service.go

package addsvc

// This file contains the Service definition, and a basic service
// implementation. It also includes service middlewares.

import (
	"errors"
	"time"

	"golang.org/x/net/context"

	"github.com/go-kit/kit/log"
	"github.com/go-kit/kit/metrics"
)

// Service describes a service that adds things together.
type Service interface {
	Sum(ctx context.Context, a, b int) (int, error)
	Concat(ctx context.Context, a, b string) (string, error)
}

// Business-domain errors like these may be served in two ways: returned
// directly by endpoints, or bundled into the response struct. Both methods can
// be made to work, but errors returned directly by endpoints are counted by
// middlewares that check errors, like circuit breakers.
//
// If you don't want that behavior -- and you probably don't -- then it's better
// to bundle errors into the response struct.

var (
	// ErrTwoZeroes is an arbitrary business rule for the Add method.
	ErrTwoZeroes = errors.New("can't sum two zeroes")

	// ErrIntOverflow protects the Add method. We've decided that this error
	// indicates a misbehaving service and should count against e.g. circuit
	// breakers. So, we return it directly in endpoints, to illustrate the
	// difference. In a real service, this probably wouldn't be the case.
	ErrIntOverflow = errors.New("integer overflow")

	// ErrMaxSizeExceeded protects the Concat method.
	ErrMaxSizeExceeded = errors.New("result exceeds maximum size")
)

// These annoying helper functions are required to translate Go error types to
// and from strings, which is the type we use in our IDLs to represent errors.
// There is special casing to treat empty strings as nil errors.

func str2err(s string) error {
	if s == "" {
		return nil
	}
	return errors.New(s)
}

func err2str(err error) string {
	if err == nil {
		return ""
	}
	return err.Error()
}

// NewBasicService returns a naïve, stateless implementation of Service.
func NewBasicService() Service {
	return basicService{}
}

type basicService struct{}

const (
	intMax = 1<<31 - 1
	intMin = -(intMax + 1)
	maxLen = 102400
)

// Sum implements Service.
func (s basicService) Sum(_ context.Context, a, b int) (int, error) {
	if a == 0 && b == 0 {
		return 0, ErrTwoZeroes
	}
	if (b > 0 && a > (intMax-b)) || (b < 0 && a < (intMin-b)) {
		return 0, ErrIntOverflow
	}
	return a + b, nil
}

// Concat implements Service.
func (s basicService) Concat(_ context.Context, a, b string) (string, error) {
	if len(a)+len(b) > maxLen {
		return "", ErrMaxSizeExceeded
	}
	return a + b, nil
}

// Middleware describes a service (as opposed to endpoint) middleware.
type Middleware func(Service) Service

// ServiceLoggingMiddleware returns a service middleware that logs the
// parameters and result of each method invocation.
func ServiceLoggingMiddleware(logger log.Logger) Middleware {
	return func(next Service) Service {
		return serviceLoggingMiddleware{
			logger: logger,
			next:   next,
		}
	}
}

type serviceLoggingMiddleware struct {
	logger log.Logger
	next   Service
}

func (mw serviceLoggingMiddleware) Sum(ctx context.Context, a, b int) (v int, err error) {
	defer func(begin time.Time) {
		mw.logger.Log(
			"method", "Sum",
			"a", a, "b", b, "result", v, "error", err,
			"took", time.Since(begin),
		)
	}(time.Now())
	return mw.next.Sum(ctx, a, b)
}

func (mw serviceLoggingMiddleware) Concat(ctx context.Context, a, b string) (v string, err error) {
	defer func(begin time.Time) {
		mw.logger.Log(
			"method", "Concat",
			"a", a, "b", b, "result", v, "error", err,
			"took", time.Since(begin),
		)
	}(time.Now())
	return mw.next.Concat(ctx, a, b)
}

// ServiceInstrumentingMiddleware returns a service middleware that instruments
// the number of integers summed and characters concatenated over the lifetime of
// the service.
func ServiceInstrumentingMiddleware(ints, chars metrics.Counter) Middleware {
	return func(next Service) Service {
		return serviceInstrumentingMiddleware{
			ints:  ints,
			chars: chars,
			next:  next,
		}
	}
}

type serviceInstrumentingMiddleware struct {
	ints  metrics.Counter
	chars metrics.Counter
	next  Service
}

func (mw serviceInstrumentingMiddleware) Sum(ctx context.Context, a, b int) (int, error) {
	v, err := mw.next.Sum(ctx, a, b)
	mw.ints.Add(float64(v))
	return v, err
}

func (mw serviceInstrumentingMiddleware) Concat(ctx context.Context, a, b string) (string, error) {
	v, err := mw.next.Concat(ctx, a, b)
	mw.chars.Add(float64(len(v)))
	return v, err
}