snapshot of https://github.com/grpc/grpc-go.git (master, 2026-09-15; blobs > 131072B omitted)

internal/xds/balancer/clusterimpl/clusterimpl.go

@@ -0,0 +1,644 @@
+/*
+ *
+ * Copyright 2020 gRPC 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.
+ *
+ */
+
+// Package clusterimpl implements the xds_cluster_impl balancing policy. It
+// handles the cluster features (e.g. circuit_breaking, RPC dropping).
+//
+// Note that it doesn't handle name resolution, which is done by policy
+// xds_cluster_resolver.
+package clusterimpl
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net"
+	"slices"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"google.golang.org/grpc/balancer"
+	"google.golang.org/grpc/balancer/weightedroundrobin"
+	"google.golang.org/grpc/connectivity"
+	"google.golang.org/grpc/credentials"
+	"google.golang.org/grpc/credentials/tls/certprovider"
+	"google.golang.org/grpc/internal/balancer/gracefulswitch"
+	"google.golang.org/grpc/internal/credentials/xds"
+	"google.golang.org/grpc/internal/grpclog"
+	"google.golang.org/grpc/internal/grpcsync"
+	"google.golang.org/grpc/internal/pretty"
+	xdsinternal "google.golang.org/grpc/internal/xds"
+
+	"google.golang.org/grpc/internal/xds/balancer/clusterimpl/internal"
+	"google.golang.org/grpc/internal/xds/balancer/loadstore"
+	"google.golang.org/grpc/internal/xds/bootstrap"
+	"google.golang.org/grpc/internal/xds/clients"
+	"google.golang.org/grpc/internal/xds/clients/lrsclient"
+	"google.golang.org/grpc/internal/xds/xdsclient"
+	"google.golang.org/grpc/internal/xds/xdsclient/xdsresource"
+	"google.golang.org/grpc/resolver"
+	"google.golang.org/grpc/serviceconfig"
+)
+
+const (
+	// Name is the name of the cluster_impl balancer.
+	Name                   = "xds_cluster_impl_experimental"
+	defaultRequestCountMax = 1024
+	loadStoreStopTimeout   = 1 * time.Second
+)
+
+var (
+	// Below function is no-op in actual code, but can be overridden in
+	// tests to give tests visibility into exactly when certain events happen.
+	clientConnUpdateHook = func() {}
+	pickerUpdateHook     = func() {}
+	buildProvider        = buildProviderFunc
+)
+
+func init() {
+	balancer.Register(bb{})
+}
+
+type bb struct{}
+
+func (bb) Build(cc balancer.ClientConn, bOpts balancer.BuildOptions) balancer.Balancer {
+	b := &clusterImplBalancer{
+		ClientConn:      cc,
+		loadWrapper:     loadstore.NewWrapper(),
+		requestCountMax: defaultRequestCountMax,
+	}
+	b.xdsHIPtr.Store(xds.NewHandshakeInfo(nil, nil, nil, "", false, false, false))
+	b.logger = prefixLogger(b)
+	b.child = gracefulswitch.NewBalancer(b, bOpts)
+	b.logger.Infof("Created")
+
+	var creds credentials.TransportCredentials
+	switch {
+	case bOpts.DialCreds != nil:
+		creds = bOpts.DialCreds
+	case bOpts.CredsBundle != nil:
+		creds = bOpts.CredsBundle.TransportCredentials()
+	}
+	if xc, ok := creds.(interface{ UsesXDS() bool }); ok && xc.UsesXDS() {
+		b.xdsCredsInUse = true
+	}
+	b.logger.Infof("xDS credentials in use: %v", b.xdsCredsInUse)
+	return b
+}
+
+func (bb) Name() string {
+	return Name
+}
+
+func (bb) ParseConfig(c json.RawMessage) (serviceconfig.LoadBalancingConfig, error) {
+	return parseConfig(c)
+}
+
+type clusterImplBalancer struct {
+	balancer.ClientConn
+
+	// The following fields are set at creation time, and are read-only after
+	// that, and therefore need not be protected by a mutex.
+	logger *grpclog.PrefixLogger
+	// TODO: #8366 -  Refactor usage of loadWrapper to easily plugin a test
+	// load reporter from tests.
+	loadWrapper *loadstore.Wrapper
+
+	// The following fields are only accessed from balancer API methods, which
+	// are guaranteed to be called serially by gRPC.
