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Sergii Zolotarov

A warrior's history

Sergii Zolotarov
Author
Sergii Zolotarov
Twenty years of Java, from Kyiv to Leeuwarden. Currently at Booking.com
I’ve been writing software for more than twenty years. Some of the tools I’ve fought with are ancient, some are bleeding edge — and I keep learning, so the list keeps growing.

20+

Years of Java

Since 2005, Kyiv → the Netherlands.

4

Sun certifications

Programmer, Developer, Web & Business Component.

1 / 70

Vendor award

Inspirational Vendor Award, out of 70 nominees.

The campaigns
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  1. Software Engineer — freelance

    2026 – now

    Booking.com, Amsterdam

    As any Software Engineer at Booking I am responsible for everything around a living service. And being a contractor I get the trickest tasks nobody wants to work on. These include: maintaining and fixing issues in .... Perl monolith on a baremetal cluster, Python data loaders in Spark, fixing / adding / removing component dependencies, configuring and optimizing APIgee routes in GCP, hardening security for external API's, optimizing metrics cardinality to save costs, looking into cache efficiency and GC pauses in a Java backend and hundreds of other tiny things. And obviously taming a fleet of AI agents 🤖️🤖️🤖️

  2. Team lead — freelance

    2025 – 2026

    CJIB, Leeuwarden

    Migrating a Eureca/Metis system to a new Spring / Angular stack and adding features along the way. At its heart: an in-house process engine — a state machine on a database, with JMS as the messaging backbone. I worked on the architecture and the development process with the team, plus the maintenance, operations and monitoring that keep a system like that alive.

  3. Java DevOps — freelance

    2020 – 2025

    FedEx, Hoofddorp

    Started as the only back-end developer on a new global customer-registration platform and wrote its first code. Spring Boot on Azure, a validation and rules engine for each market's quirks (tax IDs, document uploads, Salesforce notifications), legacy SOAP/EJB/mainframe systems wrapped in modern REST APIs, and a CI/CD pipeline for every service. Won the Inspirational Vendor Award — chosen from 70 nominees, and the first time it went to a freelancer.

  4. Java DevOps — freelance

    2019 – 2020

    ING, Leeuwarden

    Rewrote the XLetter template-management system: Polymer/Redux front end replacing JSP, Spring Boot replacing WebSphere. Selenium tests on ING's grid, integration tests and a mock server in Kotlin.

  5. Java DevOps engineer — freelance

    2018 – 2019

    Bol.com, Utrecht

    Split a monolithic recommendations backend into microservices and moved it from the datacenter to Google Cloud: gRPC/REST services on Kubernetes, Datastore, Spinnaker, Dataflow (Apache Beam) data loaders, Prometheus and Gatling. Everything automated and in source control.

  6. Dev engineer — freelance

    2017 – 2018

    ING, Amsterdam

    Middleware and reporting for ING's pricing solution: Apache Ignite as the data layer, gRPC on top, OSGi (Karaf), ActiveMQ, Tibco and Protocol Buffers.

  7. Technical lead

    2009 – 2017

    Asset Control, Beetsterzwaag

    Eight years of integration middleware that let customers consume data from a legacy backend in every format imaginable — Camel, Spring, AspectJ, CXF, Jetty. Finished with an Akka-based scheduling and orchestration system tying the product suite together.

  8. Java architect

    2007 – 2009

    Mirasoft, Kyiv

    Replaced a huge Swing front end with IceFaces (JSF) and Spring MVC, still talking to the original EJB backend. On the side: a Groupon-style platform for the Indian market (Spring MVC, Hibernate, Lucene).

  9. Senior Java developer

    2005 – 2007

    Ciklum & Luxoft, Kyiv

    Trade monitoring for Deutsche Bank (JSF, JPA, WebLogic). Before that, a mobile SMS communications and advertising platform built with Spring, Struts and Eclipse RCP.

  10. First blood

    2004 – 2005

    Mirasoft, Kyiv

    Web services that combined hotel, ticket, taxi and insurance providers into one product. SOAP, REST, Tomcat — and Firebird, which later ran the first version of this site on a Raspberry Pi 1.

  11. BSc Physics

    1999 – 2005

    National University of Kyiv-Mohyla Academy

The arsenal
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Java EE, SE, ME, OSGi · Spring / Spring Boot · Camel · Akka · JPA / Hibernate · Kotlin · Groovy

Cloud & ops Azure · Google Cloud · Kubernetes · Docker · Spinnaker · GitLab CI · GitHub Actions · Jenkins

Front end Angular · TypeScript · Polymer / lit-html · GWT · Vaadin · JSF

Messaging & data JMS / ActiveMQ · Azure Event Hubs · gRPC · Cassandra · Apache Ignite · Oracle · Postgres · Firebird

Desktop Eclipse RCP · SWT / JFace · Swing · JavaFX

Spoken English · Dutch · Ukrainian · Russian

What others say
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Sergey is my go-to person for both front-end and back-end development of all our apps. Someone with sharp insight, lots of experience and up-to-date knowledge of the latest tech stack.

— IT manager at FedEx, in Spilberg’s interview (translated from Dutch)

He built the whole prototype in a short amount of time and the quality was excellent. I later used him for 3 more projects and every time he has gone above and beyond what is required.

— Omair Aasim, client