app/vmauth: automatically retry failing GET requests on the remaining backends

app/vmauth/main.go

@@ -107,11 +107,6 @@ func requestHandler(w http.ResponseWriter, r *http.Request) bool {
 		return true
 	}
 	ui.requests.Inc()
-	targetURL, headers, err := createTargetURL(ui, r.URL)
-	if err != nil {
-		httpserver.Errorf(w, r, "cannot determine targetURL: %s", err)
-		return true
-	}
 
 	// Limit the concurrency of requests to backends
 	concurrencyLimitOnce.Do(concurrencyLimitInit)
@@ -128,13 +123,34 @@ func requestHandler(w http.ResponseWriter, r *http.Request) bool {
 		handleConcurrencyLimitError(w, r, err)
 		return true
 	}
-	processRequest(w, r, targetURL, headers)
+	processRequest(w, r, ui)
 	ui.endConcurrencyLimit()
 	<-concurrencyLimitCh
 	return true
 }
 
-func processRequest(w http.ResponseWriter, r *http.Request, targetURL *url.URL, headers []Header) {
+func processRequest(w http.ResponseWriter, r *http.Request, ui *UserInfo) {
+	u := normalizeURL(r.URL)
+	up, headers, err := ui.getURLPrefix(u)
+	if err != nil {
+		httpserver.Errorf(w, r, "cannot determine targetURL: %s", err)
+		return
+	}
+	maxAttempts := up.getBackendsCount()
+	for i := 0; i < maxAttempts; i++ {
+		targetURL := up.mergeURLs(u)
+		if tryProcessingRequest(w, r, targetURL, headers) {
+			return
+		}
+	}
+	err = &httpserver.ErrorWithStatusCode{
+		Err:        fmt.Errorf("all the backends for the user %q are unavailable", ui.name()),
+		StatusCode: http.StatusServiceUnavailable,
+	}
+	httpserver.Errorf(w, r, "%s", err)
+}
+
+func tryProcessingRequest(w http.ResponseWriter, r *http.Request, targetURL *url.URL, headers []Header) bool {
 	// This code has been copied from net/http/httputil/reverseproxy.go
 	req := sanitizeRequestHeaders(r)
 	req.URL = targetURL
@@ -144,12 +160,20 @@ func processRequest(w http.ResponseWriter, r *http.Request, targetURL *url.URL,
 	transportOnce.Do(transportInit)
 	res, err := transport.RoundTrip(req)
 	if err != nil {
-		err = &httpserver.ErrorWithStatusCode{
+		remoteAddr := httpserver.GetQuotedRemoteAddr(r)
-			Err:        fmt.Errorf("error when proxying the request to %q: %s", targetURL, err),
+		requestURI := httpserver.GetRequestURI(r)
-			StatusCode: http.StatusBadGateway,
+		if r.Method == "POST" || r.Method == "PUT" {
+			// It is impossible to retry POST and PUT requests,
+			// since we already proxied the request body to the backend.
+			err = &httpserver.ErrorWithStatusCode{
+				Err:        fmt.Errorf("cannot proxy the request to %q: %w", targetURL, err),
+				StatusCode: http.StatusServiceUnavailable,
+			}
+			httpserver.Errorf(w, r, "%s", err)
+			return true
 		}
-		httpserver.Errorf(w, r, "%s", err)
+		logger.Warnf("remoteAddr: %s; requestURI: %s; error when proxying the request to %q: %s", remoteAddr, requestURI, targetURL, err)
-		return
+		return false
 	}
 	removeHopHeaders(res.Header)
 	copyHeader(w.Header(), res.Header)
@@ -164,8 +188,9 @@ func processRequest(w http.ResponseWriter, r *http.Request, targetURL *url.URL,
 		remoteAddr := httpserver.GetQuotedRemoteAddr(r)
 		requestURI := httpserver.GetRequestURI(r)
 		logger.Warnf("remoteAddr: %s; requestURI: %s; error when proxying response body from %s: %s", remoteAddr, requestURI, targetURL, err)
-		return
+		return true
 	}
+	return true
 }
 
 var copyBufPool bytesutil.ByteBufferPool

app/vmauth/target_url.go

@@ -12,6 +12,10 @@ func (up *URLPrefix) mergeURLs(requestURI *url.URL) *url.URL {
 	return mergeURLs(pu, requestURI)
 }
 
