Open to opportunities

Oleh Kostiuk

@olehkostiuk

Message

Senior software engineer specializing in Java distributed systems and event-driven payments.

Poland

Message

What I'm looking for

I’m looking for a remote-first, B2B-ready role where I can architect reliable Java backends and event-driven payment systems, lead migrations with no downtime, and mentor engineers to deliver high-quality production results.

I’m a Senior Software Engineer with 10+ years designing and delivering Java-based distributed systems. My work centers on Spring Boot, JPA/Hibernate, RESTful services, and event-driven architectures with Kafka—especially for high-scale payment engines.

I’ve led and shipped complex modernization efforts, including no-downtime monolith-to-microservice migrations. I’ve also built and integrated payment processing systems, designing checkout, user management, and payment flow architecture end-to-end.

Beyond delivery, I bring leadership and quality discipline. For 5 years, I’ve mentored engineers and managed teams of 6–8, driving code reviews, technical documentation, and clear client communication while keeping production stability front and center.

I’m also comfortable operating in cloud-native environments, building containerized deployments with Docker and Kubernetes and supporting CI/CD using Jenkins and GitHub Actions. I’m remote-first and B2B-ready in Poland, and I’m excited to bring this same reliability and architectural clarity to new payment and platform challenges.

Experience

Work history, roles, and key accomplishments

Senior Backend Engineer (Java)

Gransoft Solutions (for Air Doctor)

Jul 2024 - Dec 2025 (1 year 5 months)

Designed and built an Airwallex payment processor integration in Java/Spring Boot, reducing manual payment processing by 80%+. Maintained and extended a legacy Java/GWT codebase and collaborated with product, QA, and finance teams on payment-flow architecture.

Java Spring Boot Payment Integration GWT Payment Flow Architectures

Senior Backend Engineer

AgileEngine (for Estee Lauder)

Jun 2020 - Jul 2024 (4 years 1 month)

Re-architected a Drupal-based e-commerce platform into a modern backend across the full software design and development lifecycle. Built checkout, user management, and payment-processing systems via RESTful APIs.

Drupal Java REST APIs Checkout Systems User Management Payment Processing

Senior Backend Engineer

Independent Contractor (oDesk / Upwork)

Oct 2008 - Dec 2019 (11 years 2 months)

Delivered 65+ projects totaling 15,000+ hours, focusing on custom application design and REST API development. Completed database modeling and full SDLC delivery for client work.

Custom Applications REST APIs Database Modeling SDLC

Senior Backend Engineer

Upwork (Billing / Payments / Accounting)

Jun 2014 - Sep 2019 (5 years 3 months)

Architected and built an escrow payment backend, including a Java transaction engine for fixed-price jobs and membership billing at scale. Led a zero-downtime migration of 13.5M users from a Perl monolith to Java microservices and integrated multi-gateway payment processing (PayPal, ACH, Wellsfargo) using Kafka-adjacent event streaming for batch charging.

Java Perl Escrow Processing Transaction Processing Microservices Kafka Payment Gateways PayPal ACH Wells Fargo

Head of Unix Administration

NewAge Opt-In

Mar 2006 - Sep 2008 (2 years 6 months)

Built, trained, and managed a 6-person engineering team, including hiring, mentoring, and code reviews. Architected custom monitoring software for 150+ remote servers.

Unix Administration Team Leadership Hiring Mentor Code Review Custom Monitoring Software Monitoring

Head of Unix Administration

NixSolutions Ltd

Nov 2004 - Dec 2005 (1 year 1 month)

Built and managed an 8-person department from scratch and mentored junior team members. Provided client-facing solution architecture, requirements gathering, and technical documentation.

Unix Administration Team Leadership Mentor Requirements Gathering Technical Documentation