# Simple web server HTTP is a higher-level protocol than TCP, and it describes how a web server communicates with client-browsers. Specifically for that purpose, Go has `net/http` package, which we will now explore. We will start with some really simple concepts. First, let’s write a __Hello world!__ web server. We import `http`, and our web server is started with the function `http.ListenAndServe("0.0.0.0:3000", nil)`, which returns _nil_ if everything is OK or an error otherwise (0.0.0.0 can be omitted from the address, 3000 is the chosen port number). A web-address is represented by the type `http.URL`, which has a `Path` field that contains the `URL` as a string. Client-requests are described by the type `http.Request`, which has a `URL` field. If the request `req` is a POST of an Html-form, and `var1` is the name of an Html input-field on that form, then the value entered by the user can be captured in Go-code with: ` req.FormValue("var1")`. This also works when `var1` is a parameter given via the `url`: https://1dkne4jl5mmmm.educative.run/?var1=value (in your case URL will be different). An alternative is to first call `request.ParseForm()`, and the value can then be retrieved as the 1st return parameter of `request.Form["var1"]`, like in: ```go var1, found := request.Form["var1"] ``` > __Remark:__ If you're running it locally, http://localhost:nnnn/?var1=value will work. The 2^nd parameter `found` is then true. If `var1` was not on the form, `found` becomes _false_. The `Form` field is in fact of type `map[string][]string`. The web server sends an `http.Response`, its output is sent on an `http.ResponseWriter` object. This object assembles the HTTP server's response; by writing to it, we send data to the HTTP client. Now we still have to program what the web server must do, how it handles a request. This is done through the function `http.HandleFunc`. In this example, it says that if the root __"/"__ is requested (or any other address on that server) the function `HelloServer` is called. This function is of the type `http.HandlerFunc`, and they are most often named `Prefhandler` with some prefix `Pref`. The `http.HandleFunc` registers a handler function (here `HelloServer`) for incoming requests on __/__. The __/__ can be replaced by more specific URL’s like __/create__, __/edit__, and so on; for each specific URL, you can then define its corresponding handler-function. This function has as 2^nd parameter the request `req`. Its first parameter is the `ResponseWriter`, to which it writes a string composed of `Hello` and `r.URL.Path[1:]`. The trailing `[1:]` means _create a sub-slice of Path from the 1^st character to the end_. It drops the leading __/__ from the pathname. This writing is done with the function `fmt.Fprintf()`; another possibility is `io.WriteString(w, "hello, world!\n")` The 1^st parameter is a requested path and the 2^nd parameter is a reference to a function to call when the path is requested.

A Simple Web Server

Simple web server