add metrics for backend_retries_total

2017-04-18 08:22:06 +02:00 · 2017-04-18 08:22:06 +02:00 · e007bb7546
commit e007bb7546
parent 7874ffd506
8 changed files with 530 additions and 125 deletions
--- a/middlewares/metrics.go
+++ b/middlewares/metrics.go
@ -1,22 +1,29 @@
 package middlewares

 import (
-	"github.com/go-kit/kit/metrics"
 	"net/http"
 	"strconv"
 	"time"
+
+	"github.com/go-kit/kit/metrics"
 )

 // Metrics is an Interface that must be satisfied by any system that
-// wants to expose and monitor metrics
+// wants to expose and monitor Metrics.
 type Metrics interface {
 	getReqsCounter() metrics.Counter
-	getLatencyHistogram() metrics.Histogram
-	handler() http.Handler
+	getReqDurationHistogram() metrics.Histogram
+	RetryMetrics
+}
+
+// RetryMetrics must be satisfied by any system that wants to collect and
+// expose retry specific Metrics.
+type RetryMetrics interface {
+	getRetryCounter() metrics.Counter
 }

 // MetricsWrapper is a Negroni compatible Handler which relies on a
-// given Metrics implementation to expose and monitor Traefik metrics
+// given Metrics implementation to expose and monitor Traefik Metrics.
 type MetricsWrapper struct {
 	Impl Metrics
 }
@ -35,17 +42,25 @@ func (m *MetricsWrapper) ServeHTTP(rw http.ResponseWriter, r *http.Request, next
 	start := time.Now()
 	prw := &responseRecorder{rw, http.StatusOK}
 	next(prw, r)
-	labels := []string{"code", strconv.Itoa(prw.StatusCode()), "method", r.Method}
+	labels := []string{"code", strconv.Itoa(prw.statusCode), "method", r.Method}
 	m.Impl.getReqsCounter().With(labels...).Add(1)
-	m.Impl.getLatencyHistogram().Observe(float64(time.Since(start).Seconds()))
+	m.Impl.getReqDurationHistogram().Observe(float64(time.Since(start).Seconds()))
 }

-func (rw *responseRecorder) StatusCode() int {
-	return rw.statusCode
+// MetricsRetryListener is an implementation of the RetryListener interface to
+// record Metrics about retry attempts.
+type MetricsRetryListener struct {
+	retryMetrics RetryMetrics
 }

-// Handler is the chance for the Metrics implementation
-// to expose its metrics on a server endpoint
-func (m *MetricsWrapper) Handler() http.Handler {
-	return m.Impl.handler()
+// Retried tracks the retry in the Metrics implementation.
+func (m *MetricsRetryListener) Retried(attempt int) {
+	if m.retryMetrics != nil {
+		m.retryMetrics.getRetryCounter().Add(1)
+	}
+}
+
+// NewMetricsRetryListener instantiates a MetricsRetryListener with the given RetryMetrics.
+func NewMetricsRetryListener(retryMetrics RetryMetrics) RetryListener {
+	return &MetricsRetryListener{retryMetrics: retryMetrics}
 }
--- a/middlewares/metrics_test.go
+++ b/middlewares/metrics_test.go
@ -0,0 +1,48 @@
+package middlewares
+
+import (
+	"testing"
+
+	"github.com/go-kit/kit/metrics"
+)
+
+func TestMetricsRetryListener(t *testing.T) {
+	// nil implementation, nothing should fail
+	retryListener := NewMetricsRetryListener(nil)
+	retryListener.Retried(1)
+
+	retryMetrics := newCollectingMetrics()
+	retryListener = NewMetricsRetryListener(retryMetrics)
+	retryListener.Retried(1)
+	retryListener.Retried(2)
+
+	wantCounterValue := float64(2)
+	if retryMetrics.retryCounter.counterValue != wantCounterValue {
+		t.Errorf("got counter value of %d, want %d", retryMetrics.retryCounter.counterValue, wantCounterValue)
+	}
+}
+
+// collectingRetryMetrics is an implementation of the RetryMetrics interface that can be used inside tests to collect the times Add() was called.
+type collectingRetryMetrics struct {
+	retryCounter *collectingCounter
+}
+
+func newCollectingMetrics() collectingRetryMetrics {
+	return collectingRetryMetrics{retryCounter: &collectingCounter{}}
+}
+
+func (metrics collectingRetryMetrics) getRetryCounter() metrics.Counter {
+	return metrics.retryCounter
+}
+
+type collectingCounter struct {
+	counterValue float64
+}
+
+func (c *collectingCounter) With(labelValues ...string) metrics.Counter {
+	panic("collectingCounter.With not implemented!")
+}
+
+func (c *collectingCounter) Add(delta float64) {
+	c.counterValue += delta
+}
--- a/middlewares/prometheus.go
+++ b/middlewares/prometheus.go
@ -1,57 +1,64 @@
 package middlewares

