Senior Backend Engineer (Java) – Freelance – Brussels

About the role

We are looking for a senior backend engineer with strong Java expertise to design and evolve the core of a digital platform ecosystem in Brussels. The role is freelance, 6‑month fixed‑term, with a hybrid work model (two days on‑site per week).

Key responsibilities

• Design and build scalable backend services and APIs.
• Develop integration layers between systems and external partners.
• Contribute to Backend‑for‑Frontend (BFF) architecture.
• Define and implement API standards and lifecycle rules.
• Build reusable backend components across multiple domains.
• Ensure service reliability, scalability, security and proper authentication flows (OAuth2, OpenID Connect, JWT).
• Work with distributed, cloud‑based systems and improve observability, monitoring and performance.
• Participate in architecture and technical design discussions.
• Support PoCs, MVP development and perform debugging with root‑cause analysis.

Required profile

• Strong backend engineering experience, preferably with Java.
• Proven ability to build production‑grade APIs using REST and API‑first design.
• Solid understanding of distributed systems and integration patterns.
• Familiarity with BFF architecture concepts.
• Experience with CI/CD pipelines and automated testing.
• Knowledge of observability, production monitoring and cloud environments.
• Agile development experience and a problem‑solving mindset.
• Ability to constructively challenge technical decisions.
• Fluency in French or Dutch (at least one) and English.

Required skills

• Java
• REST
• OAuth2
• OpenID Connect
• JWT
• CI/CD
• Automated testing
• Observability and production monitoring
• Cloud platforms
• Distributed systems
• Integration patterns
• Backend‑for‑Frontend (BFF) architecture