func (e *Exporter) scrape(ch chan<- prometheus.Metric) {
	e.up.Set(1)
	e.totalScrapes.Inc()

	if time.Now().Sub(tempUpdated) < staleInterval {
		ch <- prometheus.MustNewConstMetric(
			prometheus.NewDesc(prometheus.BuildFQName(namespace, "sensor", "temperature"),
				"Current temperature.", []string{"metric"}, nil),
			prometheus.GaugeValue, celsius, "celsius",
		)
		ch <- prometheus.MustNewConstMetric(
			prometheus.NewDesc(prometheus.BuildFQName(namespace, "sensor", "temperature"),
				"Temperature in C and F.", []string{"metric"}, nil),
			prometheus.GaugeValue, fahrenheit, "fahrenheit",
		)
	}
	if time.Now().Sub(soundUpdated) < staleInterval {
		ch <- prometheus.MustNewConstMetric(
			prometheus.NewDesc(prometheus.BuildFQName(namespace, "sensor", "sound"),
				"Sound (noise) level.", nil, nil),
			prometheus.GaugeValue, float64(sound),
		)
	}
	if time.Now().Sub(lightUpdated) < staleInterval {
		ch <- prometheus.MustNewConstMetric(
			prometheus.NewDesc(prometheus.BuildFQName(namespace, "sensor", "light"),
				"Luminous flux per unit area.", nil, nil),
			prometheus.GaugeValue, float64(light),
		)
	}
}

// Takes a prometheus registry and returns a new Collector exposing
// Device stats.
func NewDevstatCollector() (Collector, error) {
	return &devstatCollector{
		devinfo: &C.struct_devinfo{},
		bytes: typedDesc{prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, devstatSubsystem, "bytes_total"),
			"The total number of bytes in transactions.",
			[]string{"device", "type"}, nil,
		), prometheus.CounterValue},
		transfers: typedDesc{prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, devstatSubsystem, "transfers_total"),
			"The total number of transactions.",
			[]string{"device", "type"}, nil,
		), prometheus.CounterValue},
		duration: typedDesc{prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, devstatSubsystem, "duration_seconds_total"),
			"The total duration of transactions in seconds.",
			[]string{"device", "type"}, nil,
		), prometheus.CounterValue},
		busyTime: typedDesc{prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, devstatSubsystem, "busy_time_seconds_total"),
			"Total time the device had one or more transactions outstanding in seconds.",
			[]string{"device"}, nil,
		), prometheus.CounterValue},
		blocks: typedDesc{prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, devstatSubsystem, "blocks_transferred_total"),
			"The total number of blocks transferred.",
			[]string{"device"}, nil,
		), prometheus.CounterValue},
	}, nil
}

// Takes a prometheus registry and returns a new Collector exposing
// kernel/system statistics.
func NewPfCollector() (Collector, error) {
	subsystem := "pf"
	return &pfCollector{
		match: prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, subsystem, "match"),
			"PF internal Match counter.",
			nil, nil,
		),
		states: prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, subsystem, "state_count"),
			"State Table entry count.",
			nil, nil,
		),
		searches: prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, subsystem, "state_search"),
			"State Table search counter.",
			nil, nil,
		),
		inserts: prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, subsystem, "state_insert"),
			"State Table insert counter.",
			nil, nil,
		),
		removals: prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, subsystem, "state_removal"),
			"State Table remove counter.",
			nil, nil,
		),
	}, nil
}

func NewRunitCollector() (Collector, error) {
	var (
		subsystem   = "service"
		constLabels = prometheus.Labels{"supervisor": "runit"}
		labelNames  = []string{"service"}
	)

	return &runitCollector{
		state: typedDesc{prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, subsystem, "state"),
			"State of runit service.",
			labelNames, constLabels,
		), prometheus.GaugeValue},
		stateDesired: typedDesc{prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, subsystem, "desired_state"),
			"Desired state of runit service.",
			labelNames, constLabels,
		), prometheus.GaugeValue},
		stateNormal: typedDesc{prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, subsystem, "normal_state"),
			"Normal state of runit service.",
			labelNames, constLabels,
		), prometheus.GaugeValue},
		stateTimestamp: typedDesc{prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, subsystem, "state_last_change_timestamp_seconds"),
			"Unix timestamp of the last runit service state change.",
			labelNames, constLabels,
		), prometheus.GaugeValue},
	}, nil
}