 import (
+	"fmt"
+
 	"github.com/containous/traefik/types"
 	"github.com/go-kit/kit/metrics"
 	"github.com/go-kit/kit/metrics/prometheus"
 	stdprometheus "github.com/prometheus/client_golang/prometheus"
-	"github.com/prometheus/client_golang/prometheus/promhttp"
-	"net/http"
 )

 const (
-	reqsName    = "traefik_requests_total"
-	latencyName = "traefik_request_duration_seconds"
+	reqsTotalName    = "traefik_requests_total"
+	reqDurationName  = "traefik_request_duration_seconds"
+	retriesTotalName = "traefik_backend_retries_total"
 )

-// Prometheus is an Implementation for Metrics that exposes prometheus metrics for the latency
-// and the number of requests partitioned by status code and method.
+// Prometheus is an Implementation for Metrics that exposes the following Prometheus metrics:
+// - number of requests partitioned by status code and method
+// - request durations
+// - amount of retries happened
 type Prometheus struct {
-	reqsCounter      metrics.Counter
-	latencyHistogram metrics.Histogram
+	reqsCounter          metrics.Counter
+	reqDurationHistogram metrics.Histogram
+	retryCounter         metrics.Counter
 }

 func (p *Prometheus) getReqsCounter() metrics.Counter {
 	return p.reqsCounter
 }

-func (p *Prometheus) getLatencyHistogram() metrics.Histogram {
-	return p.latencyHistogram
+func (p *Prometheus) getReqDurationHistogram() metrics.Histogram {
+	return p.reqDurationHistogram
 }

-// NewPrometheus returns a new prometheus Metrics implementation.
-func NewPrometheus(name string, config *types.Prometheus) *Prometheus {
-	var m Prometheus
+func (p *Prometheus) getRetryCounter() metrics.Counter {
+	return p.retryCounter
+}
+
+// NewPrometheus returns a new Prometheus Metrics implementation.
+// With the returned collectors you have the possibility to clean up the internal Prometheus state by unsubscribing the collectors.
+// This is for example useful while testing the Prometheus implementation.
+// If any of the Prometheus Metrics can not be registered an error will be returned and the returned Metrics implementation will be nil.
+func NewPrometheus(name string, config *types.Prometheus) (*Prometheus, []stdprometheus.Collector, error) {
+	var prom Prometheus
+	var collectors []stdprometheus.Collector

 	cv := stdprometheus.NewCounterVec(
 		stdprometheus.CounterOpts{
-			Name:        reqsName,
+			Name:        reqsTotalName,
 			Help:        "How many HTTP requests processed, partitioned by status code and method.",
 			ConstLabels: stdprometheus.Labels{"service": name},
 		},
 		[]string{"code", "method"},
 	)
-
-	err := stdprometheus.Register(cv)
+	cv, err := registerCounterVec(cv)
 	if err != nil {
-		e, ok := err.(stdprometheus.AlreadyRegisteredError)
-		if !ok {
-			panic(err)
-		}
-		m.reqsCounter = prometheus.NewCounter(e.ExistingCollector.(*stdprometheus.CounterVec))
-	} else {
-		m.reqsCounter = prometheus.NewCounter(cv)
+		return nil, collectors, err
 	}
+	prom.reqsCounter = prometheus.NewCounter(cv)
+	collectors = append(collectors, cv)

