Unfolding Your Next RoleWe are looking for an experienced Senior .NET Developer to design, build and support system integrations between a registry platform application and multiple third-party systems, including document generation platforms and payment service providers.You will work closely with architecture, engineering, QA, and external stakeholders to deliver robust, secure and maintainable integrations that support key customer workflows.You will work on a project leveraging modern architectures and technologies such as modular monolith, C#, .NET 9, CQRS, DDD, Azure, and Docker.IN SHORT:🌎 100% remote or from the office (Wrocław, Bielsko-Biała) - You choose!💰 120 - 150 PLN/h☑️ B2B contract💻 >4 years of commercial programming experienceYour Daily Missions Designing and implementing REST/FTP/SOAP API integrations between a registry platform and third-party systems. Developing and maintaining integrations for: Documatrix (document generation / templating workflows), Payrexx (payment processing / checkout flows), Other customer-specific back-office systems and external service providers. Building integration services for synchronous and asynchronous processing, including webhook-based event handling. Troubleshooting integration issues across environments and supporting incident resolution when required. Collaborating with interdisciplinary teams in creating web applications in a modular monolith architecture. Designing, developing, and maintaining scalable and maintainable backend solutions. Writing clear, maintainable, and testable code with an emphasis on good testing practices. Participating in code reviews and contributing to knowledge-sharing within the team. Applying coding standards, design patterns, and best practices to ensure high-quality solutions. Engaging in architectural discussions and proposing improvements to enhance system performance and scalability. Working in an Agile SCRUM environment, contributing to sprint planning, daily stand-ups, and retrospectives. Supporting and mentoring other developers when needed. The Essentials We're Seeking Over 4 years of commercial programming experience as a .NET Developer. Strong understanding of software architecture principles and backend design patterns (e.g., dependency injection, repository, mediator, factory). Practical knowledge of cloud environments, ideally Azure. Fluency in English and Polish (minimum B2+) – a must. Solid eperiance with Authentication mechanisms (i.e. OAuth2, JWT, API keys). Proven track record of building and maintaining API-based integrations using REST, SOAP, or FTP. Familiarity with message-driven or async patterns (queues / background processing). Hands-on experience with CQRS and Domain-Driven Design (DDD), including aggregates, entities, value objects, and bounded contexts. Proven ability to build systems with clear separation of concerns, scalability, and maintainability. Solid knowledge of clean architecture principles. Experience with PostgreSQL and modern ORM frameworks (e.g. Entity Framework or equivalent). Exposure to CosmosDB or similar NoSQL/document databases. Familiarity with Docker for local development and deployments. Nice to Have Integration experience with payment providers such as Payrexx, Stripe, Adyen, etc. Document generation / templating integration experience with Documatrix.