+	xdsClient        xdsclient.XDSClient     // Sent down in ResolverState attributes.
+	cancelLoadReport func(context.Context)   // To stop reporting load through the above xDS client.
+	edsServiceName   string                  // EDS service name to report load for.
+	lrsServer        *bootstrap.ServerConfig // Load reporting server configuration.
+	dropCategories   []DropConfig            // The categories for drops.
+	child            *gracefulswitch.Balancer
+	xdsHIPtr         atomic.Pointer[grpcsync.RefCounted[*xds.HandshakeInfo]] // Accessed atomically as it is shared between the balancer and the transport.
+	xdsCredsInUse    bool
+
+	// The active security configuration received from the control plane.
+	securityConfig *xdsresource.SecurityConfig
+
+	// The following fields are protected by mu, since they are accessed in
+	// balancer API methods and in methods called from the child policy.
+	mu                       sync.Mutex
+	clusterName              string                                      // The cluster name for credentials handshaking.
+	inhibitPickerUpdates     bool                                        // Inhibits state updates from child policy when processing an update from the parent.
+	pendingPickerUpdates     bool                                        // True if a picker update from the child policy was inhibited when processing an update from the parent.
+	childState               balancer.State                              // Most recent state update from the child policy.
+	drops                    []*dropper                                  // Drops implementation.
+	requestCounterCluster    string                                      // The cluster name for the request counter, from LB config.
+	requestCounterService    string                                      // The service name for the request counter, from LB config.
+	requestCountMax          uint32                                      // Max concurrent requests, from LB config.
+	requestCounter           *xdsclient.ClusterRequestsCounter           // Tracks total inflight requests for a given service.
+	telemetryLabels          map[string]string                           // Telemetry labels to set on picks, from LB config.
+	lrsReportEndpointMetrics *xdsresource.LRSReportEndpointMetricsConfig // LRS metrics to propagate.
+}
+
+// dropRequestsPerMillion scales a drop overload's numerator and denominator to
+// a number of requests to drop per million. numerator can be as large as a
+// million (a 100% drop), so the multiplication is done in uint64 to avoid
+// overflowing a uint32, and the result is capped at a million because a drop
+// ratio above 100% is treated as 100%.
+func dropRequestsPerMillion(numerator, denominator uint32) uint32 {
+	rpm := uint64(numerator) * million / uint64(denominator)
+	if rpm > million {
+		rpm = million
+	}
+	return uint32(rpm)
+}
+
+// handleClusterConfigLocked updates the internal state of the balancer with the
+// new cluster configuration. It returns true if a new picker needs to be
+// generated as a result of these changes. It must be called with b.mu held.
+func (b *clusterImplBalancer) handleClusterConfigLocked(clusterConfig xdsresource.ClusterConfig) bool {
+	clusterUpdate := clusterConfig.Cluster
+	var updatePicker bool
+
+	b.telemetryLabels = clusterUpdate.TelemetryLabels
+
+	if !b.lrsReportEndpointMetrics.Equal(clusterUpdate.LRSReportEndpointMetrics) {
+		b.lrsReportEndpointMetrics = clusterUpdate.LRSReportEndpointMetrics
+		updatePicker = true
+	}
+
+	var newDrops []DropConfig
+	if clusterUpdate.ClusterType == xdsresource.ClusterTypeEDS {
+		edsUpdate := clusterConfig.EndpointConfig.EDSUpdate
+		newDrops = make([]DropConfig, 0, len(edsUpdate.Drops))
+		for _, d := range edsUpdate.Drops {
+			newDrops = append(newDrops, DropConfig{
+				Category:           d.Category,
+				RequestsPerMillion: dropRequestsPerMillion(d.Numerator, d.Denominator),
+			})
+		}
+		if !slices.Equal(b.dropCategories, newDrops) {
+			b.dropCategories = newDrops
+			b.drops = make([]*dropper, 0, len(newDrops))
+			for _, c := range newDrops {
+				b.drops = append(b.drops, newDropper(c))
+			}
+			updatePicker = true
+		}
+	}
+
+	if b.requestCounterCluster != clusterUpdate.ClusterName || b.requestCounterService != clusterUpdate.EDSServiceName {
+		b.requestCounterCluster = clusterUpdate.ClusterName
+		b.requestCounterService = clusterUpdate.EDSServiceName
+		b.requestCounter = xdsclient.GetClusterRequestsCounter(clusterUpdate.ClusterName, clusterUpdate.EDSServiceName)
+		updatePicker = true
+	}
+	var newRequestCountMax uint32 = 1024
+	if clusterUpdate.MaxRequests != nil {
+		newRequestCountMax = *clusterUpdate.MaxRequests
+	}
+	if b.requestCountMax != newRequestCountMax {
+		b.requestCountMax = newRequestCountMax
+		updatePicker = true
+	}
+
+	return updatePicker
+}
+
+func (b *clusterImplBalancer) newPickerLocked() *picker {
+	return &picker{
+		drops:           b.drops,
+		s:               b.childState,
+		loadStore:       b.loadWrapper,
+		counter:         b.requestCounter,
+		countMax:        b.requestCountMax,
+		telemetryLabels: b.telemetryLabels,
+		clusterName:     b.clusterName,
+		metrics:         b.lrsReportEndpointMetrics,
+	}
+}
+
+// updateLoadStore checks the config for load store, and decides whether it
+// needs to restart the load reporting stream.
