diff --git a/vendor/google.golang.org/grpc/clientconn.go b/vendor/google.golang.org/grpc/clientconn.go
new file mode 100644
index 0000000..4414ba8
--- /dev/null
+++ b/vendor/google.golang.org/grpc/clientconn.go
@@ -0,0 +1,1539 @@
+/*
+ *
+ * Copyright 2014 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 grpc
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"math"
+	"net"
+	"reflect"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"google.golang.org/grpc/balancer"
+	"google.golang.org/grpc/balancer/base"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/connectivity"
+	"google.golang.org/grpc/credentials"
+	"google.golang.org/grpc/grpclog"
+	"google.golang.org/grpc/internal/backoff"
+	"google.golang.org/grpc/internal/channelz"
+	"google.golang.org/grpc/internal/grpcsync"
+	"google.golang.org/grpc/internal/transport"
+	"google.golang.org/grpc/keepalive"
+	"google.golang.org/grpc/resolver"
+	"google.golang.org/grpc/serviceconfig"
+	"google.golang.org/grpc/status"
+
+	_ "google.golang.org/grpc/balancer/roundrobin"           // To register roundrobin.
+	_ "google.golang.org/grpc/internal/resolver/dns"         // To register dns resolver.
+	_ "google.golang.org/grpc/internal/resolver/passthrough" // To register passthrough resolver.
+)
+
+const (
+	// minimum time to give a connection to complete
+	minConnectTimeout = 20 * time.Second
+	// must match grpclbName in grpclb/grpclb.go
+	grpclbName = "grpclb"
+)
+
+var (
+	// ErrClientConnClosing indicates that the operation is illegal because
+	// the ClientConn is closing.
+	//
+	// Deprecated: this error should not be relied upon by users; use the status
+	// code of Canceled instead.
+	ErrClientConnClosing = status.Error(codes.Canceled, "grpc: the client connection is closing")
+	// errConnDrain indicates that the connection starts to be drained and does not accept any new RPCs.
+	errConnDrain = errors.New("grpc: the connection is drained")
+	// errConnClosing indicates that the connection is closing.
+	errConnClosing = errors.New("grpc: the connection is closing")
+	// errBalancerClosed indicates that the balancer is closed.
+	errBalancerClosed = errors.New("grpc: balancer is closed")
+	// invalidDefaultServiceConfigErrPrefix is used to prefix the json parsing error for the default
+	// service config.
+	invalidDefaultServiceConfigErrPrefix = "grpc: the provided default service config is invalid"
+)
+
+// The following errors are returned from Dial and DialContext
+var (
+	// errNoTransportSecurity indicates that there is no transport security
+	// being set for ClientConn. Users should either set one or explicitly
+	// call WithInsecure DialOption to disable security.
+	errNoTransportSecurity = errors.New("grpc: no transport security set (use grpc.WithInsecure() explicitly or set credentials)")
+	// errTransportCredsAndBundle indicates that creds bundle is used together
+	// with other individual Transport Credentials.
+	errTransportCredsAndBundle = errors.New("grpc: credentials.Bundle may not be used with individual TransportCredentials")
+	// errTransportCredentialsMissing indicates that users want to transmit security
+	// information (e.g., OAuth2 token) which requires secure connection on an insecure
+	// connection.
+	errTransportCredentialsMissing = errors.New("grpc: the credentials require transport level security (use grpc.WithTransportCredentials() to set)")
+	// errCredentialsConflict indicates that grpc.WithTransportCredentials()
+	// and grpc.WithInsecure() are both called for a connection.
+	errCredentialsConflict = errors.New("grpc: transport credentials are set for an insecure connection (grpc.WithTransportCredentials() and grpc.WithInsecure() are both called)")
+)
+
+const (
+	defaultClientMaxReceiveMessageSize = 1024 * 1024 * 4
+	defaultClientMaxSendMessageSize    = math.MaxInt32
+	// http2IOBufSize specifies the buffer size for sending frames.
+	defaultWriteBufSize = 32 * 1024
+	defaultReadBufSize  = 32 * 1024
+)
+
+// Dial creates a client connection to the given target.
+func Dial(target string, opts ...DialOption) (*ClientConn, error) {
+	return DialContext(context.Background(), target, opts...)
+}
+
+// DialContext creates a client connection to the given target. By default, it's
+// a non-blocking dial (the function won't wait for connections to be
+// established, and connecting happens in the background). To make it a blocking
+// dial, use WithBlock() dial option.
+//
+// In the non-blocking case, the ctx does not act against the connection. It
+// only controls the setup steps.
