Backend Developer – .NET & Java

Backend Developer – .NET & Java About Polaris Advisors Polaris Advisors is a technology and professional services firm connecting skilled professionals across Latin America with opportunities to work with leading U.S.-based organizations. We focus on building strong technical teams and creating long-term partnerships between our consultants and clients. About Our Client Our client is a U.S.-based technology organization undergoing a significant backend modernization initiative. The team is transitioning existing .NET services to modern Java/Spring Boot architecture while maintaining system stability, API compatibility, and production performance. This is an opportunity for engineers who enjoy understanding complex legacy systems and modernizing them thoughtfully, not simply replacing code, but ensuring the new platform behaves reliably in production. About the Role We are looking for Backend Developers with strong hands-on experience in both .NET and Java to join a large-scale backend migration initiative. You will reverse-engineer existing .NET services, design equivalent Java/Spring Boot services, rebuild persistence layers, preserve API contracts, and establish automated testing to prove behavioral parity between the old and new systems. What You'll Do Reverse-engineer existing C#/.NET services, APIs, and data access layers, including undocumented behavior and edge cases.Design and build equivalent services using Java and Spring Boot.Migrate systems incrementally while allowing legacy and modern services to run side by side.Preserve existing REST API contracts and integrations.Rebuild SQL Server persistence layers, stored procedures, and Entity Framework mappings using JPA/Hibernate.Develop automated unit, integration, and contract tests to validate behavioral parity.Own CI/CD, containerization, and deployment of backend services.Participate in architecture and technical design decisions.Mentor other engineers and contribute to reusable migration patterns and best practices. Required Qualifications 4+ years of professional experience building and supporting production backend systems.Strong hands-on experience with C# and .NET, including .NET Framework, .NET Core/.NET 6+, ASP.NET Web API, and Entity Framework.Strong hands-on experience with Java 17 or 21, Spring Boot, Spring Data, and Maven or Gradle.Solid understanding of REST API design, API versioning, and OpenAPI/Swagger.Strong relational database experience with SQL Server and/or PostgreSQL, including schema design, indexing, query optimization, and stored procedures.Experience with AWS or Azure, Docker, and CI/CD pipelines such as Azure DevOps, GitHub Actions, or Jenkins.Strong automated testing experience with tools such as JUnit, Mockito, Testcontainers, xUnit, or NUnit.English proficiency at B2/C1 level, with the ability to participate in daily meetings and technical discussions with U.S.-based teams. Nice to Have Experience with legacy modernization, re-platforming, or .NET-to-Java migrations.Experience with multi-tenant SaaS architectures.Experience with Kafka, RabbitMQ, or Azure Service Bus.Experience integrating with Salesforce, Microsoft 365/Graph, Slack, ServiceNow, or Splunk.Kubernetes and production observability experience.Experience with performance profiling and optimization under production workloads.Previous experience working directly with U.S.-based clients or nearshore teams. Who We're Looking For We're looking for engineers who enjoy solving complex problems and understanding how systems actually work. If you are comfortable reading unfamiliar legacy code, questioning assumptions, identifying hidden edge cases, and building a modern solution without breaking existing functionality, this could be a great opportunity for you. If you're interested, apply or send us your resume to learn more.