Chapter 24: Middleware (The Onion)

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

24.1 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.

24.2 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.

24.3 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.