 	var buckets []float64
 	if config.Buckets != nil {
@ -59,31 +66,64 @@ func NewPrometheus(name string, config *types.Prometheus) *Prometheus {
 	} else {
 		buckets = []float64{0.1, 0.3, 1.2, 5}
 	}
-
 	hv := stdprometheus.NewHistogramVec(
 		stdprometheus.HistogramOpts{
-			Name:        latencyName,
+			Name:        reqDurationName,
 			Help:        "How long it took to process the request.",
 			ConstLabels: stdprometheus.Labels{"service": name},
 			Buckets:     buckets,
 		},
 		[]string{},
 	)
+	hv, err = registerHistogramVec(hv)
+	if err != nil {
+		return nil, collectors, err
+	}
+	prom.reqDurationHistogram = prometheus.NewHistogram(hv)
+	collectors = append(collectors, hv)
+
+	cv = stdprometheus.NewCounterVec(
+		stdprometheus.CounterOpts{
+			Name:        retriesTotalName,
+			Help:        "How many request retries happened in total.",
+			ConstLabels: stdprometheus.Labels{"service": name},
+		},
+		[]string{},
+	)
+	cv, err = registerCounterVec(cv)
+	if err != nil {
+		return nil, collectors, err
+	}
+	prom.retryCounter = prometheus.NewCounter(cv)
+	collectors = append(collectors, cv)
+
+	return &prom, collectors, nil
+}
+
+func registerCounterVec(cv *stdprometheus.CounterVec) (*stdprometheus.CounterVec, error) {
+	err := stdprometheus.Register(cv)

-	err = stdprometheus.Register(hv)
 	if err != nil {
 		e, ok := err.(stdprometheus.AlreadyRegisteredError)
 		if !ok {
-			panic(err)
+			return nil, fmt.Errorf("error registering CounterVec: %s", e)
 		}
-		m.latencyHistogram = prometheus.NewHistogram(e.ExistingCollector.(*stdprometheus.HistogramVec))
-	} else {
-		m.latencyHistogram = prometheus.NewHistogram(hv)
+		cv = e.ExistingCollector.(*stdprometheus.CounterVec)
 	}

-	return &m
+	return cv, nil
 }

-func (p *Prometheus) handler() http.Handler {
-	return promhttp.Handler()
+func registerHistogramVec(hv *stdprometheus.HistogramVec) (*stdprometheus.HistogramVec, error) {
+	err := stdprometheus.Register(hv)
+
+	if err != nil {
+		e, ok := err.(stdprometheus.AlreadyRegisteredError)
+		if !ok {
+			return nil, fmt.Errorf("error registering HistogramVec: %s", e)
+		}
+		hv = e.ExistingCollector.(*stdprometheus.HistogramVec)
+	}
+
+	return hv, nil
 }
--- a/middlewares/prometheus_test.go
+++ b/middlewares/prometheus_test.go
@ -2,6 +2,8 @@ package middlewares

 import (
 	"fmt"
+	"io"
+	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@ -16,55 +18,37 @@ import (
 )

 func TestPrometheus(t *testing.T) {
-	metricsFamily, err := prometheus.DefaultGatherer.Gather()
+	defer resetPrometheusValues()
+
+	metricsFamilies, err := prometheus.DefaultGatherer.Gather()
 	if err != nil {
 		t.Fatalf("could not gather metrics family: %s", err)
 	}
-	initialMetricsFamilyCount := len(metricsFamily)
+	initialMetricsFamilyCount := len(metricsFamilies)

 	recorder := httptest.NewRecorder()

-	n := negroni.New()
-	metricsMiddlewareBackend := NewMetricsWrapper(NewPrometheus("test", &types.Prometheus{}))
-	n.Use(metricsMiddlewareBackend)
-	r := http.NewServeMux()
-	r.Handle("/metrics", promhttp.Handler())
-	r.HandleFunc(`/ok`, func(w http.ResponseWriter, r *http.Request) {
-		w.WriteHeader(http.StatusOK)
-		fmt.Fprintln(w, "ok")
-	})
-	n.UseHandler(r)
+	req1 := mustNewRequest("GET", "http://localhost:3000/ok", ioutil.NopCloser(nil))
+	req2 := mustNewRequest("GET", "http://localhost:3000/metrics", ioutil.NopCloser(nil))

-	req1, err := http.NewRequest("GET", "http://localhost:3000/ok", nil)
-	if err != nil {
-		t.Error(err)
-	}
-	req2, err := http.NewRequest("GET", "http://localhost:3000/metrics", nil)
-	if err != nil {
-		t.Error(err)
-	}
+	httpHandler := setupTestHTTPHandler()
+	httpHandler.ServeHTTP(recorder, req1)
+	httpHandler.ServeHTTP(recorder, req2)