+func (b *clusterImplBalancer) updateLoadStore(clusterUpdate *xdsresource.ClusterUpdate) error {
+	var updateLoadClusterAndService bool
+
+	// ClusterName is different, restart. ClusterName is from ClusterName and
+	// EDSServiceName.
+	clusterName := b.getClusterName()
+	if clusterName != clusterUpdate.ClusterName {
+		updateLoadClusterAndService = true
+		b.setClusterName(clusterUpdate.ClusterName)
+		clusterName = clusterUpdate.ClusterName
+	}
+	if b.edsServiceName != clusterUpdate.EDSServiceName {
+		updateLoadClusterAndService = true
+		b.edsServiceName = clusterUpdate.EDSServiceName
+	}
+	if updateLoadClusterAndService {
+		// This updates the clusterName and serviceName that will be reported
+		// for the loads. The update here is too early, the perfect timing is
+		// when the picker is updated with the new connection. But from this
+		// balancer's point of view, it's impossible to tell.
+		//
+		// On the other hand, this will almost never happen. Each LRS policy
+		// shouldn't get updated config. The parent should do a graceful switch
+		// when the clusterName or serviceName is changed.
+		b.loadWrapper.UpdateClusterAndService(clusterName, b.edsServiceName)
+	}
+
+	var (
+		stopOldLoadReport  bool
+		startNewLoadReport bool
+	)
+
+	// Check if it's necessary to restart load report.
+	if b.lrsServer == nil {
+		if clusterUpdate.LRSServerConfig != nil {
+			// Old is nil, new is not nil, start new LRS.
+			b.lrsServer = clusterUpdate.LRSServerConfig
+			startNewLoadReport = true
+		}
+		// Old is nil, new is nil, do nothing.
+	} else if clusterUpdate.LRSServerConfig == nil {
+		// Old is not nil, new is nil, stop old, don't start new.
+		b.lrsServer = nil
+		stopOldLoadReport = true
+	} else {
+		// Old is not nil, new is not nil, compare string values, if
+		// different, stop old and start new.
+		if !b.lrsServer.Equal(clusterUpdate.LRSServerConfig) {
+			b.lrsServer = clusterUpdate.LRSServerConfig
+			stopOldLoadReport = true
+			startNewLoadReport = true
+		}
+	}
+
+	if stopOldLoadReport {
+		if b.cancelLoadReport != nil {
+			stopCtx, stopCancel := context.WithTimeout(context.Background(), loadStoreStopTimeout)
+			defer stopCancel()
+			b.cancelLoadReport(stopCtx)
+			b.cancelLoadReport = nil
+			if !startNewLoadReport {
+				// If a new LRS stream will be started later, no need to update
+				// it to nil here.
+				b.loadWrapper.UpdateLoadStore(nil)
+			}
+		}
+	}
+	if startNewLoadReport {
+		var loadStore *lrsclient.LoadStore
+		if b.xdsClient != nil {
+			loadStore, b.cancelLoadReport = b.xdsClient.ReportLoad(b.lrsServer)
+		}
+		b.loadWrapper.UpdateLoadStore(loadStore)
+	}
+
+	return nil
+}
+
+func buildProviderFunc(configs map[string]*certprovider.BuildableConfig, instanceName, certName string, wantIdentity, wantRoot bool) (certprovider.Provider, error) {
+	cfg := configs[instanceName]
+	provider, err := cfg.Build(certprovider.BuildOptions{
+		CertName:     certName,
+		WantIdentity: wantIdentity,
+		WantRoot:     wantRoot,
+	})
+	if err != nil {
+		// This error is not expected since the bootstrap process parses the
+		// config and makes sure that it is acceptable to the plugin. Still, it
+		// is possible that the plugin parses the config successfully, but its
+		// Build() method errors out.
