(2.12) Optimised meta snapshot encoding when GOEXPERIMENT=jsonv2 enabled

scripts/runTestsOnTravis.sh

@@ -2,6 +2,8 @@
 
 set -ex
 
+export GOEXPERIMENT=jsonv2
+
 if [ "$1" = "compile" ]; then
     # First check that NATS builds.
     go build -v;

server/events.go

@@ -315,7 +315,7 @@ type ClientInfo struct {
 	Name       string        `json:"name,omitempty"`
 	Lang       string        `json:"lang,omitempty"`
 	Version    string        `json:"ver,omitempty"`
-	RTT        time.Duration `json:"rtt,omitempty"`
+	RTT        time.Duration `json:"rtt,omitempty,format:units"`
 	Server     string        `json:"server,omitempty"`
 	Cluster    string        `json:"cluster,omitempty"`
 	Alternates []string      `json:"alts,omitempty"`

server/jetstream_cluster.go

@@ -31,6 +31,7 @@ import (
 	"strings"
 	"sync/atomic"
 	"time"
+	"weak"
 
 	"github.com/klauspost/compress/s2"
 	"github.com/minio/highwayhash"
@@ -69,6 +70,9 @@ type jetStreamCluster struct {
 	peerStreamCancelMove *subscription
 	// To pop out the monitorCluster before the raft layer.
 	qch chan struct{}
+	// Meta snapshot encode buffers. Weakly held so GC can collect at any time, but
+	// can help us optimistically to reduce allocations on future snapshot calls.
+	lastsnap weak.Pointer[bytes.Buffer] // nolint:unused
 }
 
 // Used to track inflight stream add requests to properly re-use same group and sync subject.
@@ -1584,7 +1588,7 @@ func (js *jetStream) metaSnapshot() ([]byte, error) {
 
 	// Track how long it took to marshal the JSON
 	mstart := time.Now()
-	b, err := json.Marshal(streams)
+	b, err := js.metaSnapshotJSON(streams)
 	mend := time.Since(mstart)
 
 	js.mu.RUnlock()

server/jetstream_cluster_snap_v1.go

@@ -0,0 +1,24 @@
+// Copyright 2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+//go:build !goexperiment.jsonv2
+
+package server
+
+import (
+	"encoding/json"
+)
+
+func (js *jetStream) metaSnapshotJSON(streams []writeableStreamAssignment) ([]byte, error) {
+	return json.Marshal(streams)
+}

server/jetstream_cluster_snap_v2.go

@@ -0,0 +1,35 @@
+// Copyright 2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+//go:build goexperiment.jsonv2
+
+package server
+
+import (
+	"bytes"
+	jsonv2 "encoding/json/v2"
+	"weak"
+)
+
+func (js *jetStream) metaSnapshotJSON(streams []writeableStreamAssignment) ([]byte, error) {
+	b := js.cluster.lastsnap.Value()
+	if b == nil {
+		b = bytes.NewBuffer(nil)
+		js.cluster.lastsnap = weak.Make(b)
+	}
+	b.Reset()
+	if err := jsonv2.MarshalWrite(b, streams); err != nil {
+		return nil, err
+	}
+	return b.Bytes(), nil
+}