An example demonstrating basic service registry implemented in Go

service-registry-example.go

// Go play example: https://play.golang.org/p/5eoIRYy5Bn
package main

import (
	"fmt"
	"reflect"
)

type Policy interface {
  IsAllowed() bool
}

type YesPolicy struct {}

func (p YesPolicy) IsAllowed() bool {
	fmt.Printf("YesPolicy!\n")
	return true
}

type NoPolicy struct {}

func (p NoPolicy) IsAllowed() bool {
	fmt.Printf("NoPolicy...\n")
	return false
}

type Authenticator struct {
	AuthPolicy Policy
}

func (a Authenticator) Authenticate() {
	p := a.AuthPolicy
	pt := reflect.TypeOf(p)
	var ptt reflect.Type
	if pt.Kind() == reflect.Ptr {
		ptt = pt
		pt = pt.Elem()
	}
	fmt.Printf(" Authenticate: policy object=%-10p\n", p)
	fmt.Printf("             : policy   type=%20s [%p]\n", pt, pt)
if ptt != nil {
	fmt.Printf("             : policy  *type=%20s [%p]\n", ptt, ptt)
}
}

func main() {
	fmt.Printf("Hard-coded use of policies:\n")
	Authenticator{&YesPolicy{}}.Authenticate()
	Authenticator{&NoPolicy{}}.Authenticate()

	fmt.Printf("\n======\n\n")

	policyType := reflect.TypeOf((*Policy)(nil)).Elem()
	fmt.Printf("Policy interface type: %s [%p]\n\n", policyType, policyType)

	serviceRegistry := map[reflect.Type]interface{}{
		policyType: &YesPolicy{},
	}
	fmt.Printf("Service Registry: %v\n\n", serviceRegistry)

	fmt.Printf("Use of policies via service regsitry:\n")
	Authenticator{serviceRegistry[policyType].(Policy)}.Authenticate()
}

service-registry-output.txt

Hard-coded use of policies:
 Authenticate: policy object=0x196adc  
             : policy   type=      main.YesPolicy [0x1026a0]
             : policy  *type=     *main.YesPolicy [0x100b60]
 Authenticate: policy object=0x196adc  
             : policy   type=       main.NoPolicy [0x102640]
             : policy  *type=      *main.NoPolicy [0x100b00]

======

Policy interface type: main.Policy [0x101520]

Service Registry: map[main.Policy:0x196adc]

Use of policies via service regsitry:
 Authenticate: policy object=0x196adc  
             : policy   type=      main.YesPolicy [0x1026a0]
             : policy  *type=     *main.YesPolicy [0x100b60]