API Developer

API Developer - 3 months - Brussels Belgium

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About the Role

RELYzIT Talent is seeking an elite API Developer for a high-impact, freelance assignment with our client in Brussels. You’ll architect and deliver a robust, secure API that ensures business continuity as a critical third-party tool is phased out. This is a builder’s challenge: design, implement, and demonstrate a production-ready API that empowers users to transition seamlessly, all while upholding the highest standards of code quality, security, and documentation.

Why This Role Matters

Our client’s business relies on a third-party API whose future is uncertain. Your mission: build a compatible, future-proof replacement that integrates with the ODS project’s database, ensuring uninterrupted access to essential metadata and data submission capabilities. The stakes are high—your work will safeguard data flows, enable smooth user transitions, and set a new standard for secure, maintainable API design. This is your opportunity to deliver lasting impact where it matters most.

Key Responsibilities

• Analyze the existing third-party API and documentation to define the critical subset for re-implementation.
• Map and align ODS project database metadata with the API’s requirements, identifying and addressing any gaps.
• Design and implement a robust security model (authentication, authorization, transport/data protection) tailored to the new API’s needs.
• Develop metadata endpoints leveraging the ODS database as the authoritative source.
• Build submission endpoints with comprehensive validation and clear, actionable error responses.
• Integrate submitted data with the ODS project’s Python loading code, ensuring compatibility and reliability.
• Deliver production-quality code, automated tests (pytest), and concise technical documentation.
• Deploy and demonstrate the solution on Ubuntu Linux, showcasing end-to-end functionality.
• Report on project limitations, open questions, and recommended next steps for full production rollout.

Technical Skills & Experience

• Expert-level Python 3.12+ development, with a focus on FastAPI for RESTful API design.
• Deep experience with SQLAlchemy and PostgreSQL, including reading and mapping existing schemas.
• Proven track record in REST API architecture, development, and compatibility projects.
• Advanced API security expertise: OAuth2/OIDC, JWT, API keys, TLS, and input validation.
• Automated testing using pytest, with a commitment to code quality and reliability.
• Experience deploying solutions on Linux (Ubuntu preferred).

Nice to Have

• Familiarity with Polars in combination with SQLAlchemy.
• Experience using uv for Python package management.
• Background in API migration or compatibility projects.
• Handling of sensitive data (e.g., health data) and GDPR compliance.

Mindset & Soft Skills

• Autonomous, proactive, and comfortable working from incomplete or evolving documentation.
• Pragmatic approach: focus on delivering critical features first, without compromising on quality.
• Clear, concise communicator—able to articulate findings, limitations, and next steps to both technical and non-technical stakeholders.
• Builder’s mindset: ownership, problem-solving, and a drive to deliver real-world impact.

Language Proficiency

• Professional proficiency in English (written and spoken) is required.

Other Requirements

• Partial remote work is possible; on-site presence in Brussels may be required for key milestones or demonstrations.
• Freelance contract: 3 months duration, with potential for extension based on project needs and performance.
• Eligibility to work in Belgium.

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