+//
+// In the blocking case, ctx can be used to cancel or expire the pending
+// connection. Once this function returns, the cancellation and expiration of
+// ctx will be noop. Users should call ClientConn.Close to terminate all the
+// pending operations after this function returns.
+//
+// The target name syntax is defined in
+// https://github.com/grpc/grpc/blob/master/doc/naming.md.
+// e.g. to use dns resolver, a "dns:///" prefix should be applied to the target.
+func DialContext(ctx context.Context, target string, opts ...DialOption) (conn *ClientConn, err error) {
+	cc := &ClientConn{
+		target:            target,
+		csMgr:             &connectivityStateManager{},
+		conns:             make(map[*addrConn]struct{}),
+		dopts:             defaultDialOptions(),
+		blockingpicker:    newPickerWrapper(),
+		czData:            new(channelzData),
+		firstResolveEvent: grpcsync.NewEvent(),
+	}
+	cc.retryThrottler.Store((*retryThrottler)(nil))
+	cc.ctx, cc.cancel = context.WithCancel(context.Background())
+
+	for _, opt := range opts {
+		opt.apply(&cc.dopts)
+	}
+
+	chainUnaryClientInterceptors(cc)
+	chainStreamClientInterceptors(cc)
+
+	defer func() {
+		if err != nil {
+			cc.Close()
+		}
+	}()
+
+	if channelz.IsOn() {
+		if cc.dopts.channelzParentID != 0 {
+			cc.channelzID = channelz.RegisterChannel(&channelzChannel{cc}, cc.dopts.channelzParentID, target)
+			channelz.AddTraceEvent(cc.channelzID, &channelz.TraceEventDesc{
+				Desc:     "Channel Created",
+				Severity: channelz.CtINFO,
+				Parent: &channelz.TraceEventDesc{
+					Desc:     fmt.Sprintf("Nested Channel(id:%d) created", cc.channelzID),
+					Severity: channelz.CtINFO,
+				},
+			})
+		} else {
+			cc.channelzID = channelz.RegisterChannel(&channelzChannel{cc}, 0, target)
+			channelz.AddTraceEvent(cc.channelzID, &channelz.TraceEventDesc{
+				Desc:     "Channel Created",
+				Severity: channelz.CtINFO,
+			})
+		}
+		cc.csMgr.channelzID = cc.channelzID
+	}
+
+	if !cc.dopts.insecure {
+		if cc.dopts.copts.TransportCredentials == nil && cc.dopts.copts.CredsBundle == nil {
+			return nil, errNoTransportSecurity
+		}
+		if cc.dopts.copts.TransportCredentials != nil && cc.dopts.copts.CredsBundle != nil {
+			return nil, errTransportCredsAndBundle
+		}
+	} else {
+		if cc.dopts.copts.TransportCredentials != nil || cc.dopts.copts.CredsBundle != nil {
+			return nil, errCredentialsConflict
+		}
+		for _, cd := range cc.dopts.copts.PerRPCCredentials {
+			if cd.RequireTransportSecurity() {
+				return nil, errTransportCredentialsMissing
+			}
+		}
+	}
+
+	if cc.dopts.defaultServiceConfigRawJSON != nil {
+		scpr := parseServiceConfig(*cc.dopts.defaultServiceConfigRawJSON)
+		if scpr.Err != nil {
+			return nil, fmt.Errorf("%s: %v", invalidDefaultServiceConfigErrPrefix, scpr.Err)
+		}
+		cc.dopts.defaultServiceConfig, _ = scpr.Config.(*ServiceConfig)
+	}
+	cc.mkp = cc.dopts.copts.KeepaliveParams
+
+	if cc.dopts.copts.Dialer == nil {
+		cc.dopts.copts.Dialer = newProxyDialer(
+			func(ctx context.Context, addr string) (net.Conn, error) {
+				network, addr := parseDialTarget(addr)
+				return (&net.Dialer{}).DialContext(ctx, network, addr)
+			},
+		)
+	}
+
+	if cc.dopts.copts.UserAgent != "" {
+		cc.dopts.copts.UserAgent += " " + grpcUA
+	} else {
+		cc.dopts.copts.UserAgent = grpcUA
+	}
+
+	if cc.dopts.timeout > 0 {
+		var cancel context.CancelFunc
+		ctx, cancel = context.WithTimeout(ctx, cc.dopts.timeout)
+		defer cancel()
+	}
+	defer func() {
+		select {
+		case <-ctx.Done():
+			conn, err = nil, ctx.Err()
+		default:
+		}
+	}()
+
+	scSet := false
+	if cc.dopts.scChan != nil {
+		// Try to get an initial service config.