+		return nil, fmt.Errorf("xds: failed to get security plugin instance (%+v): %v", cfg, err)
+	}
+	return provider, nil
+}
+
+func (b *clusterImplBalancer) buildProviders(config *xdsresource.SecurityConfig) (certprovider.Provider, certprovider.Provider, error) {
+	cpc := b.xdsClient.BootstrapConfig().CertProviderConfigs()
+	var rootProvider certprovider.Provider
+	if config.UseSystemRootCerts {
+		rootProvider = systemRootCertsProvider{}
+	} else {
+		rp, err := buildProvider(cpc, config.RootInstanceName, config.RootCertName, false, true)
+		if err != nil {
+			return nil, nil, err
+		}
+		rootProvider = rp
+	}
+
+	var identityProvider certprovider.Provider
+	if name, cert := config.IdentityInstanceName, config.IdentityCertName; name != "" {
+		ip, err := buildProvider(cpc, name, cert, true, false)
+		if err != nil {
+			if rootProvider != nil {
+				rootProvider.Close()
+			}
+			return nil, nil, err
+		}
+		identityProvider = ip
+	}
+	return rootProvider, identityProvider, nil
+}
+
+// handleSecurityConfig processes the security configuration received from the
+// management server, creates appropriate certificate provider plugins, and
+// updates the HandshakeInfo which is added as an address attribute in
+// NewSubConn() calls.
+func (b *clusterImplBalancer) handleSecurityConfig(config *xdsresource.SecurityConfig) error {
+	// If xdsCredentials are not in use, i.e, the user did not want to get
+	// security configuration from an xDS server, we should not be acting on the
+	// received security config here. Doing so poses a security threat.
+	if !b.xdsCredsInUse || config.Equal(b.securityConfig) {
+		return nil
+	}
+
+	// Security config being nil is a valid case where the management server has
+	// not sent any security configuration. The xdsCredentials implementation
+	// handles this by delegating to its fallback credentials.
+	if config == nil {
+		b.securityConfig = nil
+		oldHI := b.xdsHIPtr.Swap(xds.NewHandshakeInfo(nil, nil, nil, "", false, false, false))
+		if oldHI != nil {
+			oldHI.Decrement()
+		}
+		return nil
+	}
+
+	rootProvider, identityProvider, err := b.buildProviders(config)
+	if err != nil {
+		return err
+	}
+
+	newHI := xds.NewHandshakeInfo(rootProvider, identityProvider, config.SubjectAltNameMatchers, config.SNI, false, config.AutoSNISANValidation, config.UseAutoHostSNI)
+	b.securityConfig = config
+	oldHI := b.xdsHIPtr.Swap(newHI)
+	if oldHI != nil {
+		oldHI.Decrement()
+	}
+	return nil
+}
+
+func (b *clusterImplBalancer) UpdateClientConnState(s balancer.ClientConnState) error {
+	defer clientConnUpdateHook()
+
+	b.mu.Lock()
+	b.inhibitPickerUpdates = true
+	b.mu.Unlock()
+	if b.logger.V(2) {
+		b.logger.Infof("Received configuration: %s", pretty.ToJSON(s.BalancerConfig))
+	}
+	newConfig, ok := s.BalancerConfig.(*LBConfig)
+	if !ok {
+		return fmt.Errorf("unexpected balancer config with type: %T", s.BalancerConfig)
+	}
+
+	// Need to check for potential errors at the beginning of this function, so
+	// that on errors, we reject the whole config, instead of applying part of
+	// it.
+	bb := balancer.Get(newConfig.ChildPolicy.Name)
+	if bb == nil {
+		return fmt.Errorf("child policy %q not registered", newConfig.ChildPolicy.Name)
+	}
+
+	if b.xdsClient == nil {
+		c := xdsclient.FromResolverState(s.ResolverState)
+		if c == nil {
+			return balancer.ErrBadResolverState
+		}
+		b.xdsClient = c
+	}
+
+	xdsConfig := xdsresource.XDSConfigFromResolverState(s.ResolverState)
+	if xdsConfig == nil {
+		b.logger.Warningf("Received balancer config with no xDS config")
+		return balancer.ErrBadResolverState
+	}
+	clusterCfg := xdsConfig.Clusters[newConfig.Cluster]
+	clusterUpdate := clusterCfg.Config.Cluster
+	if err := b.handleSecurityConfig(clusterUpdate.SecurityCfg); err != nil {
+		// If the security config is invalid, for example, if the provider
+		// instance is not found in the bootstrap config, we need to put the
+		// channel in transient failure.
