middleware-pattern.md

Middleware Pattern (Chain of Responsibility)

Goal

Build production-grade request pipelines without framework helpers.

Pattern Overview

Middleware is a sequence of focused handlers. Each handler can:

• allow request to continue
• modify request context
• stop request with an error response

Planned Taskflow Middleware Chain

1. Authentication middleware
2. Permission/role middleware
3. Validation middleware
4. Route handler

Why Chain of Responsibility Works Well

• separation of concerns
• reusable logic across routes
• easier testing by layer

Manual Middleware Runner Idea

Pseudo-flow:

1. keep array of middleware functions
2. execute middleware[index]
3. pass next function to move forward
4. handle thrown errors centrally

Design Rules

• middleware should do one job
• return early on failure
• keep shared context explicit
• do not hide side effects

Exercise

Implement middleware for POST /tasks:

• require fake token header (for learning)
• validate title type and max length
• reject invalid status values

Return structured error JSON consistently.