// Take a prometheus registry and return a new Collector exposing load average.
func NewLoadavgCollector() (Collector, error) {
	return &loadavgCollector{
		metric: []typedDesc{
			{prometheus.NewDesc(Namespace+"_load1", "1m load average.", nil, nil), prometheus.GaugeValue},
			{prometheus.NewDesc(Namespace+"_load5", "5m load average.", nil, nil), prometheus.GaugeValue},
			{prometheus.NewDesc(Namespace+"_load15", "15m load average.", nil, nil), prometheus.GaugeValue},
		},
	}, nil
}

func NewExporter(uri string) *Exporter {
	return &Exporter{
		URI: uri,
		up: prometheus.NewDesc(
			prometheus.BuildFQName(namespace, "", "up"),
			"Could the apache server be reached",
			nil,
			nil),
		scrapeFailures: prometheus.NewCounter(prometheus.CounterOpts{
			Namespace: namespace,
			Name:      "exporter_scrape_failures_total",
			Help:      "Number of errors while scraping apache.",
		}),
		accessesTotal: prometheus.NewDesc(
			prometheus.BuildFQName(namespace, "", "accesses_total"),
			"Current total apache accesses",
			nil,
			nil),
		kBytesTotal: prometheus.NewDesc(
			prometheus.BuildFQName(namespace, "", "sent_kilobytes_total"),
			"Current total kbytes sent",
			nil,
			nil),
		uptime: prometheus.NewDesc(
			prometheus.BuildFQName(namespace, "", "uptime_seconds_total"),
			"Current uptime in seconds",
			nil,
			nil),
		workers: prometheus.NewGaugeVec(prometheus.GaugeOpts{
			Namespace: namespace,
			Name:      "workers",
			Help:      "Apache worker statuses",
		},
			[]string{"state"},
		),
		scoreboard: prometheus.NewGaugeVec(prometheus.GaugeOpts{
			Namespace: namespace,
			Name:      "scoreboard",
			Help:      "Apache scoreboard statuses",
		},
			[]string{"state"},
		),
		connections: prometheus.NewGaugeVec(prometheus.GaugeOpts{
			Namespace: namespace,
			Name:      "connections",
			Help:      "Apache connection statuses",
		},
			[]string{"state"},
		),
		client: &http.Client{
			Transport: &http.Transport{
				TLSClientConfig: &tls.Config{InsecureSkipVerify: *insecure},
			},
		},
	}
}

func (c *PrometheusCollector) collectContainersInfo(ch chan<- prometheus.Metric) {
	containers, err := c.infoProvider.SubcontainersInfo("/", &info.ContainerInfoRequest{NumStats: 1})
	if err != nil {
		c.errors.Set(1)
		glog.Warningf("Couldn't get containers: %s", err)
		return
	}
	for _, container := range containers {
		baseLabels := []string{"id"}
		id := container.Name
		name := id
		if len(container.Aliases) > 0 {
			name = container.Aliases[0]
			baseLabels = append(baseLabels, "name")
		}
		image := container.Spec.Image
		if len(image) > 0 {
			baseLabels = append(baseLabels, "image")
		}
		baseLabelValues := []string{id, name, image}[:len(baseLabels)]

		if c.containerNameToLabels != nil {
			newLabels := c.containerNameToLabels(name)
			for k, v := range newLabels {
				baseLabels = append(baseLabels, k)
				baseLabelValues = append(baseLabelValues, v)
			}
		}

		// Container spec
		desc := prometheus.NewDesc("container_start_time_seconds", "Start time of the container since unix epoch in seconds.", baseLabels, nil)
		ch <- prometheus.MustNewConstMetric(desc, prometheus.GaugeValue, float64(container.Spec.CreationTime.Unix()), baseLabelValues...)