-	n.ServeHTTP(recorder, req1)
-	n.ServeHTTP(recorder, req2)
 	body := recorder.Body.String()
-	if !strings.Contains(body, reqsName) {
-		t.Errorf("body does not contain request total entry '%s'", reqsName)
+	if !strings.Contains(body, reqsTotalName) {
+		t.Errorf("body does not contain request total entry '%s'", reqsTotalName)
 	}
-	if !strings.Contains(body, latencyName) {
-		t.Errorf("body does not contain request duration entry '%s'", latencyName)
+	if !strings.Contains(body, reqDurationName) {
+		t.Errorf("body does not contain request duration entry '%s'", reqDurationName)
+	}
+	if !strings.Contains(body, retriesTotalName) {
+		t.Errorf("body does not contain total retries entry '%s'", retriesTotalName)
 	}

-	// Register the same metrics again
-	metricsMiddlewareBackend = NewMetricsWrapper(NewPrometheus("test", &types.Prometheus{}))
-	n = negroni.New()
-	n.Use(metricsMiddlewareBackend)
-	n.UseHandler(r)
-
-	n.ServeHTTP(recorder, req2)
-
-	metricsFamily, err = prometheus.DefaultGatherer.Gather()
+	metricsFamilies, err = prometheus.DefaultGatherer.Gather()
 	if err != nil {
-		t.Fatalf("could not gather metrics family: %s", err)
+		t.Fatalf("could not gather metrics families: %s", err)
 	}

 	tests := []struct {
@ -73,7 +57,7 @@ func TestPrometheus(t *testing.T) {
 		assert func(*dto.MetricFamily)
 	}{
 		{
-			name: reqsName,
+			name: reqsTotalName,
 			labels: map[string]string{
 				"code":    "200",
 				"method":  "GET",
@ -81,29 +65,44 @@ func TestPrometheus(t *testing.T) {
 			},
 			assert: func(family *dto.MetricFamily) {
 				cv := uint(family.Metric[0].Counter.GetValue())
-				if cv != 3 {
-					t.Errorf("gathered metrics do not contain correct value for total requests, got %d", cv)
+				expectedCv := uint(2)
+				if cv != expectedCv {
+					t.Errorf("gathered metrics do not contain correct value for total requests, got %d expected %d", cv, expectedCv)
 				}
 			},
 		},
 		{
-			name: latencyName,
+			name: reqDurationName,
 			labels: map[string]string{
 				"service": "test",
 			},
 			assert: func(family *dto.MetricFamily) {
 				sc := family.Metric[0].Histogram.GetSampleCount()
-				if sc != 3 {
-					t.Errorf("gathered metrics do not contain correct sample count for request duration, got %d", sc)
+				expectedSc := uint64(2)
+				if sc != expectedSc {
+					t.Errorf("gathered metrics do not contain correct sample count for request duration, got %d expected %d", sc, expectedSc)
+				}
+			},
+		},
+		{
+			name: retriesTotalName,
+			labels: map[string]string{
+				"service": "test",
+			},
+			assert: func(family *dto.MetricFamily) {
+				cv := uint(family.Metric[0].Counter.GetValue())
+				expectedCv := uint(1)
+				if cv != expectedCv {
+					t.Errorf("gathered metrics do not contain correct value for total retries, got '%d' expected '%d'", cv, expectedCv)
 				}
 			},
 		},
 	}

-	assert.Equal(t, len(tests), len(metricsFamily)-initialMetricsFamilyCount, "gathered traefic metrics count does not match tests count")
+	assert.Equal(t, len(tests), len(metricsFamilies)-initialMetricsFamilyCount, "gathered traefic metrics count does not match tests count")

 	for _, test := range tests {
-		family := findMetricFamily(test.name, metricsFamily)
+		family := findMetricFamily(test.name, metricsFamilies)
 		if family == nil {
 			t.Errorf("gathered metrics do not contain '%s'", test.name)
 			continue
@ -120,6 +119,74 @@ func TestPrometheus(t *testing.T) {
 	}
 }