+func (up *URLPrefix) getBackendsCount() int {
+	return len(up.urls)
+}
+
 func mergeURLs(uiURL, requestURI *url.URL) *url.URL {
 	targetURL := *uiURL
 	targetURL.Path += requestURI.Path
@@ -35,7 +39,22 @@ func mergeURLs(uiURL, requestURI *url.URL) *url.URL {
 	return &targetURL
 }
 
-func createTargetURL(ui *UserInfo, uOrig *url.URL) (*url.URL, []Header, error) {
+func (ui *UserInfo) getURLPrefix(u *url.URL) (*URLPrefix, []Header, error) {
+	for _, e := range ui.URLMaps {
+		for _, sp := range e.SrcPaths {
+			if sp.match(u.Path) {
+				return e.URLPrefix, e.Headers, nil
+			}
+		}
+	}
+	if ui.URLPrefix != nil {
+		return ui.URLPrefix, ui.Headers, nil
+	}
+	missingRouteRequests.Inc()
+	return nil, nil, fmt.Errorf("missing route for %q", u.String())
+}
+
+func normalizeURL(uOrig *url.URL) *url.URL {
 	u := *uOrig
 	// Prevent from attacks with using `..` in r.URL.Path
 	u.Path = path.Clean(u.Path)
@@ -52,16 +71,5 @@ func createTargetURL(ui *UserInfo, uOrig *url.URL) (*url.URL, []Header, error) {
 		// See https://github.com/VictoriaMetrics/VictoriaMetrics/pull/1554
 		u.Path = ""
 	}
-	for _, e := range ui.URLMaps {
+	return &u
-		for _, sp := range e.SrcPaths {
-			if sp.match(u.Path) {
-				return e.URLPrefix.mergeURLs(&u), e.Headers, nil
-			}
-		}
-	}
-	if ui.URLPrefix != nil {
-		return ui.URLPrefix.mergeURLs(&u), ui.Headers, nil
-	}
-	missingRouteRequests.Inc()
-	return nil, nil, fmt.Errorf("missing route for %q", u.String())
 }

app/vmauth/target_url_test.go

@@ -13,10 +13,12 @@ func TestCreateTargetURLSuccess(t *testing.T) {
 		if err != nil {
 			t.Fatalf("cannot parse %q: %s", requestURI, err)
 		}
-		target, headers, err := createTargetURL(ui, u)
+		u = normalizeURL(u)
+		up, headers, err := ui.getURLPrefix(u)
 		if err != nil {
 			t.Fatalf("unexpected error: %s", err)
 		}
+		target := up.mergeURLs(u)
 		if target.String() != expectedTarget {
 			t.Fatalf("unexpected target; got %q; want %q", target, expectedTarget)
 		}
@@ -119,15 +121,16 @@ func TestCreateTargetURLFailure(t *testing.T) {
 		if err != nil {
 			t.Fatalf("cannot parse %q: %s", requestURI, err)
 		}
-		target, headers, err := createTargetURL(ui, u)
+		u = normalizeURL(u)
+		up, headers, err := ui.getURLPrefix(u)
 		if err == nil {
 			t.Fatalf("expecting non-nil error")
 		}
-		if target != nil {
+		if up != nil {
-			t.Fatalf("unexpected target=%q; want empty string", target)
+			t.Fatalf("unexpected non-empty up=%q", up)
 		}
 		if headers != nil {
-			t.Fatalf("unexpected headers=%q; want empty string", headers)
+			t.Fatalf("unexpected non-empty headers=%q", headers)
 		}
 	}
 	f(&UserInfo{}, "/foo/bar")

docs/CHANGELOG.md

@@ -16,6 +16,7 @@ The following tip changes can be tested by building VictoriaMetrics components f
 ## tip
 
 * FEATURE: [vmauth](https://docs.victoriametrics.com/vmauth.html): add the ability to limit the number of concurrent requests on a per-user basis via `max_concurrent_requests` option. See [this feature request](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3346) and [these docs](https://docs.victoriametrics.com/vmauth.html#auth-config).
+* FEATURE: [vmauth](https://docs.victoriametrics.com/vmauth.html): automatically retry failing GET requests on all the configured backends.
 * FEATURE: [vmalert enterprise](https://docs.victoriametrics.com/vmalert.html): add ability to read alerting and recording rules from S3, GCS or S3-compatible object storage. See [these docs](https://docs.victoriametrics.com/vmalert.html#reading-rules-from-object-storage).
 
 ## [v1.87.1](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.87.1)