+		return fmt.Errorf("received Cluster resource that contains invalid security config: %v", err)
+
+	}
+	// Update load reporting config. This needs to be done before updating the
+	// child policy because we need the loadStore from the updated client to be
+	// passed to the ccWrapper, so that the next picker from the child policy
+	// will pick up the new loadStore.
+	if err := b.updateLoadStore(clusterUpdate); err != nil {
+		return err
+	}
+
+	// Build config for the gracefulswitch balancer. It is safe to ignore JSON
+	// marshaling errors here, since the config was already validated as part of
+	// ParseConfig().
+	cfg := []map[string]any{{newConfig.ChildPolicy.Name: newConfig.ChildPolicy.Config}}
+	cfgJSON, _ := json.Marshal(cfg)
+	parsedCfg, err := gracefulswitch.ParseConfig(cfgJSON)
+	if err != nil {
+		return err
+	}
+
+	// Addresses and sub-balancer config are sent to sub-balancer.
+	err = b.child.UpdateClientConnState(balancer.ClientConnState{
+		ResolverState:  weightedroundrobin.SetBackendService(s.ResolverState, b.clusterName),
+		BalancerConfig: parsedCfg,
+	})
+
+	b.mu.Lock()
+	updatePicker := b.handleClusterConfigLocked(clusterCfg.Config)
+	// We want to send a picker update to the parent if one of the two
+	// conditions are met:
+	// - drop/request count config or LRS metrics config has changed *and* there
+	//   is already a picker from the child, or
+	// - there is a pending picker update from the child (and this covers the
+	//   case where the drop/request config has not changed, but the child sent
+	//   a picker update while we were still processing config from our parent).
+	if (updatePicker && b.childState.Picker != nil) || b.pendingPickerUpdates {
+		b.pendingPickerUpdates = false
+		b.ClientConn.UpdateState(balancer.State{
+			ConnectivityState: b.childState.ConnectivityState,
+			Picker:            b.newPickerLocked(),
+		})
+	}
+	b.inhibitPickerUpdates = false
+	b.mu.Unlock()
+	pickerUpdateHook()
+	return err
+}
+
+func (b *clusterImplBalancer) ResolverError(err error) {
+	b.child.ResolverError(err)
+}
+
+func (b *clusterImplBalancer) updateSubConnState(_ balancer.SubConn, s balancer.SubConnState, cb func(balancer.SubConnState)) {
+	// Trigger re-resolution when a SubConn turns transient failure. This is
+	// necessary for the LogicalDNS in cluster_resolver policy to re-resolve.
+	//
+	// Note that this happens not only for the addresses from DNS, but also for
+	// EDS (cluster_impl doesn't know if it's DNS or EDS, only the parent
+	// knows). The parent priority policy is configured to ignore re-resolution
+	// signal from the EDS children.
+	if s.ConnectivityState == connectivity.TransientFailure {
+		b.ClientConn.ResolveNow(resolver.ResolveNowOptions{})
+	}
+
+	if cb != nil {
+		cb(s)
+	}
+}
+
+func (b *clusterImplBalancer) UpdateSubConnState(sc balancer.SubConn, s balancer.SubConnState) {
+	b.logger.Errorf("UpdateSubConnState(%v, %+v) called unexpectedly", sc, s)
+}
+
+func (b *clusterImplBalancer) Close() {
+	b.child.Close()
+	b.childState = balancer.State{}
+
+	if b.cancelLoadReport != nil {
+		stopCtx, stopCancel := context.WithTimeout(context.Background(), loadStoreStopTimeout)
+		defer stopCancel()
+		b.cancelLoadReport(stopCtx)
+		b.cancelLoadReport = nil
+	}
+	oldHI := b.xdsHIPtr.Swap(nil)
+	if oldHI != nil {
+		oldHI.Decrement()
+	}
+	b.securityConfig = nil
+	b.logger.Infof("Shutdown")
+}
+
+func (b *clusterImplBalancer) ExitIdle() {
+	b.child.ExitIdle()
+}
+
+// Override methods to accept updates from the child LB.