+func TestPrometheusRegisterMetricsMultipleTimes(t *testing.T) {
+	defer resetPrometheusValues()
+
+	recorder := httptest.NewRecorder()
+	req1 := mustNewRequest("GET", "http://localhost:3000/ok", ioutil.NopCloser(nil))
+
+	httpHandler := setupTestHTTPHandler()
+	httpHandler.ServeHTTP(recorder, req1)
+
+	httpHandler = setupTestHTTPHandler()
+	httpHandler.ServeHTTP(recorder, req1)
+
+	metricsFamilies, err := prometheus.DefaultGatherer.Gather()
+	if err != nil {
+		t.Fatalf("could not gather metrics families: %s", err)
+	}
+
+	reqsTotalFamily := findMetricFamily(reqsTotalName, metricsFamilies)
+	if reqsTotalFamily == nil {
+		t.Fatalf("gathered metrics do not contain '%s'", reqsTotalName)
+	}
+
+	cv := uint(reqsTotalFamily.Metric[0].Counter.GetValue())
+	expectedCv := uint(2)
+	if cv != expectedCv {
+		t.Errorf("wrong counter value when registering metrics multiple times, got '%d' expected '%d'", cv, expectedCv)
+	}
+}
+
+func setupTestHTTPHandler() http.Handler {
+	serveMux := http.NewServeMux()
+	serveMux.Handle("/metrics", promhttp.Handler())
+	serveMux.Handle("/ok", &networkFailingHTTPHandler{failAtCalls: []int{1}})
+
+	metrics, _ := newPrometheusMetrics()
+
+	n := negroni.New()
+	n.Use(NewMetricsWrapper(metrics))
+	n.UseHandler(NewRetry(2, serveMux, NewMetricsRetryListener(metrics)))
+
+	return n
+}
+
+// mustNewRequest is like http.NewRequest but panics if an error occurs.
+func mustNewRequest(method, urlStr string, body io.Reader) *http.Request {
+	req, err := http.NewRequest(method, urlStr, body)
+	if err != nil {
+		panic(fmt.Sprintf("NewRequest(%s, %s, %+v): %s", method, urlStr, body, err))
+	}
+	return req
+}
+
+func resetPrometheusValues() {
+	_, collectors := newPrometheusMetrics()
+
+	for _, collector := range collectors {
+		prometheus.Unregister(collector)
+	}
+}
+
+func newPrometheusMetrics() (*Prometheus, []prometheus.Collector) {
+	prom, collectors, err := NewPrometheus("test", &types.Prometheus{})
+	if err != nil {
+		panic(fmt.Sprintf("Error creating Prometheus Metrics: %s", err))
+	}
+	return prom, collectors
+}
+
 func findMetricFamily(name string, families []*dto.MetricFamily) *dto.MetricFamily {
 	for _, family := range families {
 		if family.GetName() == name {
--- a/middlewares/retry.go
+++ b/middlewares/retry.go
@ -11,21 +11,24 @@ import (
 	"github.com/vulcand/oxy/utils"
 )

+// Compile time validation responseRecorder implements http interfaces correctly.
 var (
-	_ Stateful = &ResponseRecorder{}
+	_ Stateful = &retryResponseRecorder{}
 )

 // Retry is a middleware that retries requests
 type Retry struct {
 	attempts int
 	next     http.Handler
+	listener RetryListener
 }

 // NewRetry returns a new Retry instance
-func NewRetry(attempts int, next http.Handler) *Retry {
+func NewRetry(attempts int, next http.Handler, listener RetryListener) *Retry {
 	return &Retry{
 		attempts: attempts,
 		next:     next,
+		listener: listener,
 	}
 }

@ -39,7 +42,7 @@ func (retry *Retry) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
 	}
 	attempts := 1
 	for {
-		recorder := NewRecorder()
+		recorder := newRetryResponseRecorder()
 		recorder.responseWriter = rw
 		retry.next.ServeHTTP(recorder, r)
 		if !isNetworkError(recorder.Code) || attempts >= retry.attempts {
@ -50,6 +53,7 @@ func (retry *Retry) ServeHTTP(rw http.ResponseWriter, r *http.Request) {
 		}
 		attempts++
 		log.Debugf("New attempt %d for request: %v", attempts, r.URL)
+		retry.listener.Retried(attempts)
 	}
 }

@ -57,9 +61,16 @@ func isNetworkError(status int) bool {
 	return status == http.StatusBadGateway || status == http.StatusGatewayTimeout
 }

-// ResponseRecorder is an implementation of http.ResponseWriter that
-// records its mutations for later inspection in tests.
-type ResponseRecorder struct {
+// RetryListener is used to inform about retry attempts.
+type RetryListener interface {
+	// Retried will be called when a retry happens, with the request attempt passed to it.
+	// For the first retry this will be attempt 2.
+	Retried(attempt int)
+}
+
+// retryResponseRecorder is an implementation of http.ResponseWriter that
+// records its mutations for later inspection.
+type retryResponseRecorder struct {
 	Code      int           // the HTTP response code from WriteHeader
 	HeaderMap http.Header   // the HTTP response headers
 	Body      *bytes.Buffer // if non-nil, the bytes.Buffer to append written data to
@ -68,9 +79,9 @@ type ResponseRecorder struct {
 	err            error
 }

