web: deduplicate series in federation

storage/interface.go

@@ -104,6 +104,13 @@ type dedupedSeriesSet struct {
 // DeduplicateSeriesSet merges two SeriesSet and removes duplicates.
 // If two series exist in both sets, their datapoints must be equal.
 func DeduplicateSeriesSet(a, b SeriesSet) SeriesSet {
+	if a == nil {
+		return b
+	}
+	if b == nil {
+		return a
+	}
+
 	s := &dedupedSeriesSet{a: a, b: b}
 	s.adone = !s.a.Next()
 	s.bdone = !s.b.Next()

web/api/v1/api.go

@@ -340,12 +340,8 @@ func (api *API) series(r *http.Request) (interface{}, *apiError) {
 	var set storage.SeriesSet
 
 	for _, mset := range matcherSets {
-		if set == nil {
-			set = q.Select(mset...)
-		} else {
 		set = storage.DeduplicateSeriesSet(set, q.Select(mset...))
 	}
-	}
 
 	metrics := []labels.Labels{}
 

web/federate.go

@@ -69,15 +69,20 @@ func (h *Handler) federation(w http.ResponseWriter, req *http.Request) {
 	}
 	defer q.Close()
 
-	// TODO(fabxc): expose merge functionality in storage interface.
-	// We just concatenate results for all sets of matchers, which may produce
-	// duplicated results.
 	vec := make(promql.Vector, 0, 8000)
 
+	var set storage.SeriesSet
+
 	for _, mset := range matcherSets {
-		series := q.Select(mset...)
+		set = storage.DeduplicateSeriesSet(set, q.Select(mset...))
-		for series.Next() {
+	}
-			s := series.At()
+	if set == nil {
+		return
+	}
+
+	for set.Next() {
+		s := set.At()
+
 		// TODO(fabxc): allow fast path for most recent sample either
 		// in the storage itself or caching layer in Prometheus.
 		it := storage.NewBuffer(s.Iterator(), int64(promql.StalenessDelta/1e6))
@@ -100,12 +105,11 @@ func (h *Handler) federation(w http.ResponseWriter, req *http.Request) {
 			Point:  promql.Point{T: t, V: v},
 		})
 	}
-		if series.Err() != nil {
+	if set.Err() != nil {
 		federationErrors.Inc()
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
-	}
 
 	sort.Sort(byName(vec))