+
+func (b *clusterImplBalancer) UpdateState(state balancer.State) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	// Inhibit sending a picker update to our parent as part of handling new
+	// state from the child, if we are currently handling an update from our
+	// parent. Update the childState field regardless.
+	b.childState = state
+	if b.inhibitPickerUpdates {
+		b.pendingPickerUpdates = true
+		if b.logger.V(2) {
+			b.logger.Infof("Received a picker update from the child when processing an update from the parent")
+		}
+		return
+	}
+
+	b.ClientConn.UpdateState(balancer.State{
+		ConnectivityState: state.ConnectivityState,
+		Picker:            b.newPickerLocked(),
+	})
+	pickerUpdateHook()
+}
+
+func (b *clusterImplBalancer) setClusterName(n string) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	b.clusterName = n
+}
+
+func (b *clusterImplBalancer) getClusterName() string {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	return b.clusterName
+}
+
+// scWrapper is a wrapper of SubConn with locality ID. The locality ID can be
+// retrieved from the addresses when creating SubConn.
+//
+// All SubConns passed to the child policies are wrapped in this, so that the
+// picker can get the localityID from the picked SubConn, and do load reporting.
+//
+// After wrapping, all SubConns to and from the parent ClientConn (e.g. for
+// SubConn state update, update/remove SubConn) must be the original SubConns.
+// All SubConns to and from the child policy (NewSubConn, forwarding SubConn
+// state update) must be the wrapper. The balancer keeps a map from the original
+// SubConn to the wrapper for this purpose.
+type scWrapper struct {
+	balancer.SubConn
+
+	localityID clients.Locality
+
+	hostname string
+}
+
+func (b *clusterImplBalancer) NewSubConn(addrs []resolver.Address, opts balancer.NewSubConnOptions) (balancer.SubConn, error) {
+	clusterName := b.getClusterName()
+	newAddrs := make([]resolver.Address, len(addrs))
+	for i, addr := range addrs {
+		newAddrs[i] = xdsinternal.SetXDSHandshakeClusterName(addr, clusterName)
+		newAddrs[i] = xds.SetHandshakeInfo(newAddrs[i], &b.xdsHIPtr)
+
+		host := xdsresource.Hostname(addr)
+		// If the hostname contains a port, strip it. Per [RFC 6066, Section
+		// 3](https://www.rfc-editor.org/rfc/rfc6066.html#section-3), the SNI
+		// may only contain a qualified DNS hostname, which excludes port
+		// numbers.
+		h, _, err := net.SplitHostPort(host)
+		if err == nil {
+			host = h
+		}
+		// Store hostname in the address attributes, so that it can be used in
+		// the client handshake.
+		newAddrs[i] = xds.SetAddressHostname(newAddrs[i], host)
+	}
+	var sc balancer.SubConn
+	scw := &scWrapper{}
+	if len(addrs) > 0 {
+		scw.hostname = xdsresource.Hostname(addrs[0])
+		scw.localityID = xdsinternal.GetLocalityID(addrs[0])
+	}
+	oldListener := opts.StateListener
+	opts.StateListener = func(state balancer.SubConnState) {
+		b.updateSubConnState(sc, state, oldListener)
+	}
+	sc, err := b.ClientConn.NewSubConn(newAddrs, opts)
+	if err != nil {
+		return nil, err
+	}
+	scw.SubConn = sc
+	return scw, nil
+}
+
+func (b *clusterImplBalancer) RemoveSubConn(sc balancer.SubConn) {
+	b.logger.Errorf("RemoveSubConn(%v) called unexpectedly", sc)
+}
+
+func (b *clusterImplBalancer) UpdateAddresses(sc balancer.SubConn, _ []resolver.Address) {
+	b.logger.Errorf("UpdateAddresses(%v) called unexpectedly", sc)
+}
+
+// systemRootCertsProvider implements a certprovider.Provider that returns the
+// system default root certificates for validation.
+type systemRootCertsProvider struct{}
+
+func (systemRootCertsProvider) Close() {}
+
+func (systemRootCertsProvider) KeyMaterial(context.Context) (*certprovider.KeyMaterial, error) {
+	rootCAs, err := internal.X509SystemCertPoolFunc()
+	if err != nil {
+		return nil, err
+	}
+	return &certprovider.KeyMaterial{Roots: rootCAs}, nil
+}