REST API Middleware Development

I need a robust REST-based middleware layer that can sit between my in-house platform and several external services, with FMS being the first mandatory integration. The core job is to build clean, well-documented endpoints that pull, push, and keep data perfectly in sync, acting as the decisioning hub for anything my clients ask to move between systems.

While FMS is the immediate priority, the architecture must stay open enough for quick add-ons—think Google Analytics, Salesforce, or Mailchimp—without major refactors. I’m aiming for a stateless service that handles authentication, rate-limits gracefully, logs every request/response for audit, and exposes clear Swagger/OpenAPI docs so downstream teams can plug in with zero guesswork.

Key deliverables:
• Production-ready middleware code (language/framework is up to you but must be common-stack and container-friendly).
• End-to-end data synchronization flows for FMS, triggered both on schedule and on demand.
• Configuration layer to map any future data source or field without code changes.
• Comprehensive API documentation and a quick-start guide.
• Deployment scripts (Docker/Kubernetes or similar) plus a brief knowledge-transfer session.

Acceptance criteria: all listed flows run error-free in a staging environment for 48 hours, data parity is ≥99.9 %, and unit/integration tests cover critical paths.

If you have solid experience designing scalable REST services and can prove prior work on similar multi-platform connectors, I’d like to see an outline of your approach and timeline.