		if container.Spec.HasCpu {
			desc := prometheus.NewDesc("container_spec_cpu_shares", "CPU share of the container.", baseLabels, nil)
			ch <- prometheus.MustNewConstMetric(desc, prometheus.GaugeValue, float64(container.Spec.Cpu.Limit), baseLabelValues...)
		}

		if container.Spec.HasMemory {
			desc := prometheus.NewDesc("container_spec_memory_limit_bytes", "Memory limit for the container.", baseLabels, nil)
			ch <- prometheus.MustNewConstMetric(desc, prometheus.GaugeValue, specMemoryValue(container.Spec.Memory.Limit), baseLabelValues...)
			desc = prometheus.NewDesc("container_spec_memory_swap_limit_bytes", "Memory swap limit for the container.", baseLabels, nil)
			ch <- prometheus.MustNewConstMetric(desc, prometheus.GaugeValue, specMemoryValue(container.Spec.Memory.SwapLimit), baseLabelValues...)
		}

		// Now for the actual metrics
		stats := container.Stats[0]
		for _, cm := range c.containerMetrics {
			desc := cm.desc(baseLabels)
			for _, metricValue := range cm.getValues(stats) {
				ch <- prometheus.MustNewConstMetric(desc, cm.valueType, float64(metricValue.value), append(baseLabelValues, metricValue.labels...)...)
			}
		}
	}
}

// Takes a prometheus registry and returns a new Collector exposing
// CPU stats.
func NewStatCollector() (Collector, error) {
	return &statCollector{
		cpu: typedDesc{prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, "cpu", "seconds_total"),
			"Seconds the CPU spent in each mode.",
			[]string{"cpu", "mode"}, nil,
		), prometheus.CounterValue},
		temp: typedDesc{prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, "cpu", "temperature_celsius"),
			"CPU temperature",
			[]string{"cpu"}, nil,
		), prometheus.GaugeValue},
	}, nil
}

// Takes a prometheus registry and returns a new Collector exposing
// conntrack stats
func NewConntrackCollector() (Collector, error) {
	return &conntrackCollector{
		current: prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, "", "nf_conntrack_entries"),
			"Number of currently allocated flow entries for connection tracking.",
			nil, nil,
		),
		limit: prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, "", "nf_conntrack_entries_limit"),
			"Maximum size of connection tracking table.",
			nil, nil,
		),
	}, nil
}

// NewBondingCollector returns a newly allocated bondingCollector.
// It exposes the number of configured and active slave of linux bonding interfaces.
func NewBondingCollector() (Collector, error) {
	return &bondingCollector{
		slaves: typedDesc{prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, "bonding", "slaves"),
			"Number of configured slaves per bonding interface.",
			[]string{"master"}, nil,
		), prometheus.GaugeValue},
		active: typedDesc{prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, "bonding", "active"),
			"Number of active slaves per bonding interface.",
			[]string{"master"}, nil,
		), prometheus.GaugeValue},
	}, nil
}