-// NewRecorder returns an initialized ResponseRecorder.
-func NewRecorder() *ResponseRecorder {
-	return &ResponseRecorder{
+// newRetryResponseRecorder returns an initialized retryResponseRecorder.
+func newRetryResponseRecorder() *retryResponseRecorder {
+	return &retryResponseRecorder{
 		HeaderMap: make(http.Header),
 		Body:      new(bytes.Buffer),
 		Code:      200,
@ -78,7 +89,7 @@ func NewRecorder() *ResponseRecorder {
 }

 // Header returns the response headers.
-func (rw *ResponseRecorder) Header() http.Header {
+func (rw *retryResponseRecorder) Header() http.Header {
 	m := rw.HeaderMap
 	if m == nil {
 		m = make(http.Header)
@ -88,7 +99,7 @@ func (rw *ResponseRecorder) Header() http.Header {
 }

 // Write always succeeds and writes to rw.Body, if not nil.
-func (rw *ResponseRecorder) Write(buf []byte) (int, error) {
+func (rw *retryResponseRecorder) Write(buf []byte) (int, error) {
 	if rw.err != nil {
 		return 0, rw.err
 	}
@ -96,27 +107,27 @@ func (rw *ResponseRecorder) Write(buf []byte) (int, error) {
 }

 // WriteHeader sets rw.Code.
-func (rw *ResponseRecorder) WriteHeader(code int) {
+func (rw *retryResponseRecorder) WriteHeader(code int) {
 	rw.Code = code
 }

 // Hijack hijacks the connection
-func (rw *ResponseRecorder) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+func (rw *retryResponseRecorder) Hijack() (net.Conn, *bufio.ReadWriter, error) {
 	return rw.responseWriter.(http.Hijacker).Hijack()
 }

 // CloseNotify returns a channel that receives at most a
 // single value (true) when the client connection has gone
 // away.
-func (rw *ResponseRecorder) CloseNotify() <-chan bool {
+func (rw *retryResponseRecorder) CloseNotify() <-chan bool {
 	return rw.responseWriter.(http.CloseNotifier).CloseNotify()
 }

