Build Backend Middleware

Build Backend Middleware

Purpose

Provide a structured approach to implementing middleware so cross-cutting concerns (auth, logging, validation, rate limits, error handling) are consistent and composable.

When to use

Use this skill when you are:

• Adding authentication/authorization enforcement
• Adding request logging, correlation IDs, and audit trails
• Validating requests before controller execution
• Implementing rate limiting or feature flags
• Introducing error boundaries for async handlers
• Debugging "middleware order" issues

Inputs

• The middleware's responsibility and scope (global vs route-specific)
• Where it should run in the request lifecycle (before/after auth, before validation, etc.)
• What context it reads/writes (e.g., user identity, request ID)

Outputs

• A middleware function with clear invariants
• A documented ordering decision
• A minimal verification plan (happy path + one failure case)

Middleware ordering (typical)

A common order for HTTP services:

1. Request context

   • correlation/request ID
   • structured logging context

2. Parsing

   • JSON/body parsing, multipart parsing

3. Authentication

   • session / token validation

4. Authorization

   • role/permission checks

5. Validation

   • params/query/body validation

6. Business handlers

   • controller execution (delegates to service)

7. Error middleware

   • final error mapping and logging/tracking

Rules

• Middleware MUST be single-purpose and composable.
• Middleware MUST NOT embed business logic (belongs in services).
• Middleware that adds context SHOULD attach the context to a typed request context (or res.locals pattern).
• Error middleware MUST be last in the chain.

Steps

1. Define the middleware contract:
   • inputs the middleware reads
   • outputs/side effects the middleware writes
   • failure modes (status code + error shape)
2. Decide where the middleware sits in the chain (and why).
3. Implement:
   • short, readable logic
   • consistent error mapping
4. Add a focused test:
   • one success case
   • one failure case
5. Verify in a running service:
   • confirm ordering is correct (logs/context present)

Verification

• Middleware executes in the documented order
• Middleware correctly calls next() or terminates the response
• Auth middleware rejects unauthenticated requests with 401
• Error middleware catches and formats all upstream errors
• Request/correlation ID is present in logs for traced requests
• One success and one failure test exist for each middleware

Boundaries

• MUST NOT embed business logic in middleware (delegate to services)
• MUST NOT modify request/response outside documented responsibilities
• MUST NOT skip error middleware registration (must be last)
• MUST NOT assume middleware order without explicit documentation
• SHOULD NOT create middleware with multiple unrelated responsibilities
• SHOULD NOT block the event loop with synchronous operations in middleware

Included assets

• Templates: ./templates/ contains middleware scaffolds and async error wrappers.
• Examples: ./examples/ contains common middleware patterns.

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