+		select {
+		case sc, ok := <-cc.dopts.scChan:
+			if ok {
+				cc.sc = &sc
+				scSet = true
+			}
+		default:
+		}
+	}
+	if cc.dopts.bs == nil {
+		cc.dopts.bs = backoff.DefaultExponential
+	}
+	if cc.dopts.resolverBuilder == nil {
+		// Only try to parse target when resolver builder is not already set.
+		cc.parsedTarget = parseTarget(cc.target)
+		grpclog.Infof("parsed scheme: %q", cc.parsedTarget.Scheme)
+		cc.dopts.resolverBuilder = resolver.Get(cc.parsedTarget.Scheme)
+		if cc.dopts.resolverBuilder == nil {
+			// If resolver builder is still nil, the parsed target's scheme is
+			// not registered. Fallback to default resolver and set Endpoint to
+			// the original target.
+			grpclog.Infof("scheme %q not registered, fallback to default scheme", cc.parsedTarget.Scheme)
+			cc.parsedTarget = resolver.Target{
+				Scheme:   resolver.GetDefaultScheme(),
+				Endpoint: target,
+			}
+			cc.dopts.resolverBuilder = resolver.Get(cc.parsedTarget.Scheme)
+		}
+	} else {
+		cc.parsedTarget = resolver.Target{Endpoint: target}
+	}
+	creds := cc.dopts.copts.TransportCredentials
+	if creds != nil && creds.Info().ServerName != "" {
+		cc.authority = creds.Info().ServerName
+	} else if cc.dopts.insecure && cc.dopts.authority != "" {
+		cc.authority = cc.dopts.authority
+	} else {
+		// Use endpoint from "scheme://authority/endpoint" as the default
+		// authority for ClientConn.
+		cc.authority = cc.parsedTarget.Endpoint
+	}
+
+	if cc.dopts.scChan != nil && !scSet {
+		// Blocking wait for the initial service config.
+		select {
+		case sc, ok := <-cc.dopts.scChan:
+			if ok {
+				cc.sc = &sc
+			}
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		}
+	}
+	if cc.dopts.scChan != nil {
+		go cc.scWatcher()
+	}
+
+	var credsClone credentials.TransportCredentials
+	if creds := cc.dopts.copts.TransportCredentials; creds != nil {
+		credsClone = creds.Clone()
+	}
+	cc.balancerBuildOpts = balancer.BuildOptions{
+		DialCreds:        credsClone,
+		CredsBundle:      cc.dopts.copts.CredsBundle,
+		Dialer:           cc.dopts.copts.Dialer,
+		ChannelzParentID: cc.channelzID,
+		Target:           cc.parsedTarget,
+	}
+
+	// Build the resolver.
+	rWrapper, err := newCCResolverWrapper(cc)
+	if err != nil {
+		return nil, fmt.Errorf("failed to build resolver: %v", err)
+	}
+
+	cc.mu.Lock()
+	cc.resolverWrapper = rWrapper
+	cc.mu.Unlock()
+	// A blocking dial blocks until the clientConn is ready.
+	if cc.dopts.block {
+		for {
+			s := cc.GetState()
+			if s == connectivity.Ready {
+				break
+			} else if cc.dopts.copts.FailOnNonTempDialError && s == connectivity.TransientFailure {
+				if err = cc.blockingpicker.connectionError(); err != nil {
+					terr, ok := err.(interface {
+						Temporary() bool
+					})
+					if ok && !terr.Temporary() {
+						return nil, err
+					}
+				}
+			}
+			if !cc.WaitForStateChange(ctx, s) {
+				// ctx got timeout or canceled.
+				return nil, ctx.Err()
+			}
+		}
+	}
+
+	return cc, nil
+}
+
+// chainUnaryClientInterceptors chains all unary client interceptors into one.
+func chainUnaryClientInterceptors(cc *ClientConn) {
+	interceptors := cc.dopts.chainUnaryInts
+	// Prepend dopts.unaryInt to the chaining interceptors if it exists, since unaryInt will
+	// be executed before any other chained interceptors.
+	if cc.dopts.unaryInt != nil {
+		interceptors = append([]UnaryClientInterceptor{cc.dopts.unaryInt}, interceptors...)
