angular UI, with Apache Camel handling integrations messaging between clients.
  • Tractor Supply — Migrated a legacy Commerce V9 platform to a purpose-built microservice. The interesting part was the data migration — moving live retail data cleanly across systems without disruption. Stack: Spring Boot, MongoDB, Azure.
03.

Writing

Kafka Architecture Java
Kafka for Java Developers: Beyond the Hello World
Producers, consumers, offsets, consumer groups, dead letter topics, and the mistakes that only show up in production.
March 2026
Architecture Spring Boot
Why I Stopped Using @Transactional on Everything (And What I Do Instead)
A deep-dive into Spring's transaction management, the hidden pitfalls of proxy-based AOP, and how to design service boundaries that actually make sense.
March 2026
Architecture System Design
CAP Theorem: What It Actually Means for the Systems You Build
Beyond the textbook triangle — the real tradeoffs between consistency, availability, and partition tolerance, with diagrams and practical system design guidance.
March 2026
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04. What's next?

Get in Touch

I'm currently open to new opportunities. Whether you have a role in mind, want to discuss architecture, or just want to say hello — my inbox is open.

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