Lesson 0: Building a Web Server

First step in our jourmey: we will build a web server. Why? Because it will teach us how to use a lot of basic building block for more complicated programs later on.

But before we dive into code, we will present some concepts we will be working on in this lesson.

Knowledge bits

• In Go, we organize our code into modules. Go modules are in turn made up of packages.

  In our case we will only define the main package. This specific package name is used by the Go compiler to know that the package should compile as an executable program.

  The main function in the package main will be the entry point of our executable.

  More info on this topic can be found in this in https://blog.golang.org/using-go-modules.

• Functions in Go can return more than one value. It is a common pattern in Go to always return an error on operations that may fail. One should then check if the error variable is nil, which means there was no error, or to handle the error if not nil.

  For example:

  host, err := os.Hostname()
  if err != nil {
      fmt.Sprintf("we saw an error: %v\n", err)
  }
  
  fmt.Println("everything is awesome")
  

• pointers: no, don't worry about the naming (you can not do pointer arithmetic as in C or C++). Pointers in Go are used to pass variables by reference to functions. This means, we will be using the exact same object that was passed.

• when using Sprintf, Fprintf or Printf (or any printing function that ends with an f) we can use several printing verbs to present the data. (official docs in https://golang.org/pkg/fmt/) In this lesson we will use:

  • %v: we will output the value in a default format
  • %s: the output would be the string format

Lesson content

Our example program lives here in main.go.

The go.mod file indicates that in this directory we have a Go module. We got that one from doing a go mod init, nothing fancy.

To better understand the mechanics of our example please go though the documentation at https://godoc.org/net/http.

Testing It

To run the app you can compile

$ go build -o app

and build it

$ ./app
starting web server at 0.0.0.0:8080

If you read through the code, you'll notice that we registered an HTTP handler (a function that processes incoming requests) for the route /. You can see this by sending a request to it

$ curl http://localhost:8080/
Greeting from alejandrox1-machine1!

Incidentally, since we only have one route registered, and that is the root, all requests will be handled by it

$ curl http://localhost:8080/hello/
Greeting from alejandrox1-machine1!

And another one

$ curl http://localhost:8080/hello/again/
Greeting from alejandrox1-machine1!

Next steps (optional)

Couple interesting things we can do:

• add more routes to the existing server
• be able to read URL params (e.g http://localhost:8080/hello?user=alejandrox&lesson=0) and output

  Greeting from alejandrox1-machine1!
  Params:
  user: alejandrox
  lesson: 0