feat: flag to enable gauge replacement

README.md

@@ -65,6 +65,7 @@ Flags:
                                   Example: "user1=pass1,user2=pass2"
       --apiListen string         Listen for API requests on this host/port. (default ":80")
       --cors string              The 'Access-Control-Allow-Origin' value to be returned. (default "*")
+      --gaugeBehavior string     How gauges are aggregated (sum or replace). (default "sum")
   -h, --help                     help for prom-aggregation-gateway
       --lifecycleListen string   Listen for lifecycle requests (health, metrics) on this host/port (default ":8888")
 
@@ -73,6 +74,15 @@ Use "prom-aggregation-gateway [command] --help" for more information about a com
 
 Any flags you see above can also be set by `ENV_VARIABLES`. ENV_VARS must have a prefix of `PAG_`, for example `PAG_AUTHUSERS=user1=pass1,user2=pass2` will start the service with basic auth. If an ENV_VARIABLE is set than it will be used over a CLI argument passed to the service.
 
+### Gauge Behavior
+
+Gauges can be aggregated in two ways:
+
+1. `sum` (default) - just like counters, by summing the values received.
+2. `replace` - every new value replaces the current value, scrape will always see the latest value.
+
+set this with the flag `--gaugeBehavior=XXX` or as an env var `PAG_GAUGEBEHAVIOR=XXX`.
+
 ## Ready-built images
 
 Container images are published here:

cmd/root.go

@@ -26,6 +26,7 @@ func Execute() {
 	rootCmd.PersistentFlags().StringVar(&cfg.ApiListen, "apiListen", ":80", "Listen for API requests on this host/port.")
 	rootCmd.PersistentFlags().StringVar(&cfg.LifecycleListen, "lifecycleListen", ":8888", "Listen for lifecycle requests (health, metrics) on this host/port")
 	rootCmd.PersistentFlags().StringVar(&cfg.CorsDomain, "cors", "*", "The 'Access-Control-Allow-Origin' value to be returned.")
+	rootCmd.PersistentFlags().StringVar(&cfg.GaugeBehavior, "gaugeBehavior", "sum", "How gauges are aggregated (sum or replace).")
 
 	if err := rootCmd.Execute(); err != nil {
 		os.Exit(1)

cmd/start.go

@@ -23,7 +23,7 @@ func startFunc(cmd *cobra.Command, args []string) error {
 		Accounts:   cfg.AuthUsers,
 	}
 
-	routers.RunServers(apiCfg, cfg.ApiListen, cfg.LifecycleListen)
+	routers.RunServers(apiCfg, cfg.ApiListen, cfg.LifecycleListen, cfg.GaugeBehavior)
 
 	return nil
 }

config/config.go

@@ -20,6 +20,7 @@ type Server struct {
 	LifecycleListen string
 	CorsDomain      string
 	AuthUsers       []string
+	GaugeBehavior   string
 }
 
 const (

metrics/aggregate.go

@@ -17,7 +17,8 @@ import (
 
 type metricFamily struct {
 	*dto.MetricFamily
-	lock sync.RWMutex
+	lock    sync.RWMutex
+	options *aggregateOptions
 }
 
 type Aggregate struct {
@@ -28,9 +29,17 @@ type Aggregate struct {
 
 type ignoredLabels []string
 
+type gaugeBehavior string
+
+const (
+	sumBehavior     gaugeBehavior = "sum"
+	replaceBehavior               = "replace"
+)
+
 type aggregateOptions struct {
 	ignoredLabels     ignoredLabels
 	metricTTLDuration *time.Duration
+	gaugeBehavior     gaugeBehavior
 }
 
 type aggregateOptionsFunc func(a *Aggregate)
@@ -47,6 +56,16 @@ func SetTTLMetricTime(duration *time.Duration) aggregateOptionsFunc {
 	}
 }
 