func TestIPVSCollector(t *testing.T) {
	if err := flag.Set("collector.procfs", "fixtures/proc"); err != nil {
		t.Fatal(err)
	}
	collector, err := newIPVSCollector()
	if err != nil {
		t.Fatal(err)
	}
	sink := make(chan prometheus.Metric)
	go func() {
		err = collector.Update(sink)
		if err != nil {
			t.Fatal(err)
		}
	}()
	for expected, got := range map[string]string{
		prometheus.NewDesc("node_ipvs_connections_total", "The total number of connections made.", nil, nil).String():                                                                                                                  (<-sink).Desc().String(),
		prometheus.NewDesc("node_ipvs_incoming_packets_total", "The total number of incoming packets.", nil, nil).String():                                                                                                             (<-sink).Desc().String(),
		prometheus.NewDesc("node_ipvs_outgoing_packets_total", "The total number of outgoing packets.", nil, nil).String():                                                                                                             (<-sink).Desc().String(),
		prometheus.NewDesc("node_ipvs_incoming_bytes_total", "The total amount of incoming data.", nil, nil).String():                                                                                                                  (<-sink).Desc().String(),
		prometheus.NewDesc("node_ipvs_outgoing_bytes_total", "The total amount of outgoing data.", nil, nil).String():                                                                                                                  (<-sink).Desc().String(),
		prometheus.NewDesc("node_ipvs_backend_connections_active", "The current active connections by local and remote address.", []string{"local_address", "local_port", "remote_address", "remote_port", "proto"}, nil).String():     (<-sink).Desc().String(),
		prometheus.NewDesc("node_ipvs_backend_connections_inactive", "The current inactive connections by local and remote address.", []string{"local_address", "local_port", "remote_address", "remote_port", "proto"}, nil).String(): (<-sink).Desc().String(),
		prometheus.NewDesc("node_ipvs_backend_weight", "The current backend weight by local and remote address.", []string{"local_address", "local_port", "remote_address", "remote_port", "proto"}, nil).String():                     (<-sink).Desc().String(),
	} {
		if expected != got {
			t.Fatalf("Expected '%s' but got '%s'", expected, got)
		}
	}
}

// Collect implements prometheus.Collector.
func (c *scollectorCollector) Collect(ch chan<- prometheus.Metric) {
	Log.Debug("Collect", "samples", len(c.samples))
	ch <- lastProcessed
	Log.Debug("Collect", "lastProcessed", lastProcessed)

	c.mu.Lock()
	samples := make([]scollectorSample, 0, len(c.samples))
	for _, sample := range c.samples {
		samples = append(samples, sample)
	}
	c.mu.Unlock()

	ageLimit := time.Now().Add(-*sampleExpiry)
	for _, sample := range samples {
		if ageLimit.After(sample.Timestamp) {
			Log.Debug("skipping old sample", "limit", ageLimit, "sample", sample)
			continue
		}
		Log.Debug("sending sample", "sample", sample)
		ch <- prometheus.MustNewConstMetric(
			prometheus.NewDesc(sample.Name, sample.Help, []string{}, sample.Labels),
			sample.Type,
			sample.Value,
		)
	}
}

func (m *Metric) PromDescription(exporterName string) *prometheus.Desc {
	return prometheus.NewDesc(
		prometheus.BuildFQName("", exporterName, m.Name),
		m.Description,
		m.LabelKeys, nil,
	)
}

func (c *netDevCollector) Update(ch chan<- prometheus.Metric) (err error) {
	netDev, err := getNetDevStats(c.ignoredDevicesPattern)
	if err != nil {
		return fmt.Errorf("couldn't get netstats: %s", err)
	}
	for dev, devStats := range netDev {
		for key, value := range devStats {
			desc, ok := c.metricDescs[key]
			if !ok {
				desc = prometheus.NewDesc(
					prometheus.BuildFQName(Namespace, c.subsystem, key),
					fmt.Sprintf("Network device statistic %s.", key),
					[]string{"device"},
					nil,
				)
				c.metricDescs[key] = desc
			}
			v, err := strconv.ParseFloat(value, 64)
			if err != nil {
				return fmt.Errorf("invalid value %s in netstats: %s", value, err)
			}
			ch <- prometheus.MustNewConstMetric(desc, prometheus.GaugeValue, v, dev)
		}
	}
	return nil
}

func (c *vmStatCollector) Update(ch chan<- prometheus.Metric) (err error) {
	file, err := os.Open(procFilePath("vmstat"))
	if err != nil {
		return err
	}
	defer file.Close()

	scanner := bufio.NewScanner(file)
	for scanner.Scan() {
		parts := strings.Fields(scanner.Text())
		value, err := strconv.ParseFloat(parts[1], 64)
		if err != nil {
			return err
		}

		ch <- prometheus.MustNewConstMetric(
			prometheus.NewDesc(
				prometheus.BuildFQName(Namespace, vmStatSubsystem, parts[0]),
				fmt.Sprintf("/proc/vmstat information field %s.", parts[0]),
				nil, nil),
			prometheus.UntypedValue,
			value,
		)
	}
	return err
}

