Chapter 10: Middleware

This is one of the most advanced concepts in Go web servers. Let's dissect it carefully.

1 Functions as Values

In Go, a function is a value. You can:

1. Store it in a variable.
2. Pass it as an argument.
3. Return it from another function.

2 The Middleware Signature

func Logging(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Pre-Processing
        fmt.Println("Start")
        
        next.ServeHTTP(w, r) // Pass the baton
        
        // Post-Processing
        fmt.Println("End")
    })
}

Line-by-Line Anatomy

1. func Logging(next http.Handler) http.Handler:
   • Input: The Next layer of the onion (the inner handler).
   • Output: A New handler that wraps the old one.
2. return http.HandlerFunc(...):
   • We are building a new handler on the fly.
3. func(w, r):
   • This is the Closure. It captures the next variable from outside.
4. next.ServeHTTP(w, r):
   • Crucial: This calls the original handler.
   • If you forget this line, the request stops here. The User never gets a response.

If Transport talks to Service, and Service talks to Repository... who checks if the user is logged in? Who measures speed? This is the Middleware.

3 The Matryoshka (Russian Doll)

Middleware is like a Matryoshka doll. You put your Handler inside a Logger. You put the Logger inside an Authenticator. When a request comes in, it has to drill through all the outer layers to reach the center (your logic).

The Chain

When we wrap handlers: Logging(Auth(HomeHandler))

1. Request arrives $\rightarrow$ Logging starts.
2. Logging calls next $\rightarrow$ Auth starts.
3. Auth calls next $\rightarrow$ HomeHandler (Business Logic).
4. HomeHandler returns.
5. Auth finishes.
6. Logging finishes.

🎓 Knowledge Check: What happens if a middleware forgets to call next.ServeHTTP(w, r)?

Answer: The request stops there! It never reaches the next middleware or your business logic. The user will likely see a blank screen or a timeout.