Add supporting multi-target dynamic scraping

README.md

@@ -199,6 +199,31 @@ The labels are:
 - rs_state: Replicaset state is an integer from `getDiagnosticData()` -> `replSetGetStatus.myState`. 
 Check [the official documentation](https://docs.mongodb.com/manual/reference/replica-states/) for details on replicaset status values.
 
+#### Prometheus Configuration to Scrape Multiple MongoDB Hosts
+The Prometheus documentation [provides](https://prometheus.io/docs/guides/multi-target-exporter/) a good example of multi-target exporters.
+
+To use `mongodb_exporter` in multi-target mode, you can use the `/scrape` endpoint with the `target` parameter.
+
+You can optionally specify initial URIs using `--mongodb.uri` (or `MONGODB_URI`) to preload a set of MongoDB instances, but it is not required. Additional targets can still be queried dynamically via `/scrape?target=...`.
+
+This allows combining static and dynamic target discovery in a flexible way.
+```
+scrape_configs:
+  - job_name: 'mongodb_exporter_targets'
+    metrics_path: /scrape
+    static_configs:
+      - targets:
+          - mongodb://mongo-host1:27017
+          - mongo-host2:27017
+    relabel_configs:
+      - source_labels: [__address__]
+        target_label: __param_target
+      - source_labels: [__param_target]
+        target_label: instance
+      - target_label: __address__
+        replacement: <<MONGODB-EXPORTER-HOSTNAME>>:9216
+```
+
 ## Usage Reference
 
 See the [Reference Guide](REFERENCE.md) for details on using the exporter.

exporter/build_uri.go

@@ -0,0 +1,130 @@
+// mongodb_exporter
+// Copyright (C) 2025 Percona LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package exporter
+
+import (
+	"fmt"
+	"log"
+	"log/slog"
+	"net/url"
+	"regexp"
+	"strings"
+)
+
+func ParseURIList(uriList []string, logger *slog.Logger, splitCluster bool) []string { //nolint:gocognit,cyclop
+	var URIs []string
+
+	// If server URI is prefixed with mongodb scheme string, then every next URI in
+	// line not prefixed with mongodb scheme string is a part of cluster. Otherwise,
+	// treat it as a standalone server
+	realURI := ""
+	matchRegexp := regexp.MustCompile(`^mongodb(\+srv)?://`)
+	for _, URI := range uriList {
+		matches := matchRegexp.FindStringSubmatch(URI)
+		if matches != nil {
+			if realURI != "" {
+				// Add the previous host buffer to the url list as we met the scheme part
+				URIs = append(URIs, realURI)
+				realURI = ""
+			}
+			if matches[1] == "" {
+				realURI = URI
+			} else {
+				// There can be only one host in SRV connection string
+				if splitCluster {
+					// In splitCluster mode we get srv connection string from SRV recors
+					URI = GetSeedListFromSRV(URI, logger)
+				}
+				URIs = append(URIs, URI)
+			}
+		} else {
+			if realURI == "" {
+				URIs = append(URIs, "mongodb://"+URI)
+			} else {
+				realURI += "," + URI
+			}
+		}
+	}
+	if realURI != "" {
+		URIs = append(URIs, realURI)
+	}
+
+	if splitCluster {
+		// In this mode we split cluster strings into separate targets
+		separateURIs := []string{}
+		for _, hosturl := range URIs {
+			urlParsed, err := url.Parse(hosturl)
+			if err != nil {
+				log.Fatalf("Failed to parse URI %s: %v", hosturl, err)
+			}
+			for _, host := range strings.Split(urlParsed.Host, ",") {
+				targetURI := "mongodb://"
+				if urlParsed.User != nil {
+					targetURI += urlParsed.User.String() + "@"
+				}
+				targetURI += host
+				if urlParsed.Path != "" {
+					targetURI += urlParsed.Path
+				}
+				if urlParsed.RawQuery != "" {
+					targetURI += "?" + urlParsed.RawQuery
+				}
+				separateURIs = append(separateURIs, targetURI)
+			}
+		}
+		return separateURIs
+	}
+	return URIs
+}
+
+// buildURIManually builds the URI manually by checking if the user and password are supplied
+func buildURIManually(uri string, user string, password string) string {
+	uriArray := strings.SplitN(uri, "://", 2) //nolint:mnd
+	prefix := uriArray[0] + "://"
+	uri = uriArray[1]
+
+	// IF user@pass not contained in uri AND custom user and pass supplied in arguments
+	// DO concat a new uri with user and pass arguments value
+	if !strings.Contains(uri, "@") && user != "" && password != "" {
+		// add user and pass to the uri
+		uri = fmt.Sprintf("%s:%s@%s", user, password, uri)
+	}
+
+	// add back prefix after adding the user and pass
+	uri = prefix + uri
+
+	return uri
+}
+
+func BuildURI(uri string, user string, password string) string {
+	defaultPrefix := "mongodb://" // default prefix
+
+	if !strings.HasPrefix(uri, defaultPrefix) && !strings.HasPrefix(uri, "mongodb+srv://") {
+		uri = defaultPrefix + uri
+	}
+	parsedURI, err := url.Parse(uri)
+	if err != nil {
+		// PMM generates URI with escaped path to socket file, so url.Parse fails
+		// in this case we build URI manually
+		return buildURIManually(uri, user, password)
+	}
+
+	if parsedURI.User == nil && user != "" && password != "" {
+		parsedURI.User = url.UserPassword(user, password)
+	}
+
+	return parsedURI.String()
+}