+	}
+	var chainedInt UnaryClientInterceptor
+	if len(interceptors) == 0 {
+		chainedInt = nil
+	} else if len(interceptors) == 1 {
+		chainedInt = interceptors[0]
+	} else {
+		chainedInt = func(ctx context.Context, method string, req, reply interface{}, cc *ClientConn, invoker UnaryInvoker, opts ...CallOption) error {
+			return interceptors[0](ctx, method, req, reply, cc, getChainUnaryInvoker(interceptors, 0, invoker), opts...)
+		}
+	}
+	cc.dopts.unaryInt = chainedInt
+}
+
+// getChainUnaryInvoker recursively generate the chained unary invoker.
+func getChainUnaryInvoker(interceptors []UnaryClientInterceptor, curr int, finalInvoker UnaryInvoker) UnaryInvoker {
+	if curr == len(interceptors)-1 {
+		return finalInvoker
+	}
+	return func(ctx context.Context, method string, req, reply interface{}, cc *ClientConn, opts ...CallOption) error {
+		return interceptors[curr+1](ctx, method, req, reply, cc, getChainUnaryInvoker(interceptors, curr+1, finalInvoker), opts...)
+	}
+}
+
+// chainStreamClientInterceptors chains all stream client interceptors into one.
+func chainStreamClientInterceptors(cc *ClientConn) {
+	interceptors := cc.dopts.chainStreamInts
+	// Prepend dopts.streamInt to the chaining interceptors if it exists, since streamInt will
+	// be executed before any other chained interceptors.
+	if cc.dopts.streamInt != nil {
+		interceptors = append([]StreamClientInterceptor{cc.dopts.streamInt}, interceptors...)
+	}
+	var chainedInt StreamClientInterceptor
+	if len(interceptors) == 0 {
+		chainedInt = nil
+	} else if len(interceptors) == 1 {
+		chainedInt = interceptors[0]
+	} else {
+		chainedInt = func(ctx context.Context, desc *StreamDesc, cc *ClientConn, method string, streamer Streamer, opts ...CallOption) (ClientStream, error) {
+			return interceptors[0](ctx, desc, cc, method, getChainStreamer(interceptors, 0, streamer), opts...)
+		}
+	}
+	cc.dopts.streamInt = chainedInt
+}
+
+// getChainStreamer recursively generate the chained client stream constructor.
+func getChainStreamer(interceptors []StreamClientInterceptor, curr int, finalStreamer Streamer) Streamer {
+	if curr == len(interceptors)-1 {
+		return finalStreamer
+	}
+	return func(ctx context.Context, desc *StreamDesc, cc *ClientConn, method string, opts ...CallOption) (ClientStream, error) {
+		return interceptors[curr+1](ctx, desc, cc, method, getChainStreamer(interceptors, curr+1, finalStreamer), opts...)
+	}
+}
+
+// connectivityStateManager keeps the connectivity.State of ClientConn.
+// This struct will eventually be exported so the balancers can access it.
+type connectivityStateManager struct {
+	mu         sync.Mutex
+	state      connectivity.State
+	notifyChan chan struct{}
+	channelzID int64
+}
+
+// updateState updates the connectivity.State of ClientConn.
+// If there's a change it notifies goroutines waiting on state change to
+// happen.
+func (csm *connectivityStateManager) updateState(state connectivity.State) {
+	csm.mu.Lock()
+	defer csm.mu.Unlock()
+	if csm.state == connectivity.Shutdown {
+		return
+	}
+	if csm.state == state {
+		return
+	}
+	csm.state = state
+	if channelz.IsOn() {
+		channelz.AddTraceEvent(csm.channelzID, &channelz.TraceEventDesc{
+			Desc:     fmt.Sprintf("Channel Connectivity change to %v", state),
+			Severity: channelz.CtINFO,
+		})
+	}
+	if csm.notifyChan != nil {
+		// There are other goroutines waiting on this channel.
+		close(csm.notifyChan)
+		csm.notifyChan = nil
+	}
+}
+
+func (csm *connectivityStateManager) getState() connectivity.State {
+	csm.mu.Lock()
+	defer csm.mu.Unlock()
+	return csm.state
+}
+
+func (csm *connectivityStateManager) getNotifyChan() <-chan struct{} {
+	csm.mu.Lock()
+	defer csm.mu.Unlock()
+	if csm.notifyChan == nil {
+		csm.notifyChan = make(chan struct{})
+	}
+	return csm.notifyChan
+}
+
+// ClientConn represents a virtual connection to a conceptual endpoint, to
+// perform RPCs.
+//
+// A ClientConn is free to have zero or more actual connections to the endpoint
+// based on configuration, load, etc. It is also free to determine which actual
+// endpoints to use and may change it every RPC, permitting client-side load
+// balancing.
