Abhishek Gupta for ITNEXT

Posted on Jan 3, 2020

Learn about the basics of server side HTTP APIs in Go

#go #tutorial #beginners #showdev

Welcome to another edition of Just Enough Go - a series of articles about the Go programming language which covers some of the most commonly used Go standard library packages e.g. encoding/json, io, net/http, sync etc. I plan to keep these relatively short and example driven.

More than happy to get your feedback via Twitter or just drop a comment 🙏🏻

In this post, we will explore net/http package which provides the server and client side APIs for HTTP services. This part will provide an overview of the important server components (client APIs will be covered in another post)

Code examples are available on GitHub

Let's start off with the fundamental building blocks - ServeMux and Server

ServeMux (multiplexer)

Simply put, ServeMux is an HTTP request multiplexer which is responsible for matching the URL in the request to an appropriate handler and executing it. You can create one by calling http.NewServeMux. The next thing you do is attach URLs and their respective handler implementations to a ServeMux instance using Handle and HandleFunc methods.

Let's see how you would use the Handle method - it accepts a String and an http.Handler

func (mux *ServeMux) Handle(pattern string, handler Handler)

http.Handler is an interface (second parameter in the Handle method) with the ServeHTTP method

type Handler interface {
    ServeHTTP(ResponseWriter, *Request)
}

We can simply use a struct to provide the implementation. e.g.

type home struct{}

func (h home) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
    rw.Write([]byte("Welcome to the Just Enough Go! blog series!"))
}

... and attach this to the multiplexer as follows:

mux := http.NewServeMux()
mux.Handle("/", home{})

Let's add another handler to our ServeMux (mux) - this time, we'll use the HandleFunc variant whose signature is the following:

func (mux *ServeMux) HandleFunc(pattern string, handler func(ResponseWriter, *Request))

Unlike the Handle method, HandleFunc accepts the handler implementation in the form of a function (along with the path for which it is to be invoked). You can use it as such

mux.HandleFunc("/posts", func(rw http.ResponseWriter, req *http.Request) {
    rw.Write([]byte("Visit http://bit.ly/just-enough-go to get started"))
})

Server (HTTP server)

Now we have a multiplexer which can respond if a user navigates to the root of our service i.e. / as well as /posts. Let's tie it all together with a Server. It's very easy to create a new instance of a Server

server := http.Server{Addr: ":8080", Handler: mux}

There are a bunch of parameters which we can define for our HTTP server, but let's look at a couple of important ones i.e. Addr and Handler (highlighted above) - Addr is the address on which the server listens e.g. http://localhost:8080 and Handler is actually an http.Handler instance.

The Handler bit is interesting because we just saw how the Handle method in ServeMux also accepts an http.Handler. So, do we pass the same instance here as we did for the Handle method in ServeMux, and what's the point of doing that (again)?

If you just had route or path which you wanted to handle, you can pass an instance of an http.Handler (e.g. home{} in this case) and skip the ServeMux altogether. Otherwise, for most cases, you can/should pass an instance of a ServeMux so that you can handle multiple routes/paths (e.g. /home, /items etc.) - and this is possible because it implements http.Handler. Internally, it works by dispatching or routing to the appropriate handler based on the path (URL) in http.Request.

It defines a ServeHTTP method as required by the http.Handler interface

func (mux *ServeMux) ServeHTTP(w ResponseWriter, r *Request)

The Handler can be nil - this scenario is discussed later in this post

Great! So far, have a ServeMux with two handlers and we have associated the Server with the multiplexer and defined where it will listen at. Finally, you just need to start it using the ListenAndServe method

server.ListenAndServe()

That's it. Here is the consolidated code (pretty small!)

package main

import (
    "log"
    "net/http"
)

func main() {
    mux := http.NewServeMux()

    mux.Handle("/", home{})

    mux.HandleFunc("/posts", func(rw http.ResponseWriter, req *http.Request) {
        rw.Write([]byte("Visit http://bit.ly/just-enough-go to get started"))

    })

    server := http.Server{Addr: ":8080", Handler: mux}
    log.Fatal(server.ListenAndServe())
}

type home struct{}

func (h home) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
    rw.Write([]byte("Welcome to the \"Just Enough Go\" blog series!!"))
}

To try this

simply save the code in a file (e.g. go-http-1.go) and
run it - go run go-http-1.go
access the endpoints - curl http://localhost:8080/ and curl http://localhost:8080/posts

Default multiplexer

To make things simpler, there is a ready-to-use multiplexer DefaultServeMux. You don't need to use an explicit ServeMux. The Handle and HandleFunc methods available in a ServeMux are also exposed as global functions in net/http package for this purpose - you can use them the same way!

http.Handle("/users",myHandler{})

http.HandleFunc("/items",func(rw http.ResponseWriter, req *http.Request){
    //handler logic
})

To start the HTTP server, you can use http.ListenAndServe function, just like you would with a Server instance.

func ListenAndServe(addr string, handler Handler) error

The handler parameter can be nil if you have used http.Handle and/or http.HandleFunc to specify the handler implementations for the respective routes.

Functions as handlers

So far, we saw how to use a struct in order to implement http.Handler interface and use it in HandleFunc. You might want to use a standalone function without declaring a struct. net/http package defines a function type for this http.HandlerFunc

type HandlerFunc func(ResponseWriter, *Request)

HandlerFunc allows you to use ordinary functions as HTTP handlers e.g.

func welcome(rw http.ResponseWriter, req *http.Request) {
    rw.Write([]byte("Welcome to Just Enough Go"))
}

welcome is a standalone function with the required signature. You can use this in Handle method which accepts a http.Handler as follows:

http.ListenAndServe(":8080", http.HandlerFunc(welcome))

HandlerFunc(f) is a Handler that calls the function f

The code looks like

package main

import "net/http"

func main() {
    http.Handle("/welcome", http.HandlerFunc(welcome))
    http.ListenAndServe(":8080", nil)
}

func welcome(rw http.ResponseWriter, req *http.Request) {
    rw.Write([]byte("Welcome to Just Enough Go"))
}

To try this: