Sebastian Kalinowski, Developer in Warsaw, Poland

Sebastian Kalinowski

Full-stack Developer

Warsaw, Poland
Toptal Member Since
October 11, 2019

Sebastian is a .NET developer specializing in back-end layer development. He has over a decade of professional experience within banking, financial, and recruitment industries. He’s also comfortable in front-end development (React), system design, and architecture. Sebastian has successfully worked with numerous clients to define requirements, design new system elements, implement new functionalities, and optimize existing systems.

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Santander Consumer Bank — Nordics
Azure, REST, TypeScript, React, .NET Core, JavaScript, C#
Microsoft SQL Server, Couchbase, Apache Kafka, RabbitMQ, .NET, C#
KMD Poland
Angular, Azure, AngularJS, SQL, IBM Db2, Web API, JavaScript, C#


Warsaw, Poland



Preferred Environment

JetBrains Rider, Visual Studio Code (VS Code), Visual Studio, Windows

The most amazing...

...project I've worked on was a highly available risk calculation system processing transactions worth billions of dollars.

Work Experience

2018 - PRESENT

Senior .NET Developer

Santander Consumer Bank — Nordics
  • Designed and implemented an internal recurring task scheduler and tracker (front end and back end).
  • Designed and implemented a tax statement's administrative app using React (front end and back end).
  • Implemented back-end parts of the PSD2 directive (Consent, Cards).
  • Configured CI/CD pipelines in Azure DevOps and Octopus Deployment.
  • Ensured high quality of the project by proposing and implementing enhancements.
  • Maintained some internal projects.
  • Completed OAuth2 and OIDC configurational tasks.
  • Worked in scrum and scrum with kanban environments.
  • Served as an API Foundation team member—providing guidelines for organizing about API interfaces.
Technologies: Azure, REST, TypeScript, React, .NET Core, JavaScript, C#
2017 - 2018

Senior .NET Developer

  • Redesigned components of the distributed batch system calculating the risk of G10 Rates trades.
  • Rearchitected and reimplemented critical component of an existing system which allowed slicing it and reducing its complexity.
  • Improved the existing codebase and paying off technical debt.
  • Investigated and solved non-trivial issues on daily basis.
  • Ensured high quality of the project by proposing and implementing enhancements.
  • Gave technical presentations for team members.
Technologies: Microsoft SQL Server, Couchbase, Apache Kafka, RabbitMQ, .NET, C#
2016 - 2017

Senior Full-stack .NET Developer

KMD Poland
  • Developed new modules for the application for Danish utility companies supporting administration, billing, metering, task management (frontend and backend).
  • Implemented new features for the application supporting job center employees in Denmark (frontend and backend).
  • Ensured high quality of the project by proposing and implementing enhancements.
  • Worked in scrum.
Technologies: Angular, Azure, AngularJS, SQL, IBM Db2, Web API, JavaScript, C#
2015 - 2016

Senior .NET Developer

Grupa Pracuj
  • Implemented internal application supporting management of job offers for the largest recruitment platform in Poland.
  • Refactored flaws of current architecture.
  • Optimized performance of existing systems.
  • Ensured high quality of projects by proposing coding conventions, service contracts, code reviews and GitHub workflow in the team.
  • Mentored junior team members.
  • Worked in a scrum environment.
Technologies: CQRS, NancyFx, AngularJS, Akka.NET, RabbitMQ, Microsoft SQL Server, Entity Framework, Web API, JavaScript, C#
2014 - 2015

.NET Developer

  • Implemented currency exchange (FX) and payment basket modules for the mBank main website.
  • Built new modules for an existing corporate CRM, optimized existing code.
Technologies: jQuery, ASP.NET MVC, Microsoft SQL Server, LINQ to SQL, Windows Forms (WinForms), JavaScript, C#
2011 - 2014

.NET Developer

  • Implemented Idea4Work: an application that simplifies the management of programmatic projects.
  • Added new features to Deal Manager: a system that helps to manage a real estate agency in Australia.
  • Implemented projects like PalletPlus (asset management and tracking) and Document Control (versioning documents) for a customer from Australia.
  • Designed parts of the system (including architecture), creating reports and unit and integrated tests.
  • Applied domain-driven design (DDD), CQRS, and event sourcing patterns.
  • Coordinated a small team of three junior programmers.
Technologies: Entity Framework, Kendo UI, Telerik Reports, Controls, Telerik, jQuery, Bootstrap, NServiceBus, SQL, RavenDB, ASP.NET MVC, Windows Presentation Foundation (WPF), JavaScript, C#
2010 - 2011

Junior .NET Developer

  • Implemented a certificate application for the Polish Office of Electronic Communications (e-learning, examination, and issuing certificates).
  • Adjusted and integrated existing open-source OCR/OMR software with the certificates app project.
  • Implemented the back end of mobile app for one of Ukrainian's banks.
  • Composed technical documentation.
Technologies: PHP, Microsoft SQL Server, Windows Communication Framework (WCF), Web Services, MySQL, C#


Scheduler and Workflow Service

An internal project used by other developer teams for scheduling and execution tracking of recurring tasks. The project was built on top of one of the open-source schedulers and .NET Core in RESTful microservices architecture with a front end built with React and TypeScript.


This was an internal project that involved a complex batch system calculating the risk of G10 Rates trades. The system had to be highly available and work 24/7. The project was built using .NET, RabbitMQ, SQL Server, and Couchbase. Since the build of the project, I've added new features and refactored one of the critical components.


This is an application for utility companies that supports administration, billing, metering, and task management. The project was supposed to replace some small legacy WinForms applications with one, coherent application built with AngularJS. The back end was built with the .NET framework and DB2 as a persistence layer.



C#, C#.NET, TypeScript, JavaScript, T-SQL (Transact-SQL), XAML, ECMAScript (ES6), CSS, HTML5, SQL, HTML, PHP


.NET Core, .NET, Entity Framework Core, Windows Presentation Foundation (WPF), ASP.NET Core, OAuth 2, Angular, NancyFx, ASP.NET MVC, Bootstrap, Windows Communication Framework (WCF), AngularJS, Kendo UI, ASP.NET


React, Entity Framework, Windows Forms (WinForms), Web API, jQuery, NServiceBus


Full-stack, APIs, RESTful Microservices, Front-end Development, Akka.NET, Controls, Web Services, Cloud, OpenID Connect (OIDC)


Visual Studio, ReSharper, C#.NET WinForms, JetBrains Rider, Telerik Reports, RabbitMQ, LINQ to SQL


Agile, Scrum, Pair Programming, CQRS, Object-oriented Programming (OOP), REST


Windows, Azure, Visual Studio Code (VS Code), Apache Kafka, Telerik


SQL Server 2016, Couchbase, Microsoft SQL Server, IBM Db2, RavenDB, MySQL


2013 - 2014

Postgraduate Coursework in Project Management

Warsaw University of Technology - Warsaw, Poland

2011 - 2012

Master's Degree in Computer Science

Warsaw University of Technology - Warsaw, Poland

2007 - 2011

Bachelor's Degree in Computer Science

Warsaw University of Technology - Warsaw, Poland



Azure Fundamentals



PRINCE2 Foundation