Disable pre-fetching, limit conn pool and reuse db connection

collector/mongodb_collector.go

@@ -15,7 +15,6 @@
 package collector
 
 import (
-	"errors"
 	"fmt"
 	"time"
 
@@ -57,6 +56,7 @@ type MongodbCollector struct {
 	scrapesTotal              prometheus.Counter
 	lastScrapeError           prometheus.Gauge
 	lastScrapeDurationSeconds prometheus.Gauge
+	mongoSess                 *mgo.Session
 }
 
 // NewMongodbCollector returns a new instance of a MongodbCollector.
@@ -87,6 +87,28 @@ func NewMongodbCollector(opts MongodbCollectorOpts) *MongodbCollector {
 	return exporter
 }
 
+// getSession returns the cached *mgo.Session (after a test ping)
+// or creates a new session and returns it. The cached session is
+// reconnected if the ping to it fails.
+func (exporter *MongodbCollector) getSession() *mgo.Session {
+	if exporter.mongoSess != nil {
+		err := exporter.mongoSess.Ping()
+		if err == nil {
+			return exporter.mongoSess
+		}
+		exporter.mongoSess.Close()
+	}
+	exporter.mongoSess = shared.MongoSession(exporter.Opts.toSessionOps())
+	return exporter.mongoSess
+}
+
+// Close cleanly closes the mongo session if it exists.
+func (exporter *MongodbCollector) Close() {
+	if exporter.mongoSess != nil {
+		exporter.mongoSess.Close()
+	}
+}
+
 // Describe sends the super-set of all possible descriptors of metrics collected by this Collector
 // to the provided channel and returns once the last descriptor has been sent.
 // Part of prometheus.Collector interface.
@@ -139,12 +161,11 @@ func (exporter *MongodbCollector) scrape(ch chan<- prometheus.Metric) {
 		}
 	}(time.Now())
 
-	mongoSess := shared.MongoSession(exporter.Opts.toSessionOps())
+	mongoSess := exporter.getSession()
 	if mongoSess == nil {
-		err = errors.New("can't create mongo session")
+		log.Errorf("can't create mongo session")
 		return
 	}
-	defer mongoSess.Close()
 
 	var serverVersion string
 	serverVersion, err = shared.MongoSessionServerVersion(mongoSess)

mongodb_exporter.go

@@ -154,8 +154,8 @@ func startWebServer() {
 	}
 
 	handler := prometheusHandler()
-
-	registerCollector()
+	collector := registerCollector()
+	defer collector.Close()
 
 	if (*sslCertFileF == "") != (*sslKeyFileF == "") {
 		log.Fatal("One of the flags -web.ssl-cert-file or -web.ssl-key-file is missing to enable HTTPS/TLS")
@@ -210,7 +210,7 @@ func startWebServer() {
 	}
 }
 
-func registerCollector() {
+func registerCollector() *collector.MongodbCollector {
 	mongodbCollector := collector.NewMongodbCollector(collector.MongodbCollectorOpts{
 		URI:                   *uriF,
 		TLSConnection:         *tlsF,
@@ -220,6 +220,7 @@ func registerCollector() {
 		TLSHostnameValidation: !(*tlsDisableHostnameValidationF),
 	})
 	prometheus.MustRegister(mongodbCollector)
+	return mongodbCollector
 }
 
 func main() {

shared/connection.go

@@ -76,6 +76,8 @@ func MongoSession(opts MongoSessionOpts) *mgo.Session {
 		return nil
 	}
 	session.SetMode(mgo.Eventual, true)
+	session.SetPoolLimit(2)
+	session.SetPrefetch(0.00)
 	session.SetSyncTimeout(syncMongodbTimeout)
 	session.SetSocketTimeout(0)
 	return session