traefik/pkg/middlewares/retry/retry.go
2021-03-04 20:08:03 +01:00

248 lines
6.7 KiB
Go

package retry
import (
"bufio"
"context"
"fmt"
"io"
"math"
"net"
"net/http"
"net/http/httptrace"
"time"
"github.com/cenkalti/backoff/v4"
"github.com/opentracing/opentracing-go/ext"
"github.com/traefik/traefik/v2/pkg/config/dynamic"
"github.com/traefik/traefik/v2/pkg/log"
"github.com/traefik/traefik/v2/pkg/middlewares"
"github.com/traefik/traefik/v2/pkg/tracing"
)
// Compile time validation that the response writer implements http interfaces correctly.
var _ middlewares.Stateful = &responseWriterWithCloseNotify{}
const (
typeName = "Retry"
)
// Listener is used to inform about retry attempts.
type Listener 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(req *http.Request, attempt int)
}
// Listeners is a convenience type to construct a list of Listener and notify
// each of them about a retry attempt.
type Listeners []Listener
// nexter returns the duration to wait before retrying the operation.
type nexter interface {
NextBackOff() time.Duration
}
// retry is a middleware that retries requests.
type retry struct {
attempts int
initialInterval time.Duration
next http.Handler
listener Listener
name string
}
// New returns a new retry middleware.
func New(ctx context.Context, next http.Handler, config dynamic.Retry, listener Listener, name string) (http.Handler, error) {
log.FromContext(middlewares.GetLoggerCtx(ctx, name, typeName)).Debug("Creating middleware")
if config.Attempts <= 0 {
return nil, fmt.Errorf("incorrect (or empty) value for attempt (%d)", config.Attempts)
}
return &retry{
attempts: config.Attempts,
initialInterval: time.Duration(config.InitialInterval),
next: next,
listener: listener,
name: name,
}, nil
}
func (r *retry) GetTracingInformation() (string, ext.SpanKindEnum) {
return r.name, tracing.SpanKindNoneEnum
}
func (r *retry) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
// if we might make multiple attempts, swap the body for an io.NopCloser
// cf https://github.com/traefik/traefik/issues/1008
if r.attempts > 1 {
body := req.Body
defer body.Close()
req.Body = io.NopCloser(body)
}
attempts := 1
backOff := r.newBackOff()
currentInterval := 0 * time.Millisecond
for {
select {
case <-time.After(currentInterval):
shouldRetry := attempts < r.attempts
retryResponseWriter := newResponseWriter(rw, shouldRetry)
// Disable retries when the backend already received request data
trace := &httptrace.ClientTrace{
WroteHeaders: func() {
retryResponseWriter.DisableRetries()
},
WroteRequest: func(httptrace.WroteRequestInfo) {
retryResponseWriter.DisableRetries()
},
}
newCtx := httptrace.WithClientTrace(req.Context(), trace)
r.next.ServeHTTP(retryResponseWriter, req.WithContext(newCtx))
if !retryResponseWriter.ShouldRetry() {
return
}
currentInterval = backOff.NextBackOff()
attempts++
log.FromContext(middlewares.GetLoggerCtx(req.Context(), r.name, typeName)).
Debugf("New attempt %d for request: %v", attempts, req.URL)
r.listener.Retried(req, attempts)
case <-req.Context().Done():
return
}
}
}
func (r *retry) newBackOff() nexter {
if r.attempts < 2 || r.initialInterval <= 0 {
return &backoff.ZeroBackOff{}
}
b := backoff.NewExponentialBackOff()
b.InitialInterval = r.initialInterval
// calculate the multiplier for the given number of attempts
// so that applying the multiplier for the given number of attempts will not exceed 2 times the initial interval
// it allows to control the progression along the attempts
b.Multiplier = math.Pow(2, 1/float64(r.attempts-1))
// according to docs, b.Reset() must be called before using
b.Reset()
return b
}
// Retried exists to implement the Listener interface. It calls Retried on each of its slice entries.
func (l Listeners) Retried(req *http.Request, attempt int) {
for _, listener := range l {
listener.Retried(req, attempt)
}
}
type responseWriter interface {
http.ResponseWriter
http.Flusher
ShouldRetry() bool
DisableRetries()
}
func newResponseWriter(rw http.ResponseWriter, shouldRetry bool) responseWriter {
responseWriter := &responseWriterWithoutCloseNotify{
responseWriter: rw,
headers: make(http.Header),
shouldRetry: shouldRetry,
}
if _, ok := rw.(http.CloseNotifier); ok {
return &responseWriterWithCloseNotify{
responseWriterWithoutCloseNotify: responseWriter,
}
}
return responseWriter
}
type responseWriterWithoutCloseNotify struct {
responseWriter http.ResponseWriter
headers http.Header
shouldRetry bool
written bool
}
func (r *responseWriterWithoutCloseNotify) ShouldRetry() bool {
return r.shouldRetry
}
func (r *responseWriterWithoutCloseNotify) DisableRetries() {
r.shouldRetry = false
}
func (r *responseWriterWithoutCloseNotify) Header() http.Header {
if r.written {
return r.responseWriter.Header()
}
return r.headers
}
func (r *responseWriterWithoutCloseNotify) Write(buf []byte) (int, error) {
if r.ShouldRetry() {
return len(buf), nil
}
return r.responseWriter.Write(buf)
}
func (r *responseWriterWithoutCloseNotify) WriteHeader(code int) {
if r.ShouldRetry() && code == http.StatusServiceUnavailable {
// We get a 503 HTTP Status Code when there is no backend server in the pool
// to which the request could be sent. Also, note that r.ShouldRetry()
// will never return true in case there was a connection established to
// the backend server and so we can be sure that the 503 was produced
// inside Traefik already and we don't have to retry in this cases.
r.DisableRetries()
}
if r.ShouldRetry() {
return
}
// In that case retry case is set to false which means we at least managed
// to write headers to the backend : we are not going to perform any further retry.
// So it is now safe to alter current response headers with headers collected during
// the latest try before writing headers to client.
headers := r.responseWriter.Header()
for header, value := range r.headers {
headers[header] = value
}
r.responseWriter.WriteHeader(code)
r.written = true
}
func (r *responseWriterWithoutCloseNotify) Hijack() (net.Conn, *bufio.ReadWriter, error) {
hijacker, ok := r.responseWriter.(http.Hijacker)
if !ok {
return nil, nil, fmt.Errorf("%T is not a http.Hijacker", r.responseWriter)
}
return hijacker.Hijack()
}
func (r *responseWriterWithoutCloseNotify) Flush() {
if flusher, ok := r.responseWriter.(http.Flusher); ok {
flusher.Flush()
}
}
type responseWriterWithCloseNotify struct {
*responseWriterWithoutCloseNotify
}
func (r *responseWriterWithCloseNotify) CloseNotify() <-chan bool {
return r.responseWriter.(http.CloseNotifier).CloseNotify()
}