decorator.go

package main

import (
	"log"
	"time"
)

// User is a user in the application.
type User struct {
	ID   int
	Name string
}

// UsersRepository an abstraction for a repository of users.
type UsersRepository interface {
	// GetUserByID returns a user by its ID.
	GetUserByID(id int) (User, error)
}

// SimpleUsersRepository is a simple implementation of UsersRepository.
type SimpleUsersRepository struct{}

// GetUserByID returns a user by its ID.
// of course, this is a dummy implementation.
func (s SimpleUsersRepository) GetUserByID(id int) (User, error) {
	return User{ID: id, Name: "John"}, nil
}

// InstrumentedUsersRepository is a wrapper for UsersRepository that adds instrumentation.
type InstrumentedUsersRepository struct {
	repo UsersRepository
}

// GetUserByID calls the underlying repository while adding basic instrumentation.
func (i InstrumentedUsersRepository) GetUserByID(id int) (User, error) {
	start := time.Now()

	// Here we can add any logic we want, for example, logging, metrics, etc.
	// In this example we are just logging the call, but we could start a new span using the open-telemetry, NewRelic, DataDog etc.
	log.Printf("start calling GetUserByID with id %d", id)
	user, err := i.repo.GetUserByID(id)
	log.Printf("finished calling GetUserByID with id %d, took: %d", id, time.Since(start))

	return user, err
}

func main() {
	var repo UsersRepository

	// Create a new instance of the simple repository.
	repo = SimpleUsersRepository{}

	// Wrap the simple repository with the instrumented one.
	repo = InstrumentedUsersRepository{repo: repo}

	// Call the instrumented repository.
	user, err := repo.GetUserByID(1)
	log.Println(user, err)
}