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de/03.4.md

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+# 3.4 Get into http package
+
+In previous sections, we learned about the work flow of the web and talked a little bit about Go's `http` package. In this section, we are going to learn about two core functions in the `http` package: Conn and ServeMux.
+
+## goroutine in Conn
+
+Unlike normal HTTP servers, Go uses goroutines for every job initiated by Conn in order to achieve high concurrency and performance, so every job is independent.
+
+Go uses the following code to wait for new connections from clients.
+
+	c, err := srv.newConn(rw)
+	if err != nil {
+    	continue
+	}
+	go c.serve()
+	
+As you can see, it creates a new goroutine for every connection, and passes the handler that is able to read data from the request to the goroutine.
+
+## Customized ServeMux
+
+We used Go's default router in previous sections when discussing conn.server, with the router passing request data to a back-end handler.
+
+The struct of the default router:
+
+	type ServeMux struct {
+    	mu sync.RWMutex   // because of concurrency, we have to use a mutex here
+    	m  map[string]muxEntry  // router rules, every string mapping to a handler
+	}
+	
+The struct of muxEntry:
+
+	type muxEntry struct {
+    	explicit bool   // exact match or not
+    	h        Handler
+	}
+	
+The interface of Handler:
+
+	type Handler interface {
+    	ServeHTTP(ResponseWriter, *Request)  // routing implementer
+	}
+	
+`Handler` is an interface, but if the function `sayhelloName` didn't implement this interface, then how did we add it as handler? The answer lies in another type called `HandlerFunc` in the `http` package. We called `HandlerFunc` to define our `sayhelloName` method, so `sayhelloName` implemented `Handler` at the same time. It's like we're calling `HandlerFunc(f)`, and the function `f` is force converted to type `HandlerFunc`.
+
+	type HandlerFunc func(ResponseWriter, *Request)
+
+	// ServeHTTP calls f(w, r).
+	func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
+    	f(w, r)
+	}
+	
+How does the router call handlers after we set the router rules?
+
+The router calls `mux.handler.ServeHTTP(w, r)` when it receives requests. In other words, it calls the `ServeHTTP` interface of the handlers which have implemented it.
+
+Now, let's see how `mux.handler` works.
+
+	func (mux *ServeMux) handler(r *Request) Handler {
+    	mux.mu.RLock()
+    	defer mux.mu.RUnlock()
+
+    	// Host-specific pattern takes precedence over generic ones
+    	h := mux.match(r.Host + r.URL.Path)
+    	if h == nil {
+        	h = mux.match(r.URL.Path)
+    	}
+    	if h == nil {
+        	h = NotFoundHandler()
+    	}
+    	return h
+	}
+	
+The router uses the request's URL as a key to find the corresponding handler saved in the map, then calls handler.ServeHTTP to execute functions to handle the data.
+
+You should understand the default router's work flow by now, and Go actually supports customized routers. The second argument of `ListenAndServe` is for configuring customized routers. It's an interface of `Handler`. Therefore, any router that implements the `Handler` interface can be used.
+
+The following example shows how to implement a simple router.
+
+	package main
+
+	import (
+    	"fmt"
+    	"net/http"
+	)
+
+	type MyMux struct {
+	}
+
+	func (p *MyMux) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+    	if r.URL.Path == "/" {
+        	sayhelloName(w, r)
+        	return
+    	}
+    	http.NotFound(w, r)
+    	return
+	}
+
+	func sayhelloName(w http.ResponseWriter, r 	*http.Request) {
+    	fmt.Fprintf(w, "Hello myroute!")
+	}
+
+	func main() {
+    	mux := &MyMux{}
+    	http.ListenAndServe(":9090", mux)
+	}
+	
+## Go code execution flow
+
+Let's take a look at the whole execution flow.
+
+- Call `http.HandleFunc`
+	1. Call HandleFunc of DefaultServeMux
+	2. Call Handle of DefaultServeMux
+	3. Add router rules to map[string]muxEntry of DefaultServeMux
+- Call `http.ListenAndServe(":9090", nil)`
+	1. Instantiate Server
+	2. Call ListenAndServe method of Server
+	3. Call net.Listen("tcp", addr) to listen to port
+	4. Start a loop and accept requests in the loop body
+	5. Instantiate a Conn and start a goroutine for every request: `go c.serve()`
+	6. Read request data: `w, err := c.readRequest()`
+	7. Check whether handler is empty or not, if it's empty then use DefaultServeMux
+	8. Call ServeHTTP of handler
+	9. Execute code in DefaultServeMux in this case
+	10. Choose handler by URL and execute code in that handler function: `mux.handler.ServeHTTP(w, r)`
+	11. How to choose handler:
+		A. Check router rules for this URL
+		B. Call ServeHTTP in that handler if there is one
+		C. Call ServeHTTP of NotFoundHandler otherwise
+			
+## Links
+
+- [Directory](preface.md)
+- Previous section: [How Go works with web](03.3.md)
+- Next section: [Summary](03.5.md)