Sergii Gulenok, Developer in Luxembourg, Luxembourg
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Sergii Gulenok

Verified Expert  in Engineering

Software Developer

Luxembourg, Luxembourg
Toptal Member Since
June 23, 2020

Sergii has more than a decade of experience in IT, working as a software engineer, project manager, and consultant. His particular strengths are JavaScript and C#. Thanks to an analytical mind and in-depth expertise, Sergii excels at problem-solving by finding the optimal, pragmatic solution that fits the time, budget, and scope. He has been an active freelance since 2015, and among his clients are a publicly-traded healthcare company, a unicorn startup, and several smaller startups.

Python 3, Amazon Web Services (AWS), MySQL, Microsoft SQL Server, Ruby...
Okami Technologies OÜ
Amazon Web Services (AWS), Webpack, .NET...
Dentsply Implants NV
.NET, Object-oriented Analysis & Design (OOAD), Design Patterns...




Preferred Environment

Jira, Slack, Vim Text Editor, Visual Studio Code (VS Code), Linux, MacOS

The most amazing...

...thing I've developed is the website and back office for a company that ranks among the top five online wholesale land marketplaces in the US.

Work Experience

Software Development Engineer II

2020 - PRESENT
  • Redesigned an internal tool for financial planning and analysis, reducing the codebase three times while achieving better performance and maintainability. It reduced the average development of new features from 2 weeks to 3-4 days.
  • Created the internal tool for headcount management in collaboration with business intelligence and data engineers. This tool reduced the number of routine tasks, provided better control over confidential data with a sophisticated permission system.
  • Managed two entry-level developers, provided mentoring and coaching, organized effective collaboration with other teams and stakeholders.
Technologies: Python 3, Amazon Web Services (AWS), MySQL, Microsoft SQL Server, Ruby, Ruby on Rails (RoR), ETL, SQL, TypeScript, JavaScript, React, REST, AWS Lambda, Docker, Python, C#, F#, Office Plugins, Continuous Delivery (CD), Continuous Integration (CI), Lean Project Management, Django, Flask, Technical Leadership, Git, .NET, Back-end, Front-end, AWS Fargate, Cloud, Java, Amazon DynamoDB

Lead Software Engineer | Project Manager

2015 - PRESENT
Okami Technologies OÜ
  • Developed a wholesale land marketplace that ranks among the five largest in the US. Transformed it from a WordPress website into a platform with many external integrations, a subscription-based payment model, social network capabilities, and more.
  • Created an internal performance and compensation review application for a unicorn startup. This custom implementation was preferable to any other tools available on the market and should become a profitable investment in less than a year.
  • Developed a dental company's system integrated with their order management and performed automatic preprocessing of input data, patient's 3D scans—saving 10-20 minutes per case (out of ≈50) on previously manual operations.
  • Built an engineering team by mentoring junior software engineers into reliable developers capable of handling complex real-life tasks with no supervision necessary.
Technologies: Amazon Web Services (AWS), Webpack, .NET, Object-oriented Analysis & Design (OOAD), Design Patterns, Continuous Integration (CI), Continuous Delivery (CD), Object-oriented Design (OOD), Gmail API, Scrapy, Terraform, Memcached, DigitalOcean, HTML, Windows Presentation Foundation (WPF), REST, Functional Programming, Object-oriented Programming (OOP), People Management, Code Architecture, Electron, Django, PostgreSQL, Redis, Express.js, Gmail, GraphQL, Node.js, SlimPHP, NoSQL, QML, Qt, F#, Web Architecture, HTML5, CSS3, CSS, Git, Jira, LINQ, SQL, C++, C#, Project Management, API Architecture, Agile Software Development, Docker, MySQL, MongoDB, Vue, React, Flask, Laravel, Clojure, PHP, Python, TypeScript, JavaScript, Technical Leadership, PHP 7, Okta, SCSS, Back-end, Front-end, Twilio API, AWS Fargate, Scraping, Web Scraping

