2. The net/http package

# The net/http Package You have probably heard that Go is fantastic for building web applications ofall shapes and sizes. This is partly due to the fantastic work that has beenput into making the standard library clean, consistent, and easy to use. Perhaps one of the most important packages for any budding Go web developer isthe `net/http` package. This package allows you to build HTTP servers in Gowith its powerful compositional constructs. Before we start coding, let's doan extremely quick overview of HTTP. ## HTTP Basics When we talk about building web applications, we usually mean that we arebuilding HTTP servers. HTTP is a protocol that was originally designed totransport HTML documents from a server to a client web browser. Today, HTTP isused to transport a whole lot more than HTML.  The important thing to notice in this diagram is the two points of interactionbetween the *Server* and the *Browser*. The *Browser* makes an HTTP requestwith some information, the *Server* then processes that request and returns a*Response*. This pattern of request-response is one of the key focal points in building webapplications in Go. In fact, the `net/http` package's most important piece isthe `http.Handler` Interface. ## The http.Handler Interface As you become more familiar with Go, you will notice how much of an impact*interfaces* make in the design of your programs. The `net/http` interfaceencapsulates the request-response pattern in one method: ~~~ type Handler interface { ServeHTTP(ResponseWriter, *Request) } ~~~ Implementors of this interface are expected to inspect and process data comingfrom the `http.Request` object and write out a response to the`http.ResponseWriter` object. The `http.ResponseWriter` interface looks like this: ~~~ type ResponseWriter interface { Header() Header Write([]byte) (int, error) WriteHeader(int) } ~~~ ## Composing Web Services Because much of the `net/http` package is built off of well defined interfacetypes, we can (and are expected to) build our web applications with compositionin mind. Each `http.Handler` implementation can be thought of as its own webserver. Many patterns can be found in that simple but powerful assumption. Throughoutthis book we will cover some of these patterns and how we can use them to solvereal world problems. ## Exercise: 1 Line File Server Let's solve a real world problem in 1 line of code. Most of the time people just need to serve static files. Maybe you have astatic HTML landing page and just want to serve up some HTML, images, and CSSand call it a day. Sure, you could pull in Apache or Python's`SimpleHTTPServer`, but Apache is too much for this little site and`SimpleHTTPServer` is, well, too slow. We will begin by creating a new project in our `GOPATH`. ~~~ cd GOPATH/src mkdir fileserver && cd fileserver ~~~ Create a **main.go** with our typical go boilerplate. ~~~ package main import "net/http" func main() { } ~~~ All we need to import is the `net/http` package for this to work. Rememberthat this is all part of the standard library in Go. Let's write our fileserver code: ~~~ http.ListenAndServe(":8080", http.FileServer(http.Dir("."))) ~~~ The `http.ListenAndServe` function is used to start the server, it will bind tothe address we gave it (`:8080`) and when it receives an HTTP request, it willhand it off to the `http.Handler` that we supply as the second argument. In ourcase it is the built-in `http.FileServer`. The `http.FileServer` function builds an `http.Handler` that will serve anentire directory of files and figure out which file to serve based on therequest path. We told the FileServer to serve the current working directorywith `http.Dir(".")`. The entire program looks like this: ~~~ package main import "net/http" func main() { http.ListenAndServe(":8080", http.FileServer(http.Dir("."))) } ~~~ Let's build and run our fileserver program: ~~~ go build ./fileserver ~~~ If we visit `localhost:8080/main.go` we should see the contents of our**main.go** file in our web browser. We can run this program from any directoryand serve the tree as a static file server. All in 1 line of Go code.