func (c *netStatCollector) Update(ch chan<- prometheus.Metric) (err error) {
	netStats, err := getNetStats(procFilePath("net/netstat"))
	if err != nil {
		return fmt.Errorf("couldn't get netstats: %s", err)
	}
	snmpStats, err := getNetStats(procFilePath("net/snmp"))
	if err != nil {
		return fmt.Errorf("couldn't get SNMP stats: %s", err)
	}
	// Merge the results of snmpStats into netStats (collisions are possible, but
	// we know that the keys are always unique for the given use case).
	for k, v := range snmpStats {
		netStats[k] = v
	}
	for protocol, protocolStats := range netStats {
		for name, value := range protocolStats {
			key := protocol + "_" + name
			v, err := strconv.ParseFloat(value, 64)
			if err != nil {
				return fmt.Errorf("invalid value %s in netstats: %s", value, err)
			}
			ch <- prometheus.MustNewConstMetric(
				prometheus.NewDesc(
					prometheus.BuildFQName(Namespace, netStatsSubsystem, key),
					fmt.Sprintf("Protocol %s statistic %s.", protocol, name),
					nil, nil,
				),
				prometheus.UntypedValue, v,
			)
		}
	}
	return nil
}

// ScrapeClientStat collects from `information_schema.client_statistics`.
func ScrapeClientStat(db *sql.DB, ch chan<- prometheus.Metric) error {
	var varName, varVal string
	err := db.QueryRow(userstatCheckQuery).Scan(&varName, &varVal)
	if err != nil {
		log.Debugln("Detailed client stats are not available.")
		return nil
	}
	if varVal == "OFF" {
		log.Debugf("MySQL @@%s is OFF.", varName)
		return nil
	}

	informationSchemaClientStatisticsRows, err := db.Query(clientStatQuery)
	if err != nil {
		return err
	}
	defer informationSchemaClientStatisticsRows.Close()

	// The client column is assumed to be column[0], while all other data is assumed to be coerceable to float64.
	// Because of the client column, clientStatData[0] maps to columnNames[1] when reading off the metrics
	// (because clientStatScanArgs is mapped as [ &client, &clientData[0], &clientData[1] ... &clientdata[n] ]
	// To map metrics to names therefore we always range over columnNames[1:]
	columnNames, err := informationSchemaClientStatisticsRows.Columns()
	if err != nil {
		return err
	}

	var (
		client             string                                // Holds the client name, which should be in column 0.
		clientStatData     = make([]float64, len(columnNames)-1) // 1 less because of the client column.
		clientStatScanArgs = make([]interface{}, len(columnNames))
	)

	clientStatScanArgs[0] = &client
	for i := range clientStatData {
		clientStatScanArgs[i+1] = &clientStatData[i]
	}

	for informationSchemaClientStatisticsRows.Next() {
		if err := informationSchemaClientStatisticsRows.Scan(clientStatScanArgs...); err != nil {
			return err
		}

		// Loop over column names, and match to scan data. Unknown columns
		// will be filled with an untyped metric number. We assume other then
		// cient, that we'll only get numbers.
		for idx, columnName := range columnNames[1:] {
			if metricType, ok := informationSchemaClientStatisticsTypes[columnName]; ok {
				ch <- prometheus.MustNewConstMetric(metricType.desc, metricType.vtype, float64(clientStatData[idx]), client)
			} else {
				// Unknown metric. Report as untyped.
				desc := prometheus.NewDesc(prometheus.BuildFQName(namespace, informationSchema, fmt.Sprintf("client_statistics_%s", strings.ToLower(columnName))), fmt.Sprintf("Unsupported metric from column %s", columnName), []string{"client"}, nil)
				ch <- prometheus.MustNewConstMetric(desc, prometheus.UntypedValue, float64(clientStatData[idx]), client)
			}
		}
	}
	return nil
}

