Wrapper

main.go

import (
  "net/http"
  "time"
)

func main() {
  httpClient := &http.Client{Timeout: time.Duration(3) * time.Second}
  cacheService := NewCache()
  service := NewService(httpClient, cacheService)
  
  // call service with wrapper attached
  stock, err := service.GetProductStock(1)
}

service.go

import "net/http"

type Service struct {
  httpClient *http.Client
}

func (service *Service) GetProductStock(id int) (int, error) {
  // Requesting Stock from Product service
}

// --- WRAPPER --- //
type CacheItf interface {
  Get(key string) (string, error)
  Set(key string, value string) error
}

// Includes any services that are used in wrapper executions
// without changing the original structure
type ServiceWrapper struct {
  service *Service
  cache CacheItf
}

func NewService(httpClient *http.Client, cache CacheItf) *ServiceWrapper {
  service := &Service{ httpClient }
  return &ServiceWrapper {
    service: service,
    cache: cache,
  }
}

// Implement any instructions like caching, validation, authorization or logging in here
// without disrupting the original flow or structure.
func (wrapper *ServiceWrapper) GetProductStock(id int) (int, error) {
  // Do Pre-function executions
  defer func () {
    // Do Post-function executions
  } ()
  
  return wrapper.service.GetProductStock(id)
}