Complete guide to creating and using middleware in backend microservices. Context propagates through entire request No need to pass context through every functi
The Middleware Guide skill provides a comprehensive framework for creating and managing middleware in backend microservices, specifically focusing on Express patterns. It covers key middleware types like authentication, audit logging with AsyncLocalStorage, error boundaries, and validation, emphasizing context propagation throughout a request without manually passing it between functions. This skill also explains middleware composition and the correct ordering required to ensure reliable request handling and error management.
The skill improves observability by integrating with tools such as Sentry for error tracking and supports scalable, type-safe context access across services and repositories. By following this guide, teams can build more maintainable and robust APIs that capture user context, handle errors gracefully, and compose reusable middleware chains efficiently.
This skill is designed for backend developers and engineering leads working in microservice architectures who need to implement consistent request handling patterns. It suits performance marketers or growth engineers collaborating with development teams to ensure reliable user data capture and error monitoring in their APIs. Additionally, agency strategists managing client platforms with complex authentication and auditing demands will find this skill valuable for aligning middleware strategies with business requirements.
Practitioners typically start by defining audit middleware that uses AsyncLocalStorage to capture user context once per request, making it accessible throughout the call stack without explicit parameter passing. Next, they implement error boundary middleware to centralize error handling, translating internal errors into user-friendly responses and capturing details in Sentry. Middleware composition is addressed by creating utility functions to combine authentication and audit middleware, enabling clean and reusable route definitions. Finally, the proper ordering of middleware is established, ensuring that request parsing, authentication, routing, error handling, and monitoring tools are layered correctly to avoid runtime issues.
How do I propagate user context across async calls without passing parameters? Use AsyncLocalStorage in audit middleware to store and retrieve context seamlessly during the entire request lifecycle. Where should error handlers be placed in the middleware chain? Always register error boundary middleware after all routes to properly catch exceptions. Can I combine multiple middleware for reuse? Yes, create composable functions that return arrays of middleware, allowing you to apply authentication and auditing consistently across endpoints.
Attach the Middleware Guide skill to backend or API-related tasks where Express.js middleware patterns are relevant. Expect detailed guidance on setting up audit context propagation, error boundaries, and middleware composition that streamline development and debugging workflows. This skill will help you standardize request handling and error management, making your service more reliable and easier to maintain. For more details on integration and usage, see the attached resources and internal documentation.
For broader context, see our roundup of claude marketing skills, and read common Claude Code content mistakes for related setup guidance.