exporter/exporter.go

@@ -84,6 +84,8 @@ type Opts struct {
 
 	URI      string
 	NodeName string
+	User     string
+	Password string
 }
 
 var (

exporter/multi_target_test.go

@@ -22,6 +22,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"regexp"
+	"sync"
 	"testing"
 
 	"github.com/prometheus/common/promslog"
@@ -69,9 +70,21 @@ func TestMultiTarget(t *testing.T) {
 		"mongodb_up{cluster_role=\"\"} 0\n",
 	}
 
+	exportersCache := make(map[string]*Exporter)
+	var cacheMutex sync.Mutex
+
+	for _, e := range exporters {
+		cacheMutex.Lock()
+		exportersCache[e.opts.URI] = e
+		cacheMutex.Unlock()
+	}
+
 	// Test all targets
 	for sn, opt := range opts {
-		assert.HTTPBodyContains(t, multiTargetHandler(serverMap), "GET", fmt.Sprintf("?target=%s", opt.URI), nil, expected[sn])
+		t.Run(fmt.Sprintf("target_%d", sn), func(t *testing.T) {
+			handler := multiTargetHandler(serverMap, opt, exportersCache, &cacheMutex, log)
+			assert.HTTPBodyContains(t, handler, "GET", fmt.Sprintf("?target=%s", opt.URI), nil, expected[sn])
+		})
 	}
 }
 

exporter/server.go

@@ -22,7 +22,7 @@ import (
 	"net/url"
 	"os"
 	"strconv"
-	"strings"
+	"sync"
 	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
@@ -44,18 +44,48 @@ type ServerOpts struct {
 }
 
 // RunWebServer runs the main web-server
-func RunWebServer(opts *ServerOpts, exporters []*Exporter, log *slog.Logger) {
+func RunWebServer(opts *ServerOpts, exporters []*Exporter, exporterOpts *Opts, log *slog.Logger) {
 	mux := http.NewServeMux()
+	serverMap := buildServerMap(exporters, log)
+
+	exportersCache := make(map[string]*Exporter)
+	var cacheMutex sync.Mutex
 
-	if len(exporters) == 0 {
-		panic("No exporters were built. You must specify --mongodb.uri command argument or MONGODB_URI environment variable")
+	// Prefill cache with existing exporters
+	for _, exp := range exporters {
+		cacheMutex.Lock()
+		cacheKey := exp.opts.URI
+		exportersCache[cacheKey] = exp
+		cacheMutex.Unlock()
 	}
 