Senior Software Engineer | People Manager | Software Project Manager

2011 - 2016
Dentsply Implants NV
  • Built two medical software applications: a design 3D CAD tool for dental technicians and the tool to preview, process, and upload the 3D scans for the Atlantis™ Web Order system.
  • Rebuilt the software core, which allowed us to cut time for building new similar software applications by 20%. It reduced the codebase significantly, eliminated lots of boilerplate code, and resolved the root cause of lingering issues.
  • Built new features from scratch to support overlay drawing in 3D along with 3D curves within the in-house DirectX renderer.
  • Introduced an automatic update system for the in-app library and software itself. The number of outdated versions in use by clients was reduced by 80%, plus it allowed clients to release software and implant libraries separately.
  • Developed three internal automation tools to ease teamwork by reducing efforts spent on routine tasks.
  • Served as a people and project manager and led the distributed teams with up to 12 people and 16 successfully delivered projects in four years.
Technologies: .NET, Object-oriented Analysis & Design (OOAD), Design Patterns, Continuous Delivery (CD), Object-oriented Design (OOD), Standard Template Library (STL), SlimDX, TFS, Windows Presentation Foundation (WPF), Mercurial, 3D, REST, Functional Programming, Object-oriented Programming (OOP), People Management, Code Architecture, Three.js, OpenGL, Microsoft SQL Server, Visual Studio Tools for Office (VSTO), QML, Qt, F#, LINQ, C++, Windows, C#, Project Management, API Architecture, Agile Software Development, DirectX, Windows Communication Foundation (WCF), Boost, Clojure, Python, JavaScript, Technical Leadership, Graphical User Interface (GUI)

Software Engineer

2009 - 2011
Materialise Dental NV
  • Developed the 3D CAD tool for dental technicians to design suprastructures, specifically tooth prostheses on top of the implants.
  • Reimplemented one of the workflows from scratch as a part of a small team of two engineers. The project had an aggressive schedule, yet the release version was finished on time and was successful at IDS, where our clients secured several contracts.
  • Introduced continuous integration and nightly builds in the department.
Technologies: .NET, Object-oriented Analysis & Design (OOAD), Design Patterns, Subversion (SVN), Continuous Delivery (CD), Object-oriented Design (OOD), Standard Template Library (STL), TFS, Windows Presentation Foundation (WPF), Mercurial, 3D, Functional Programming, Object-oriented Programming (OOP), OpenGL, QML, Qt, F#, LINQ, C++, C#, Agile Software Development, Computational Geometry, Windows Forms (WinForms), Boost, Graphical User Interface (GUI)

Senior Developer

2008 - 2009
CommerceLab Ltd.
  • Designed and launched the simple in-house alternative of the VirusTotal website.
  • Developed an internal tool that allowed the installation and issue reporting for alpha releases of the software. Alpha testers could log issues semiautomatically with system information, screenshots, and descriptions to the Jira bug-tracking system.
  • Served as one of the evangelists and lead developers for the common toolkit, a set of small modules to be used in various software projects developed by the company.
  • Developed a registry cleaning application for Windows that sped up the system by removing the old and broken registry keys.
Technologies: Object-oriented Analysis & Design (OOAD), Design Patterns, Subversion (SVN), Object-oriented Design (OOD), Standard Template Library (STL), Mercurial, Object-oriented Programming (OOP), OpenGL, C++, Agile Software Development, UML, SQLite, SOAP, COM, WTL, ATL, WinAPI, Microsoft Foundation Classes (MFC), Microsoft Foundation Class (MFC) Library, Boost, Graphical User Interface (GUI)

Software Developer

2006 - 2008
  • Released several applications designed for securing, protecting, and maintaining the Windows operating system and internet security tools, specifically local transparent proxy.
  • Supported some of the engines used in our products and internal projects via bug fixes and implementing feature requests.
  • Developed antispyware software for Windows that actively monitored the system for various root-kits, keyloggers, and other forms of spyware.
Technologies: Object-oriented Analysis & Design (OOAD), Design Patterns, Subversion (SVN), Object-oriented Design (OOD), Standard Template Library (STL), Object-oriented Programming (OOP), Jira, C++, Windows, UML, SQLite, SOAP, XML, COM, WTL, ATL, WinAPI, Microsoft Foundation Classes (MFC), Microsoft Foundation Class (MFC) Library, Boost, Graphical User Interface (GUI)

Online Marketplace for Lands and Houses
I had taken over this project about four years ago. I converted a simple WordPress-based website into the platform with a dedicated back office for listing management for the staff and several client-facing sites, the biggest one of which is This redesign allowed us to extend the business model to provide subscription services to third-party sellers (increasing revenue by 25%) and internal ad network (which also increased the revenue by 10%) while at the same time reducing the operational cost.

I developed the back-office using PHP 7 with Slim 3 framework, MySQL, and Redis databases and lots of external APIs integration for the back end and Vue.js for the front end. I also built the LandCentury website using the Laravel framework for the back end and Vue.js for buyer and seller dashboards.