 // Flush sends any buffered data to the client.
-func (rw *ResponseRecorder) Flush() {
+func (rw *retryResponseRecorder) Flush() {
 	_, err := rw.responseWriter.Write(rw.Body.Bytes())
 	if err != nil {
-		log.Errorf("Error writing response in ResponseRecorder: %s", err)
+		log.Errorf("Error writing response in retryResponseRecorder: %s", err)
 		rw.err = err
 	}
 	rw.Body.Reset()
--- a/middlewares/retry_test.go
+++ b/middlewares/retry_test.go
@ -0,0 +1,91 @@
+package middlewares
+
+import (
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestRetry(t *testing.T) {
+	testCases := []struct {
+		failAtCalls    []int
+		attempts       int
+		responseStatus int
+		listener       *countingRetryListener
+		retriedCount   int
+	}{
+		{
+			failAtCalls:    []int{1, 2},
+			attempts:       3,
+			responseStatus: http.StatusOK,
+			listener:       &countingRetryListener{},
+			retriedCount:   2,
+		},
+		{
+			failAtCalls:    []int{1, 2},
+			attempts:       2,
+			responseStatus: http.StatusBadGateway,
+			listener:       &countingRetryListener{},
+			retriedCount:   1,
+		},
+	}
+
+	for _, tc := range testCases {
+		// bind tc locally
+		tc := tc
+		tcName := fmt.Sprintf("FailAtCalls(%v) RetryAttempts(%v)", tc.failAtCalls, tc.attempts)
+
+		t.Run(tcName, func(t *testing.T) {
+			t.Parallel()
+
+			var httpHandler http.Handler
+			httpHandler = &networkFailingHTTPHandler{failAtCalls: tc.failAtCalls}
+			httpHandler = NewRetry(tc.attempts, httpHandler, tc.listener)
+
+			recorder := httptest.NewRecorder()
+			req, err := http.NewRequest("GET", "http://localhost:3000/ok", ioutil.NopCloser(nil))
+			if err != nil {
+				t.Fatalf("could not create request: %+v", err)
+			}
+
+			httpHandler.ServeHTTP(recorder, req)
+
+			if tc.responseStatus != recorder.Code {
+				t.Errorf("wrong status code %d, want %d", recorder.Code, tc.responseStatus)
+			}
+			if tc.retriedCount != tc.listener.timesCalled {
+				t.Errorf("RetryListener called %d times, want %d times", tc.listener.timesCalled, tc.retriedCount)
+			}
+		})
+	}
+}
+
+// networkFailingHTTPHandler is an http.Handler implementation you can use to test retries.
+type networkFailingHTTPHandler struct {
+	failAtCalls []int
+	callNumber  int
+}
+
+func (handler *networkFailingHTTPHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	handler.callNumber++
+
+	for _, failAtCall := range handler.failAtCalls {
+		if handler.callNumber == failAtCall {
+			w.WriteHeader(http.StatusBadGateway)
+			return
+		}
+	}
+
+	w.WriteHeader(http.StatusOK)
+}
+
+// countingRetryListener is a RetryListener implementation to count the times the Retried fn is called.
+type countingRetryListener struct {
+	timesCalled int
+}
+
+func (l *countingRetryListener) Retried(attempt int) {
+	l.timesCalled++
+}
--- a/server/server.go
+++ b/server/server.go
@ -192,11 +192,9 @@ func (server *Server) startHTTPServers() {
 		if server.accessLoggerMiddleware != nil {
 			serverMiddlewares = append(serverMiddlewares, server.accessLoggerMiddleware)
 		}
-		if server.globalConfiguration.Web != nil && server.globalConfiguration.Web.Metrics != nil {
-			if server.globalConfiguration.Web.Metrics.Prometheus != nil {
-				metricsMiddleware := middlewares.NewMetricsWrapper(middlewares.NewPrometheus(newServerEntryPointName, server.globalConfiguration.Web.Metrics.Prometheus))
-				serverMiddlewares = append(serverMiddlewares, metricsMiddleware)
-			}
+		metrics := newMetrics(server.globalConfiguration, newServerEntryPointName)
+		if metrics != nil {
+			serverMiddlewares = append(serverMiddlewares, middlewares.NewMetricsWrapper(metrics))
 		}
 		if server.globalConfiguration.Web != nil && server.globalConfiguration.Web.Statistics != nil {
 			statsRecorder = middlewares.NewStatsRecorder(server.globalConfiguration.Web.Statistics.RecentErrors)
@ -726,21 +724,15 @@ func (server *Server) loadConfig(configurations configs, globalConfiguration Glo
 							continue frontend
 						}
 					}
-					// retry ?
-					if globalConfiguration.Retry != nil {
-						retries := len(configuration.Backends[frontend.Backend].Servers)
-						if globalConfiguration.Retry.Attempts > 0 {
-							retries = globalConfiguration.Retry.Attempts
-						}
-						lb = middlewares.NewRetry(retries, lb)
-						log.Debugf("Creating retries max attempts %d", retries)
-					}

-					if server.globalConfiguration.Web != nil && server.globalConfiguration.Web.Metrics != nil {
-						if server.globalConfiguration.Web.Metrics.Prometheus != nil {
-							metricsMiddlewareBackend := middlewares.NewMetricsWrapper(middlewares.NewPrometheus(frontend.Backend, server.globalConfiguration.Web.Metrics.Prometheus))
-							negroni.Use(metricsMiddlewareBackend)
-						}
+					metrics := newMetrics(server.globalConfiguration, frontend.Backend)
+
+					if globalConfiguration.Retry != nil {
+						retryListener := middlewares.NewMetricsRetryListener(metrics)
+						lb = registerRetryMiddleware(lb, globalConfiguration, configuration, frontend.Backend, retryListener)
+					}
+					if metrics != nil {
+						negroni.Use(middlewares.NewMetricsWrapper(metrics))
 					}