-	serverMap := buildServerMap(exporters, log)
+	mux.HandleFunc(opts.Path, func(w http.ResponseWriter, r *http.Request) {
+		targetHost := r.URL.Query().Get("target")
+
+		if targetHost == "" {
+			if len(exporters) > 0 {
+				defaultExporter := exporters[0]
+				defaultExporter.Handler().ServeHTTP(w, r)
+				return
+			}
+
+			cacheMutex.Lock()
+			defer cacheMutex.Unlock()
+			for _, exp := range exportersCache {
+				exp.Handler().ServeHTTP(w, r)
+				return
+			}
 
-	defaultExporter := exporters[0]
-	mux.Handle(opts.Path, defaultExporter.Handler())
-	mux.HandleFunc(opts.MultiTargetPath, multiTargetHandler(serverMap))
+			reg := prometheus.NewRegistry()
+			h := promhttp.HandlerFor(reg, promhttp.HandlerOpts{})
+			h.ServeHTTP(w, r)
+			return
+		}
+
+		multiTargetHandler(serverMap, exporterOpts, exportersCache, &cacheMutex, log).ServeHTTP(w, r)
+	})
+
+	mux.HandleFunc(opts.MultiTargetPath, multiTargetHandler(serverMap, exporterOpts, exportersCache, &cacheMutex, log))
 	mux.HandleFunc(opts.OverallTargetPath, OverallTargetsHandler(exporters, log))
 
 	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
@@ -85,21 +115,62 @@ func RunWebServer(opts *ServerOpts, exporters []*Exporter, log *slog.Logger) {
 	}
 }
 
-func multiTargetHandler(serverMap ServerMap) http.HandlerFunc {
+// multiTargetHandler returns a handler that scrapes metrics from a target specified by the 'target' query parameter.
+// It validates the URI and caches dynamic exporters by target.
+func multiTargetHandler(serverMap ServerMap, exporterOpts *Opts, exportersCache map[string]*Exporter, cacheMutex *sync.Mutex, logger *slog.Logger) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		targetHost := r.URL.Query().Get("target")
-		if targetHost != "" {
-			if !strings.HasPrefix(targetHost, "mongodb://") {
-				targetHost = "mongodb://" + targetHost
-			}
-			if uri, err := url.Parse(targetHost); err == nil {
-				if e, ok := serverMap[uri.Host]; ok {
-					e.ServeHTTP(w, r)
-					return
-				}
-			}
+		if targetHost == "" {
+			logger.Warn("Missing target parameter")
+			http.Error(w, "Missing target parameter", http.StatusBadRequest)
+			return
 		}
-		http.Error(w, "Unable to find target", http.StatusNotFound)
+
+		parsed, err := url.Parse(targetHost)
+		if err != nil {
+			logger.Warn("Invalid target parameter", "target", targetHost, "error", err)
+			http.Error(w, "Invalid target parameter", http.StatusBadRequest)
+			return
+		}
+
+		fullURI := targetHost
+		if parsed.User == nil && exporterOpts.User != "" {
+			fullURI = BuildURI(targetHost, exporterOpts.User, exporterOpts.Password)
+		}
+
+		uri, err := url.Parse(fullURI)
+		if err != nil {
+			logger.Warn("Invalid full URI", "target", targetHost, "error", err)
+			http.Error(w, "Invalid target parameter", http.StatusBadRequest)
+			return
+		}
+
+		if handler, ok := serverMap[uri.Host]; ok {
+			logger.Debug("Serving from static serverMap", "host", uri.Host)
+			handler.ServeHTTP(w, r)
+			return
+		}
+
+		cacheMutex.Lock()
+		exp, ok := exportersCache[fullURI]
+		cacheMutex.Unlock()
+
+		if !ok {
+			logger.Info("Creating new exporter for target", "target", targetHost)
+			opts := *exporterOpts
+			opts.URI = fullURI
+			opts.Logger = logger
+
+			exp = New(&opts)
+
+			cacheMutex.Lock()
+			exportersCache[fullURI] = exp
+			cacheMutex.Unlock()
+		} else {
+			logger.Debug("Serving from cache", "target", targetHost)
+		}
+
+		exp.Handler().ServeHTTP(w, r)
 	}
 }