As the project grew more significant, I then hired several subcontractors, educated them, and managed their work productivity as a product and project manager. Under my leadership, they had added more features to the product, among which is a simple Google Firebase-based chat system.

Affiliate Marketing Network
I developed a network that connects brands with social media influencers. The website had reached the first 1,000 users in a bit more than a month using itself as a marketing tool and is on its way to returning initial investments by the end of this year.

I built this website using Node.js and Express for the back end, GraphQL as an API layer, MongoDB as a database, and React on the front end. The challenge with this project was to effectively and securely process the enormous number of action tracking events (clicks, leads, and sales) for tens of thousands of promoted links.

Compensation Review Tool for a Unicorn Startup

I developed an internal application for compensation reviews for a unicorn startup company. They have a highly tailored compensation review process with a lot of steps, and none of the existing out-of-the-box solutions available on the market was not a perfect fit. The two main challenges of the project are:

1. A lot of exceptions to the overall process that required designing a highly flexible and customizable system.
2. The system constraint was having all compensation and evaluation data secure from being read even with direct access to the database and application servers. I designed and implemented a unique system with data encryption on the client-side to support this requirement.

It was a full-stack project, and I implemented it using TypeScript, Node.js, MongoDB/DocumentDB, GraphQL, and React.

3D CAD System for Dental Implantology

I developed a customized 3D CAD tool that was used to design dental implant-based prostheses based on patients' teeth scans. This tool enforced clinical rules, which reduced the number of prostheses returned by warranty. In some cases, it cut the time that dental technicians spent on designing the prosthesis from 1.5 hours (in other software) to under 30 minutes.

The tool was integrated into the customer's order processing system, which sped up the order processing and reduced the number of human errors and waste cases. The version my team released in 2011 also had some UX innovations that were cloned by our competitors in their products.

Dental Order Submission and Preprocessing Application

I developed software for dental labs and scan centers that allowed the submission of prepared and validated orders with attached 3D scans directly into the client's order processing system.

In the first year, this tool increased the number of incoming orders by 5% and by 15% in the next year. The order pre-processing and validation embedded in this tool freed up the order processing system from bad input. This tool did not allow customers to submit orders with poor-quality 3D scans.
2002 - 2007

Master's Degree in Computer Science

Taras Shevchenko National University of Kyiv - Kyiv, Ukraine


Functional Programming Principles in Scala



React, Node.js, LINQ, Vue, DirectX, OpenGL, Three.js, Gmail API, Google Maps, Standard Template Library (STL), Microsoft Foundation Class (MFC) Library, WinAPI, ATL, WTL, Windows Forms (WinForms), Google API, Twilio API, Microsoft Foundation Classes (MFC)


Jira, Git, Mercurial, TFS, Visual Studio Tools for Office (VSTO), AWS Fargate, Terraform, Vim Text Editor, Slack, Subversion (SVN), Webpack


.NET, Windows Presentation Foundation (WPF), Qt, Laravel, Express.js, Flask, Django, Scrapy, Boost, Electron, SlimDX, Ruby on Rails (RoR), Qt 5, Material UI


Functional Programming, Agile Software Development, Object-oriented Programming (OOP), API Architecture, REST, Web Architecture, Object-oriented Analysis & Design (OOAD), Design Patterns, Continuous Integration (CI), Continuous Delivery (CD), Object-oriented Design (OOD), ETL, Model View ViewModel (MVVM)


JavaScript, C#, PHP, C++, Python, TypeScript, SQL, GraphQL, F#, QML, CSS, HTML, CSS3, HTML5, Python 3, PHP 7, SCSS, Clojure, XML, UML, Ruby, Prolog, Java


Linux, Windows, Docker, AWS Lambda, Visual Studio Code (VS Code), DigitalOcean, MacOS, Amazon Web Services (AWS)

Industry Expertise

Project Management


MySQL, MongoDB, NoSQL, SlimPHP, Redis, Memcached, Microsoft SQL Server, PostgreSQL, SQLite, Amazon DynamoDB


People Management, Code Architecture, Technical Leadership, Computational Geometry, 3D, Graphical User Interface (GUI), Lean Project Management, Back-end, Front-end, Cloud, Gmail, COM, SOAP, Windows Communication Foundation (WCF), Office Plugins, Computer Science, User Experience (UX), Okta, Geometry, Algorithms, Scraping, Web Scraping

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