Add support for selecting collectors at buildtime

This works by using a global array with references to NewXCollector
functions. Each collector appends to that array in it's init() function.

Which file gets build depends on the build tags:

To build only the ganglia exporter, you can do:

    go build -tags nonative,ganglia

By default it will build only the native collector.

README.md

@@ -1,4 +1,31 @@
-node_exporter
-=============
+# node_exporter
+
+Prometheus exporter with plugable metric collectors.
+
+
+
+## Available collectors
+
+By default it will only include the NativeCollector.
+
+To include other collectors, specify the build tags accordently.
+
+
+### NativeCollector
+
+Provides metrics for load, seconds since last login and a list of tags
+read from `node_exporter.conf`.
+
+To disable the native collector, use build tag `nonative`.
+
+
+### GmondCollector
+
+Talks to a local gmond and provide it's metrics.
+
+
+### RunitCollector
+
+Provides metrics for each runit services like state and how long it
+has been in that state.
 
-Simple exporter, exposing load, seconds since last login and a list of tags for the node it's running on.

exporter/exporter.go

@@ -1,4 +1,4 @@
-// Exporter is a prometheus exporter using multiple collectors to collect and export system metrics.
+// Exporter is a prometheus exporter using multiple collectorFactories to collect and export system metrics.
 package exporter
 
 import (
@@ -19,6 +19,7 @@ import (
 )
 
 var verbose = flag.Bool("verbose", false, "Verbose output.")
+var collectorFactories []func(config, prometheus.Registry) (Collector, error)
 
 // Interface a collector has to implement.
 type Collector interface {
@@ -33,7 +34,6 @@ type config struct {
 	Attributes       map[string]string `json:"attributes"`
 	ListeningAddress string            `json:"listeningAddress"`
 	ScrapeInterval   int               `json:"scrapeInterval"`
-	Collectors       []string          `json:"collectors"`
 }
 
 func (e *exporter) loadConfig() (err error) {
@@ -54,7 +54,7 @@ type exporter struct {
 	metricsUpdated   prometheus.Gauge
 	config           config
 	registry         prometheus.Registry
-	collectors       []Collector
+	Collectors       []Collector
 	MemProfile       string
 }
 
@@ -74,24 +74,14 @@ func New(configFile string) (e exporter, err error) {
 	if err != nil {
 		return e, fmt.Errorf("Couldn't read config: %s", err)
 	}
-
-	cn, err := NewNativeCollector(e.config, e.registry)
-	if err != nil {
-		log.Fatalf("Couldn't attach collector: %s", err)
-	}
-
-	cr, err := NewRunitCollector(e.config, e.registry)
-	if err != nil {
-		log.Fatalf("Couldn't attach collector: %s", err)
-	}
-
-	cg, err := NewGmondCollector(e.config, e.registry)
-	if err != nil {
-		log.Fatalf("Couldn't attach collector: %s", err)
+	for _, fn := range collectorFactories {
+		c, err := fn(e.config, e.registry)
+		if err != nil {
+			return e, err
+		}
+		e.Collectors = append(e.Collectors, c)
 	}
 
-	e.collectors = []Collector{&cn, &cr, &cg}
-
 	if e.config.ListeningAddress != "" {
 		e.listeningAddress = e.config.ListeningAddress
 	}
@@ -152,8 +142,8 @@ func (e *exporter) Loop() {
 		case <-tick:
 			log.Printf("Starting new scrape interval")
 			wg := sync.WaitGroup{}
-			wg.Add(len(e.collectors))
-			for _, c := range e.collectors {
+			wg.Add(len(e.Collectors))
+			for _, c := range e.Collectors {
 				go func(c Collector) {
 					e.Execute(c)
 					wg.Done()

exporter/gmond_collector.go

@@ -1,3 +1,5 @@
+// +build ganglia
+
 package exporter
 
 import (
@@ -25,18 +27,22 @@ type gmondCollector struct {
 	registry prometheus.Registry
 }
 