// Turn the MetricMap column mapping into a prometheus descriptor mapping.
func makeDescMap(metricMaps map[NamespaceAndQuery]map[string]ColumnMapping) map[NamespaceAndQuery]MetricMapNamespace {
	var metricMap = make(map[NamespaceAndQuery]MetricMapNamespace)

	for namespaceAndQuery, mappings := range metricMaps {
		namespace := namespaceAndQuery.namespace
		thisMap := make(map[string]MetricMap)

		// Get the constant labels
		var constLabels []string
		for columnName, columnMapping := range mappings {
			if columnMapping.usage == LABEL {
				constLabels = append(constLabels, columnName)
			}
		}

		for columnName, columnMapping := range mappings {
			switch columnMapping.usage {
			case DISCARD, LABEL:
				thisMap[columnName] = MetricMap{
					discard: true,
				}
			case COUNTER:
				thisMap[columnName] = MetricMap{
					vtype: prometheus.CounterValue,
					desc:  prometheus.NewDesc(fmt.Sprintf("%s_%s", namespace, columnName), columnMapping.description, constLabels, nil),
				}
			case GAUGE:
				thisMap[columnName] = MetricMap{
					vtype: prometheus.GaugeValue,
					desc:  prometheus.NewDesc(fmt.Sprintf("%s_%s", namespace, columnName), columnMapping.description, constLabels, nil),
				}
			case MAPPEDMETRIC:
				thisMap[columnName] = MetricMap{
					vtype:   prometheus.GaugeValue,
					desc:    prometheus.NewDesc(fmt.Sprintf("%s_%s", namespace, columnName), columnMapping.description, constLabels, nil),
					mapping: columnMapping.mapping,
				}
			}
		}

		metricMap[namespaceAndQuery] = MetricMapNamespace{constLabels, thisMap}
	}

	return metricMap
}

// Collect implements Prometheus.Collector.
func (c collector) Collect(ch chan<- prometheus.Metric) {
	start := time.Now()
	pdus, err := ScrapeTarget(c.target, c.module)
	if err != nil {
		log.Errorf("Error scraping target %s: %s", c.target, err)
		ch <- prometheus.NewInvalidMetric(prometheus.NewDesc("snmp_error", "Error scraping target", nil, nil), err)
		return
	}
	ch <- prometheus.MustNewConstMetric(
		prometheus.NewDesc("snmp_scrape_walk_duration_seconds", "Time SNMP walk/bulkwalk took.", nil, nil),
		prometheus.GaugeValue,
		float64(time.Since(start).Seconds()))
	ch <- prometheus.MustNewConstMetric(
		prometheus.NewDesc("snmp_scrape_pdus_returned", "PDUs returned from walk.", nil, nil),
		prometheus.GaugeValue,
		float64(len(pdus)))
	oidToPdu := make(map[string]gosnmp.SnmpPDU, len(pdus))
	for _, pdu := range pdus {
		oidToPdu[pdu.Name[1:]] = pdu
	}

	metricTree := buildMetricTree(c.module.Metrics)
	// Look for metrics that match each pdu.
PduLoop:
	for oid, pdu := range oidToPdu {
		head := metricTree
		oidList := oidToList(oid)
		for i, o := range oidList {
			var ok bool
			head, ok = head.children[o]
			if !ok {
				continue PduLoop
			}
			if head.metric != nil {
				// Found a match.
				ch <- pduToSample(oidList[i+1:], &pdu, head.metric, oidToPdu)
				break
			}
		}
	}
	ch <- prometheus.MustNewConstMetric(
		prometheus.NewDesc("snmp_scrape_duration_seconds", "Total SNMP time scrape took (walk and processing).", nil, nil),
		prometheus.GaugeValue,
		float64(time.Since(start).Seconds()))
}

// Takes a prometheus registry and returns a new Collector exposing
// entropy stats
func NewEntropyCollector() (Collector, error) {
	return &entropyCollector{
		entropy_avail: prometheus.NewDesc(
			prometheus.BuildFQName(Namespace, "", "entropy_available_bits"),
			"Bits of available entropy.",
			nil, nil,
		),
	}, nil
}