Back End Developer

Empiric; Amsterdam; Híbrido; EUR 96.000 - 165.000; Jornada completa

Descripción de la vacante

Empiric is seeking a senior freelance Back End Developer to own a consumer-facing backend service built on C#, .NET 8 and AWS. This hands-on role requires you to own architecture, releases and production quality, not just deliver features within a larger team.

You will work on an existing codebase, improve it and maintain API contracts used by mobile apps and partners. Expect a hybrid pattern with two onsite days per week in Amsterdam.

Formación

• 7+ years backend development with 5+ years in C# and .NET.
• Experience with consumer-facing backend at scale (100k+ active users).
• End-to-end ownership of architecture, releases and production quality.
• Experience taking over an existing codebase and showing concrete improvements.
• Experience designing or maintaining an API contract for clients you do not control.

Responsabilidades

• Own the architecture, release process and production quality of a core backend service.
• Take over and improve an existing codebase, raising its quality and testability.
• Design and maintain API contracts consumed by clients we do not control.
• Build and maintain partner integrations with clear boundaries.
• Develop and run the service on AWS, with infrastructure as code and automated pipelines.
• Monitor service health and performance and respond to production metrics.

Herramientas

ASP.NET Core

MediatR

FluentValidation

NSwag

Polly

IHttpClientFactory

JWT

OpenID Connect

Terraform

GitLab CI

Descripción del empleo

Location: Amsterdam, Netherlands – hybrid, 2 days per week onsite

About the role

We are looking for a senior freelance Back End Developer to take responsibility for a consumer-facing backend service built on C#, .NET 8 and AWS. This is a hands-on ownership role: you will be accountable for the architecture, releases and production quality of the service, not only for delivering features inside a larger team. You will work with an existing codebase, improve it, and maintain the API contracts that mobile apps and partners rely on.

What you will do

• Own the architecture, release process and production quality of a core backend service.
• Take over and improve an existing codebase, raising its quality, testability and reliability.
• Design and maintain API contracts consumed by clients we do not control (mobile apps, partners), including versioning and backward compatibility.
• Build and maintain partner integrations while keeping third-party types behind clear boundaries.
• Develop and run the service on AWS, with infrastructure as code and automated delivery pipelines.
• Monitor service health and performance, and act on production metrics and alerts.

Experience required

• 7+ years of backend development, of which 5+ years in C# and .NET.
• At least one consumer-facing backend at scale (100k+ active users), where you worked on the core service for 1+ year. Short modules or proofs of concept do not count.
• Proven end-to-end ownership: you were the (lead) developer responsible for architecture, releases and production quality of a service.
• Experience taking over an existing codebase from another team, and the ability to show concretely what you improved.
• Experience designing or maintaining an API contract used by clients you do not control, including versioning and backward compatibility.

Technical skills

.NET and application layer

• C# on .NET 8 and ASP.NET Core MVC: dependency injection and lifetimes, options and configuration, middleware and filters.
• Authentication with several JWT bearer schemes and OpenID Connect.
• MediatR handlers and pipeline behaviours, with FluentValidation.

Integrations and contracts

• Typed HttpClients using IHttpClientFactory and Polly; NSwag-generated REST clients; a GraphQL client.
• Keeping generated partner types behind a boundary rather than letting them spread through the application.
• JSON as a contract: Newtonsoft.Json, polymorphic types with discriminators, enum handling and JSON Schema validation.

Testing

• xUnit, NSubstitute and FluentAssertions.
• Gherkin integration tests (SpecFlow) and WireMock.Net stubs.
• MiniStack for running AWS services locally and in CI.

AWS and delivery

• ECS Fargate and a .NET Lambda triggered by SQS.
• DynamoDB, Redis, SNS and SQS.
• Parameter Store and Secrets Manager.
• Terraform and GitLab CI.
• Health checks and Datadog metrics.