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

collector/mongodb_collector.go

@@ -15,8 +15,8 @@
 package collector
 
 import (
-	"errors"
 	"fmt"
+	"sync"
 	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
@@ -38,6 +38,7 @@ type MongodbCollectorOpts struct {
 	TLSPrivateKeyFile     string
 	TLSCaFile             string
 	TLSHostnameValidation bool
+	DbPoolLimit           int
 }
 
 func (in MongodbCollectorOpts) toSessionOps() shared.MongoSessionOpts {
@@ -48,6 +49,7 @@ func (in MongodbCollectorOpts) toSessionOps() shared.MongoSessionOpts {
 		TLSPrivateKeyFile:     in.TLSPrivateKeyFile,
 		TLSCaFile:             in.TLSCaFile,
 		TLSHostnameValidation: in.TLSHostnameValidation,
+		PoolLimit:             in.DbPoolLimit,
 	}
 }
 
@@ -57,6 +59,8 @@ type MongodbCollector struct {
 	scrapesTotal              prometheus.Counter
 	lastScrapeError           prometheus.Gauge
 	lastScrapeDurationSeconds prometheus.Gauge
+	mongoSess                 *mgo.Session
+	mongoSessLock             sync.Mutex
 }
 
 // NewMongodbCollector returns a new instance of a MongodbCollector.
@@ -87,6 +91,31 @@ func NewMongodbCollector(opts MongodbCollectorOpts) *MongodbCollector {
 	return exporter
 }
 
+// getSession returns the cached *mgo.Session or creates a new session and returns it.
+// Use sync.Mutex to avoid race condition around session creation.
+func (exporter *MongodbCollector) getSession() *mgo.Session {
+	exporter.mongoSessLock.Lock()
+	defer exporter.mongoSessLock.Unlock()
+
+	if exporter.mongoSess != nil {
+		return exporter.mongoSess.Copy()
+	}
+	exporter.mongoSess = shared.MongoSession(exporter.Opts.toSessionOps())
+	if exporter.mongoSess != nil {
+		return exporter.mongoSess.Copy()
+	}
+	return nil
+}
+
+// Close cleanly closes the mongo session if it exists.
+func (exporter *MongodbCollector) Close() {
+	if exporter.mongoSess != nil {
+		exporter.mongoSessLock.Lock()
+		exporter.mongoSess.Close()
+		exporter.mongoSessLock.Unlock()
+	}
+}
+
 // 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.
@@ -139,9 +168,9 @@ 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 to %s", exporter.Opts.URI)
 		return
 	}
 	defer mongoSess.Close()

mongodb_exporter.go

@@ -63,6 +63,8 @@ var (
 		"    \tIf not provided: System default CAs are used.")
 	tlsDisableHostnameValidationF = flag.Bool("mongodb.tls-disable-hostname-validation", false, "Do hostname validation for server connection.")
 
+	dbPoolLimit = flag.Int("mongodb.max-connections", 1, "Max number of pooled connections to the database.")
+
 	// FIXME currently ignored
 	enabledGroupsFlag = flag.String("groups.enabled", "asserts,durability,background_flushing,connections,extra_info,global_lock,index_counters,network,op_counters,op_counters_repl,memory,locks,metrics", "Comma-separated list of groups to use, for more info see: docs.mongodb.org/manual/reference/command/serverStatus/")
 )
@@ -154,8 +156,8 @@ 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")
@@ -210,16 +212,18 @@ func startWebServer() {
 	}
 }
 
-func registerCollector() {
+func registerCollector() *collector.MongodbCollector {
 	mongodbCollector := collector.NewMongodbCollector(collector.MongodbCollectorOpts{
 		URI:                   *uriF,
 		TLSConnection:         *tlsF,
 		TLSCertificateFile:    *tlsCertF,
 		TLSPrivateKeyFile:     *tlsPrivateKeyF,
 		TLSCaFile:             *tlsCAF,
 		TLSHostnameValidation: !(*tlsDisableHostnameValidationF),
+		DbPoolLimit:           *dbPoolLimit,
 	})
 	prometheus.MustRegister(mongodbCollector)
+	return mongodbCollector
 }
 
 func main() {

shared/connection.go

@@ -52,6 +52,7 @@ type MongoSessionOpts struct {
 	TLSPrivateKeyFile     string
 	TLSCaFile             string
 	TLSHostnameValidation bool
+	PoolLimit             int
 }
 
 func MongoSession(opts MongoSessionOpts) *mgo.Session {
@@ -76,8 +77,10 @@ func MongoSession(opts MongoSessionOpts) *mgo.Session {
 		return nil
 	}
 	session.SetMode(mgo.Eventual, true)
+	session.SetPoolLimit(opts.PoolLimit)
+	session.SetPrefetch(0.00)
 	session.SetSyncTimeout(syncMongodbTimeout)
-	session.SetSocketTimeout(0)
+	session.SetSocketTimeout(dialMongodbTimeout)
 	return session
 }