+func init() {
+	collectors = append(collectors, NewGmondCollector)
+}
+
 var illegalCharsRE = regexp.MustCompile(`[^a-zA-Z0-9_]`)
 
 // Takes a config struct and prometheus registry and returns a new Collector scraping ganglia.
-func NewGmondCollector(config config, registry prometheus.Registry) (collector gmondCollector, err error) {
-	collector = gmondCollector{
+func NewGmondCollector(config config, registry prometheus.Registry) (Collector, error) {
+	c := gmondCollector{
 		name:     "gmond_collector",
 		config:   config,
 		Metrics:  make(map[string]prometheus.Gauge),
 		registry: registry,
 	}
 
-	return collector, nil
+	return &c, nil
 }
 
 func (c *gmondCollector) Name() string { return c.name }

exporter/native_collector.go

@@ -1,3 +1,5 @@
+// +build !nonative
+
 package exporter
 
 import (
@@ -24,10 +26,14 @@ type nativeCollector struct {
 	config     config
 }
 
+func init() {
+	collectorFactories = append(collectorFactories, NewNativeCollector)
+}
+
 // Takes a config struct and prometheus registry and returns a new Collector exposing
 // load, seconds since last login and a list of tags as specified by config.
-func NewNativeCollector(config config, registry prometheus.Registry) (collector nativeCollector, err error) {
-	collector = nativeCollector{
+func NewNativeCollector(config config, registry prometheus.Registry) (Collector, error) {
+	c := nativeCollector{
 		name:       "native_collector",
 		config:     config,
 		loadAvg:    prometheus.NewGauge(),
@@ -39,24 +45,24 @@ func NewNativeCollector(config config, registry prometheus.Registry) (collector
 		"node_load",
 		"node_exporter: system load.",
 		prometheus.NilLabels,
-		collector.loadAvg,
+		c.loadAvg,
 	)
 
 	registry.Register(
 		"node_last_login_seconds",
 		"node_exporter: seconds since last login.",
 		prometheus.NilLabels,
-		collector.lastSeen,
+		c.lastSeen,
 	)
 
 	registry.Register(
 		"node_attributes",
 		"node_exporter: system attributes.",
 		prometheus.NilLabels,
-		collector.attributes,
+		c.attributes,
 	)
 
-	return collector, nil
+	return &c, nil
 }
 
 func (c *nativeCollector) Name() string { return c.name }

exporter/runit_collector.go

@@ -1,3 +1,5 @@
+// +build runit
+
 package exporter
 
 import (
@@ -13,7 +15,11 @@ type runitCollector struct {
 	stateNormal  prometheus.Gauge
 }
 
-func NewRunitCollector(config config, registry prometheus.Registry) (runitCollector, error) {
+func init() {
+	collectors = append(collectors, NewRunitCollector)
+}
+
+func NewRunitCollector(config config, registry prometheus.Registry) (Collector, error) {
 	c := runitCollector{
 		name:         "runit_collector",
 		config:       config,
@@ -43,7 +49,7 @@ func NewRunitCollector(config config, registry prometheus.Registry) (runitCollec
 		c.stateNormal,
 	)
 
-	return c, nil
+	return &c, nil
 }
 
 func (c *runitCollector) Name() string { return c.name }

node_exporter.conf

@@ -1,10 +1,5 @@
 {
   "scrapeInterval": 10,
-  "collectors": [
-    "NativeCollector",
-    "GmondCollector",
-    "MuninCollector"
-   ],
    "attributes" : {
       "web-server" : "1",
       "zone" : "a",

node_exporter.go

@@ -2,8 +2,9 @@ package main
 
 import (
 	"flag"
-	"github.com/prometheus/node_exporter/exporter"
 	"log"
+
+	"github.com/prometheus/node_exporter/exporter"
 )
 
 var (
@@ -18,5 +19,9 @@ func main() {
 	if err != nil {
 		log.Fatalf("Couldn't instantiate exporter: %s", err)
 	}
+	log.Printf("Registered collectors:")
+	for _, c := range exporter.Collectors {
+		log.Print(" - ", c.Name())
+	}
 	exporter.Loop()
 }