web: add basic federation support.

This commit adds a federation handler on /federate. It accepts `match[]`
query parameters containing vector selectors. Their intersection determines
the in-memory metrics that are returned in the same way as the
/metrics endpoint does (modulo sorting).

web/federate.go

@@ -0,0 +1,108 @@
+package web
+
+import (
+	"io"
+	"net/http"
+
+	"bitbucket.org/ww/goautoneg"
+	"github.com/golang/protobuf/proto"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/text"
+
+	"github.com/prometheus/prometheus/promql"
+	"github.com/prometheus/prometheus/storage/local"
+
+	clientmodel "github.com/prometheus/client_golang/model"
+	dto "github.com/prometheus/client_model/go"
+)
+
+type Federation struct {
+	Storage local.Storage
+}
+
+func (fed *Federation) ServeHTTP(w http.ResponseWriter, req *http.Request) {
+	req.ParseForm()
+
+	metrics := map[clientmodel.Fingerprint]clientmodel.COWMetric{}
+
+	for _, s := range req.Form["match[]"] {
+		matchers, err := promql.ParseMetricSelector(s)
+		if err != nil {
+			http.Error(w, err.Error(), http.StatusBadRequest)
+			return
+		}
+		for fp, met := range fed.Storage.MetricsForLabelMatchers(matchers...) {
+			metrics[fp] = met
+		}
+	}
+
+	enc, contentType := chooseEncoder(req)
+	w.Header().Set("Content-Type", contentType)
+
+	protMetric := &dto.Metric{
+		Label:   []*dto.LabelPair{},
+		Untyped: &dto.Untyped{},
+	}
+	protMetricFam := &dto.MetricFamily{
+		Metric: []*dto.Metric{protMetric},
+		Type:   dto.MetricType_UNTYPED.Enum(),
+	}
+
+	for fp, met := range metrics {
+		sp := fed.Storage.LastSamplePairForFingerprint(fp)
+		if sp == nil {
+			continue
+		}
+
+		// Reset label slice.
+		protMetric.Label = protMetric.Label[:0]
+
+		for ln, lv := range met.Metric {
+			if ln == clientmodel.MetricNameLabel {
+				protMetricFam.Name = proto.String(string(lv))
+				continue
+			}
+			protMetric.Label = append(protMetric.Label, &dto.LabelPair{
+				Name:  proto.String(string(ln)),
+				Value: proto.String(string(lv)),
+			})
+		}
+		protMetric.TimestampMs = (*int64)(&sp.Timestamp)
+		protMetric.Untyped.Value = (*float64)(&sp.Value)
+
+		if _, err := enc(w, protMetricFam); err != nil {
+			http.Error(w, err.Error(), http.StatusInternalServerError)
+			return
+		}
+	}
+}
+
+type encoder func(w io.Writer, p *dto.MetricFamily) (int, error)
+
+func chooseEncoder(req *http.Request) (encoder, string) {
+	accepts := goautoneg.ParseAccept(req.Header.Get("Accept"))
+	for _, accept := range accepts {
+		switch {
+		case accept.Type == "application" &&
+			accept.SubType == "vnd.google.protobuf" &&
+			accept.Params["proto"] == "io.prometheus.client.MetricFamily":
+			switch accept.Params["encoding"] {
+			case "delimited":
+				return text.WriteProtoDelimited, prometheus.DelimitedTelemetryContentType
+			case "text":
+				return text.WriteProtoText, prometheus.ProtoTextTelemetryContentType
+			case "compact-text":
+				return text.WriteProtoCompactText, prometheus.ProtoCompactTextTelemetryContentType
+			default:
+				continue
+			}
+		case accept.Type == "text" &&
+			accept.SubType == "plain" &&
+			(accept.Params["version"] == "0.0.4" || accept.Params["version"] == ""):
+			return text.MetricFamilyToText, prometheus.TextTelemetryContentType
+		default:
+			continue
+		}
+	}
+	return text.MetricFamilyToText, prometheus.TextTelemetryContentType
+}

web/web.go

@@ -56,6 +56,7 @@ type Handler struct {
 
 	apiV1      *v1.API
 	apiLegacy  *legacy.API
+	federation *Federation
 
 	router     *route.Router
 	quitCh     chan struct{}
@@ -125,6 +126,9 @@ func New(st local.Storage, qe *promql.Engine, rm *rules.Manager, status *Prometh
 			Storage:     st,
 			Now:         clientmodel.Now,
 		},
+		federation: &Federation{
+			Storage: st,
+		},
 	}
 
 	if o.PathPrefix != "" {
@@ -145,6 +149,7 @@ func New(st local.Storage, qe *promql.Engine, rm *rules.Manager, status *Prometh
 
 	router.Get("/heap", instrf("heap", dumpHeap))
 
+	router.Get("/federate", instrh("federate", h.federation))
 	router.Get(o.MetricsPath, prometheus.Handler().ServeHTTP)
 
 	h.apiLegacy.Register(router.WithPrefix("/api"))