+func SetGaugeBehavior(behavior string) aggregateOptionsFunc {
+	return func(a *Aggregate) {
+		if behavior == replaceBehavior {
+			a.options.gaugeBehavior = replaceBehavior
+		} else {
+			a.options.gaugeBehavior = sumBehavior
+		}
+	}
+}
+
 func NewAggregate(opts ...aggregateOptionsFunc) *Aggregate {
 	a := &Aggregate{
 		families: map[string]*metricFamily{},
@@ -91,7 +110,7 @@ func (a *Aggregate) setFamilyOrGetExistingFamily(familyName string, family *dto.
 	defer a.familiesLock.Unlock()
 	existingFamily, ok := a.families[familyName]
 	if !ok {
-		a.families[familyName] = &metricFamily{MetricFamily: family}
+		a.families[familyName] = &metricFamily{MetricFamily: family, options: &a.options}
 		return nil
 	}
 	return existingFamily

metrics/aggregate_test.go

@@ -82,7 +82,28 @@ histogram_bucket{job="test",le="+Inf"} 9
 histogram_sum{job="test"} 7
 histogram_count{job="test"} 2
 `
-
+	wantReplace = `# HELP counter A counter
+# TYPE counter counter
+counter{job="test"} 60
+# HELP gauge A gauge
+# TYPE gauge gauge
+gauge{job="test"} 57
+# HELP histogram A histogram
+# TYPE histogram histogram
+histogram_bucket{job="test",le="1"} 0
+histogram_bucket{job="test",le="2"} 0
+histogram_bucket{job="test",le="3"} 3
+histogram_bucket{job="test",le="4"} 8
+histogram_bucket{job="test",le="5"} 9
+histogram_bucket{job="test",le="6"} 9
+histogram_bucket{job="test",le="7"} 9
+histogram_bucket{job="test",le="8"} 9
+histogram_bucket{job="test",le="9"} 9
+histogram_bucket{job="test",le="10"} 9
+histogram_bucket{job="test",le="+Inf"} 9
+histogram_sum{job="test"} 7
+histogram_count{job="test"} 2
+`
 	multilabel1 = `# HELP counter A counter
 # TYPE counter counter
 counter{a="a",b="b", ignore_label="ignore_value"} 1
@@ -121,6 +142,12 @@ ui_external_lib_loaded{name="mixpanel",loaded="true"} 1
 ui_external_lib_loaded{job="test",loaded="true",name="Intercom"} 2
 ui_external_lib_loaded{job="test",loaded="true",name="ga"} 2
 ui_external_lib_loaded{job="test",loaded="true",name="mixpanel"} 2
+`
+	gaugeOutputReplace = `# HELP ui_external_lib_loaded A gauge with entries in un-sorted order
+# TYPE ui_external_lib_loaded gauge
+ui_external_lib_loaded{job="test",loaded="true",name="Intercom"} 1
+ui_external_lib_loaded{job="test",loaded="true",name="ga"} 1
+ui_external_lib_loaded{job="test",loaded="true",name="mixpanel"} 1
 `
 	duplicateLabels = `
 # HELP ui_external_lib_loaded Test with duplicate values
@@ -179,17 +206,20 @@ func TestAggregate(t *testing.T) {
 		a, b          string
 		want          string
 		ignoredLabels []string
+		behavior      string
 	}{
-		{"simpleGauge", gaugeInput, gaugeInput, gaugeOutput, []string{}},
-		{"in", in1, in2, want, []string{}},
-		{"multilabel", multilabel1, multilabel2, multilabelResult, []string{"ignore_label"}},
-		{"labelFields", labelFields1, labelFields2, labelFieldResult, []string{}},
-		{"reorderedLabels", reorderedLabels1, reorderedLabels2, reorderedLabelsResult, []string{}},
-		{"ignoredLabels", ignoredLabels1, ignoredLabels2, ignoredLabelsResult, []string{"ignore_me"}},
-		{"summary", summaryInput, summaryInput, summaryOutput, []string{}},
+		{"simpleGauge", gaugeInput, gaugeInput, gaugeOutput, []string{}, "sum"},
+		{"simpleGaugeReplace", gaugeInput, gaugeInput, gaugeOutputReplace, []string{}, "replace"},
+		{"in", in1, in2, want, []string{}, "sum"},
+		{"inReplace", in1, in2, wantReplace, []string{}, "replace"},
+		{"multilabel", multilabel1, multilabel2, multilabelResult, []string{"ignore_label"}, "sum"},
+		{"labelFields", labelFields1, labelFields2, labelFieldResult, []string{}, "sum"},
+		{"reorderedLabels", reorderedLabels1, reorderedLabels2, reorderedLabelsResult, []string{}, "sum"},
+		{"ignoredLabels", ignoredLabels1, ignoredLabels2, ignoredLabelsResult, []string{"ignore_me"}, "sum"},
+		{"summary", summaryInput, summaryInput, summaryOutput, []string{}, "sum"},
 	} {
 		t.Run(c.testName, func(t *testing.T) {
-			agg := NewAggregate(AddIgnoredLabels(c.ignoredLabels...))
+			agg := NewAggregate(AddIgnoredLabels(c.ignoredLabels...), SetGaugeBehavior(c.behavior))
 