 					ipWhitelistMiddleware, err := configureIPWhitelistMiddleware(frontend.WhitelistSourceRange)
@ -961,3 +953,37 @@ func (*Server) configureBackends(backends map[string]*types.Backend) {
 		}
 	}
 }
+
+// newMetrics instantiates the proper Metrics implementation, depending on the global configuration.
+// Note that given there is no metrics instrumentation configured, it will return nil.
+func newMetrics(globalConfig GlobalConfiguration, name string) middlewares.Metrics {
+	metricsEnabled := globalConfig.Web != nil && globalConfig.Web.Metrics != nil
+	if metricsEnabled && globalConfig.Web.Metrics.Prometheus != nil {
+		metrics, _, err := middlewares.NewPrometheus(name, globalConfig.Web.Metrics.Prometheus)
+		if err != nil {
+			log.Errorf("Error creating Prometheus Metrics implementation: %s", err)
+			return nil
+		}
+		return metrics
+	}
+
+	return nil
+}
+
+func registerRetryMiddleware(
+	httpHandler http.Handler,
+	globalConfig GlobalConfiguration,
+	config *types.Configuration,
+	backend string,
+	listener middlewares.RetryListener,
+) http.Handler {
+	retries := len(config.Backends[backend].Servers)
+	if globalConfig.Retry.Attempts > 0 {
+		retries = globalConfig.Retry.Attempts
+	}
+
+	httpHandler = middlewares.NewRetry(retries, httpHandler, listener)
+	log.Debugf("Creating retries max attempts %d", retries)
+
+	return httpHandler
+}
--- a/server/server_test.go
+++ b/server/server_test.go
@ -11,6 +11,7 @@ import (
 	"github.com/containous/flaeg"
 	"github.com/containous/mux"
 	"github.com/containous/traefik/healthcheck"
+	"github.com/containous/traefik/middlewares"
 	"github.com/containous/traefik/testhelpers"
 	"github.com/containous/traefik/types"
 	"github.com/davecgh/go-spew/spew"
@ -409,3 +410,109 @@ func TestConfigureBackends(t *testing.T) {
 		})
 	}
 }
+
+func TestNewMetrics(t *testing.T) {
+	testCases := []struct {
+		desc         string
+		globalConfig GlobalConfiguration
+	}{
+		{
+			desc:         "metrics disabled",
+			globalConfig: GlobalConfiguration{},
+		},
+		{
+			desc: "prometheus metrics",
+			globalConfig: GlobalConfiguration{
+				Web: &WebProvider{
+					Metrics: &types.Metrics{
+						Prometheus: &types.Prometheus{
+							Buckets: types.Buckets{0.1, 0.3, 1.2, 5.0},
+						},
+					},
+				},
+			},
+		},
+	}
+
+	for _, tc := range testCases {
+		tc := tc
+		t.Run(tc.desc, func(t *testing.T) {
+			t.Parallel()
+
+			metricsImpl := newMetrics(tc.globalConfig, "test1")
+			if metricsImpl != nil {
+				if _, ok := metricsImpl.(*middlewares.Prometheus); !ok {
+					t.Errorf("invalid metricsImpl type, got %T want %T", metricsImpl, &middlewares.Prometheus{})
+				}
+			}
+		})
+	}
+}
+
+func TestRegisterRetryMiddleware(t *testing.T) {
+	testCases := []struct {
+		name            string
+		globalConfig    GlobalConfiguration
+		countServers    int
+		expectedRetries int
+	}{
+		{
+			name: "configured retry attempts",
+			globalConfig: GlobalConfiguration{
+				Retry: &Retry{
+					Attempts: 3,
+				},
+			},
+			expectedRetries: 3,
+		},
+		{
+			name: "retry attempts defaults to server amount",
+			globalConfig: GlobalConfiguration{
+				Retry: &Retry{},
+			},
+			expectedRetries: 2,
+		},
+	}
+
+	for _, tc := range testCases {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			var retryListener middlewares.RetryListener
+			httpHandler := okHTTPHandler{}
+			dynamicConfig := &types.Configuration{
+				Backends: map[string]*types.Backend{
+					"backend": {
+						Servers: map[string]types.Server{
+							"server": {
+								URL: "http://localhost",
+							},
+							"server2": {
+								URL: "http://localhost",
+							},
+						},
+					},
+				},
+			}
+
+			httpHandlerWithRetry := registerRetryMiddleware(httpHandler, tc.globalConfig, dynamicConfig, "backend", retryListener)
+
+			retry, ok := httpHandlerWithRetry.(*middlewares.Retry)
+			if !ok {
+				t.Fatalf("httpHandler was not decorated with retry httpHandler, got %#v", httpHandlerWithRetry)
+			}
+
+			expectedRetry := middlewares.NewRetry(tc.expectedRetries, httpHandler, retryListener)
+			if !reflect.DeepEqual(retry, expectedRetry) {
+				t.Errorf("retry httpHandler was not instantiated correctly, got %#v want %#v", retry, expectedRetry)
+			}
+		})
+	}
+}
+
+type okHTTPHandler struct{}
+
+func (okHTTPHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	w.WriteHeader(http.StatusOK)
+}