https://aarynsmith.github.io/blog/go-rest-api-part-3/

main.go

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"log"
	"net/http"
	"strconv"

	"github.com/gorilla/mux"
)

type app struct {
	Router *mux.Router
	UserDB map[int]user
}

type user struct {
	ID   int
	Name string
	Role string
}

func main() {
	a := app{}
	a.Router = mux.NewRouter()
	a.UserDB = map[int]user{
		1: user{
			ID:   1,
			Name: "User 1",
			Role: "Admin",
		},
	}
	a.Run(":3001")
}

func (a *app) Run(addr string) (err error) {
	log.Printf("Listening on %v", addr)
	a.Router.HandleFunc("/user/{id:[0-9]+}", a.GetUser).Methods("GET")
	a.Router.HandleFunc("/user", a.CreateUser).Methods("POST")
	a.Router.HandleFunc("/user/{id:[0-9]+}", a.DeleteUser).Methods("DELETE")
	a.Router.HandleFunc("/user/{id:[0-9]+}", a.ReplaceUser).Methods("PUT")
	a.Router.HandleFunc("/user/{id:[0-9]+}", a.UpdateUser).Methods("PATCH")
	return http.ListenAndServe(addr, a.Router)
}

func (a *app) GetUser(w http.ResponseWriter, req *http.Request) {
	vars := mux.Vars(req)
	id, err := strconv.Atoi(vars["id"])
	if err != nil {
		w.WriteHeader(500)
		fmt.Fprintf(w, "500 Internal Server Error\n")
		return
	}
	_, ok := a.UserDB[id]
	if !ok {
		w.WriteHeader(404)
		fmt.Fprintf(w, "404 page not found\n")
		return
	}
	j, err := json.Marshal(a.UserDB[id])
	if err != nil {
		w.WriteHeader(500)
		fmt.Fprintf(w, "500 Internal Server Error\n")
		return
	}
	fmt.Fprint(w, string(j))
}

func maxID(d map[int]user) (m int) {
	for k := range d {
		if k > m {
			m = k
		}
	}
	return m
}

func (a *app) CreateUser(w http.ResponseWriter, req *http.Request) {
	nextID := maxID(a.UserDB) + 1
	p := user{}
	buf := new(bytes.Buffer)
	buf.ReadFrom(req.Body)
	err := json.Unmarshal(buf.Bytes(), &p)
	if err != nil {
		w.WriteHeader(500)
		fmt.Fprintf(w, "500 Internal Server Error\n")
		return
	}
	a.UserDB[nextID] = p
	j, err := json.Marshal(a.UserDB[nextID])
	if err != nil {
		w.WriteHeader(500)
		fmt.Fprintf(w, "500 Internal Server Error\n")
		return
	}
	w.Header().Set("Location",
		fmt.Sprintf("http://%s/user/%d", req.Host, nextID))
	w.WriteHeader(200)
}

func (a *app) DeleteUser(w http.ResponseWriter, req *http.Request) {
	vars := mux.Vars(req)
	id, err := strconv.Atoi(vars["id"])
	if err != nil {
		w.WriteHeader(500)
		fmt.Fprintf(w, "Could not delete user: %v", err.Error())
		return
	}
	_, ok := a.UserDB[id]
	if !ok {
		w.WriteHeader(404)
		fmt.Fprintf(w, "404 page not found\n")
		return
	}
	delete(a.UserDB, id)
	w.WriteHeader(200)
}

func (a *app) ReplaceUser(w http.ResponseWriter, req *http.Request) {
	vars := mux.Vars(req)
	id, err := strconv.Atoi(vars["id"])
	if err != nil {
		w.WriteHeader(500)
		fmt.Fprintf(w, "Could not delete user: %v", err.Error())
		return
	}
	p := user{}
	buf := new(bytes.Buffer)
	buf.ReadFrom(req.Body)
	err = json.Unmarshal(buf.Bytes(), &p)
	if err != nil {
		w.WriteHeader(500)
		fmt.Fprintf(w, "500 Internal Server Error\n")
		return
	}
	_, ok := a.UserDB[id]
	if !ok {
		w.WriteHeader(404)
		fmt.Fprintf(w, "404 page not found\n")
		return
	}
	p.ID = id
	a.UserDB[id] = p
	w.WriteHeader(200)
}

func (a *app) UpdateUser(w http.ResponseWriter, req *http.Request) {
	vars := mux.Vars(req)
	id, err := strconv.Atoi(vars["id"])
	if err != nil {
		w.WriteHeader(500)
		fmt.Fprintf(w, "Could not delete user: %v", err.Error())
		return
	}
	pNew := user{}
	buf := new(bytes.Buffer)
	buf.ReadFrom(req.Body)
	err = json.Unmarshal(buf.Bytes(), &pNew)
	if err != nil {
		w.WriteHeader(500)
		fmt.Fprintf(w, "500 Internal Server Error\n")
		return
	}
	_, ok := a.UserDB[id]
	if !ok {
		w.WriteHeader(404)
		fmt.Fprintf(w, "404 page not found\n")
		return
	}
	pNew.ID = id
	if pNew.Name == "" {
		pNew.Name = a.UserDB[id].Name
	}
	if pNew.Role == "" {
		pNew.Role = a.UserDB[id].Role
	}
	a.UserDB[id] = pNew
	w.WriteHeader(200)
}