fix(origin): reduce blobrefresh namespace metric tag cardinality (#575)

Author: thijmv

lib/blobrefresh/refresher.go

@@ -16,6 +16,7 @@ package blobrefresh
 import (
 	"errors"
 	"fmt"
+	"strings"
 	"time"
 
 	"github.com/uber/kraken/core"
@@ -112,14 +113,13 @@ func (r *Refresher) Refresh(namespace string, d core.Digest, hooks ...PostHook)
 			return err
 		}
 		t := time.Since(start)
-		r.stats.Tagged(map[string]string{"namespace": namespace}).
-			Timer("download_remote_blob").Record(t)
+		stats := r.stats.Tagged(map[string]string{"namespace": extractPrefix(namespace)})
+		stats.Timer("download_remote_blob").Record(t)
+		stats.Counter("downloads").Inc(1)
 		log.With(
 			"namespace", namespace,
 			"name", d.Hex(),
 			"download_time", t).Info("Downloaded remote blob")
-		r.stats.Tagged(map[string]string{"namespace": namespace}).
-			Counter("downloads").Inc(1)
 		for _, h := range hooks {
 			h.Run(d)
 		}
@@ -143,3 +143,14 @@ func (r *Refresher) download(client backend.Client, namespace string, d core.Dig
 		return client.Download(namespace, name, w)
 	}, pieceLength)
 }
+
+// extractPrefix extracts the prefix from the namespace to decrease the cardinality of metrics.
+func extractPrefix(namespace string) string {
+	if prefix, _, ok := strings.Cut(namespace, "/"); ok {
+		return prefix
+	}
+	if prefix, _, ok := strings.Cut(namespace, "_"); ok {
+		return prefix
+	}
+	return namespace
+}

lib/blobrefresh/refresher_test.go

@@ -212,3 +212,41 @@ func TestRefreshWithMemoryCache(t *testing.T) {
 	require.NoError(err)
 	require.Equal(blob.Content, diskData)
 }
+
+func TestExtractPrefix(t *testing.T) {
+	tests := map[string]struct {
+		give string
+		want string
+	}{
+		"slash delimiter returns prefix": {
+			give: "foo/bar",
+			want: "foo",
+		},
+		"underscore delimiter returns prefix": {
+			give: "foo_bar",
+			want: "foo",
+		},
+		"no delimiter returns whole string": {
+			give: "foo",
+			want: "foo",
+		},
+		"empty prefix returns empty string": {
+			give: "/foo",
+			want: "",
+		},
+		"empty string returns empty string": {
+			give: "",
+			want: "",
+		},
+	}
+
+	for name, tt := range tests {
+		t.Run(name, func(t *testing.T) {
+			require := require.New(t)
+
+			prefix := extractPrefix(tt.give)
+
+			require.Equal(tt.want, prefix)
+		})
+	}
+}