 			err := agg.parseAndMerge(strings.NewReader(c.a), testLabels)
 			require.NoError(t, err)

metrics/merge.go

@@ -71,7 +71,7 @@ func mergeBuckets(a, b []*dto.Bucket) []*dto.Bucket {
 	return output
 }
 
-func mergeMetric(ty dto.MetricType, a, b *dto.Metric) *dto.Metric {
+func mergeMetric(ty dto.MetricType, a, b *dto.Metric, options *aggregateOptions) *dto.Metric {
 	switch ty {
 	case dto.MetricType_COUNTER:
 		return &dto.Metric{
@@ -82,9 +82,14 @@ func mergeMetric(ty dto.MetricType, a, b *dto.Metric) *dto.Metric {
 		}
 
 	case dto.MetricType_GAUGE:
-		// No very meaningful way for us to merge gauges.  We'll sum them
-		// and clear out any gauges on scrape, as a best approximation, but
-		// this relies on client pushing with the same interval as we scrape.
+		if options.gaugeBehavior == replaceBehavior {
+			return &dto.Metric{
+				Label: a.Label,
+				Gauge: &dto.Gauge{
+					Value: float64ptr(*b.Gauge.Value),
+				},
+			}
+		}
 		return &dto.Metric{
 			Label: a.Label,
 			Gauge: &dto.Gauge{
@@ -143,7 +148,7 @@ func (mf *metricFamily) mergeFamily(b *dto.MetricFamily) error {
 			newMetric = append(newMetric, b.Metric[j])
 			j++
 		} else {
-			merged := mergeMetric(*mf.Type, mf.Metric[i], b.Metric[j])
+			merged := mergeMetric(*mf.Type, mf.Metric[i], b.Metric[j], mf.options)
 			if merged != nil {
 				newMetric = append(newMetric, merged)
 			}

routers/server.go

@@ -11,11 +11,11 @@ import (
 	"github.com/zapier/prom-aggregation-gateway/metrics"
 )
 
-func RunServers(cfg ApiRouterConfig, apiListen string, lifecycleListen string) {
+func RunServers(cfg ApiRouterConfig, apiListen string, lifecycleListen string, gaugeBehavior string) {
 	sigChannel := make(chan os.Signal, 1)
 	signal.Notify(sigChannel, syscall.SIGTERM, syscall.SIGINT)
 
-	agg := metrics.NewAggregate()
+	agg := metrics.NewAggregate(metrics.SetGaugeBehavior(gaugeBehavior))
 
 	promMetricsConfig := promMetrics.Config{
 		Registry: metrics.PromRegistry,