+//
+// A ClientConn encapsulates a range of functionality including name
+// resolution, TCP connection establishment (with retries and backoff) and TLS
+// handshakes. It also handles errors on established connections by
+// re-resolving the name and reconnecting.
+type ClientConn struct {
+	ctx    context.Context
+	cancel context.CancelFunc
+
+	target       string
+	parsedTarget resolver.Target
+	authority    string
+	dopts        dialOptions
+	csMgr        *connectivityStateManager
+
+	balancerBuildOpts balancer.BuildOptions
+	blockingpicker    *pickerWrapper
+
+	mu              sync.RWMutex
+	resolverWrapper *ccResolverWrapper
+	sc              *ServiceConfig
+	conns           map[*addrConn]struct{}
+	// Keepalive parameter can be updated if a GoAway is received.
+	mkp             keepalive.ClientParameters
+	curBalancerName string
+	balancerWrapper *ccBalancerWrapper
+	retryThrottler  atomic.Value
+
+	firstResolveEvent *grpcsync.Event
+
+	channelzID int64 // channelz unique identification number
+	czData     *channelzData
+}
+
+// WaitForStateChange waits until the connectivity.State of ClientConn changes from sourceState or
+// ctx expires. A true value is returned in former case and false in latter.
+// This is an EXPERIMENTAL API.
+func (cc *ClientConn) WaitForStateChange(ctx context.Context, sourceState connectivity.State) bool {
+	ch := cc.csMgr.getNotifyChan()
+	if cc.csMgr.getState() != sourceState {
+		return true
+	}
+	select {
+	case <-ctx.Done():
+		return false
+	case <-ch:
+		return true
+	}
+}
+
+// GetState returns the connectivity.State of ClientConn.
+// This is an EXPERIMENTAL API.
+func (cc *ClientConn) GetState() connectivity.State {
+	return cc.csMgr.getState()
+}
+
+func (cc *ClientConn) scWatcher() {
+	for {
+		select {
+		case sc, ok := <-cc.dopts.scChan:
+			if !ok {
+				return
+			}
+			cc.mu.Lock()
+			// TODO: load balance policy runtime change is ignored.
+			// We may revisit this decision in the future.
+			cc.sc = &sc
+			cc.mu.Unlock()
+		case <-cc.ctx.Done():
+			return
+		}
+	}
+}
+
+// waitForResolvedAddrs blocks until the resolver has provided addresses or the
+// context expires.  Returns nil unless the context expires first; otherwise
+// returns a status error based on the context.
+func (cc *ClientConn) waitForResolvedAddrs(ctx context.Context) error {
+	// This is on the RPC path, so we use a fast path to avoid the
+	// more-expensive "select" below after the resolver has returned once.
+	if cc.firstResolveEvent.HasFired() {
+		return nil
+	}
+	select {
+	case <-cc.firstResolveEvent.Done():
+		return nil
+	case <-ctx.Done():
+		return status.FromContextError(ctx.Err()).Err()
+	case <-cc.ctx.Done():
+		return ErrClientConnClosing
+	}
+}
+
+var emptyServiceConfig *ServiceConfig
+
+func init() {
+	cfg := parseServiceConfig("{}")
+	if cfg.Err != nil {
+		panic(fmt.Sprintf("impossible error parsing empty service config: %v", cfg.Err))
+	}
+	emptyServiceConfig = cfg.Config.(*ServiceConfig)
+}
+
+func (cc *ClientConn) maybeApplyDefaultServiceConfig(addrs []resolver.Address) {
+	if cc.sc != nil {
+		cc.applyServiceConfigAndBalancer(cc.sc, addrs)
+		return
+	}
+	if cc.dopts.defaultServiceConfig != nil {
+		cc.applyServiceConfigAndBalancer(cc.dopts.defaultServiceConfig, addrs)
+	} else {
+		cc.applyServiceConfigAndBalancer(emptyServiceConfig, addrs)
+	}
+}
+
+func (cc *ClientConn) updateResolverState(s resolver.State, err error) error {
+	defer cc.firstResolveEvent.Fire()
+	cc.mu.Lock()
+	// Check if the ClientConn is already closed. Some fields (e.g.
+	// balancerWrapper) are set to nil when closing the ClientConn, and could
+	// cause nil pointer panic if we don't have this check.
+	if cc.conns == nil {
+		cc.mu.Unlock()
+		return nil
+	}
+
+	if err != nil {
+		// May need to apply the initial service config in case the resolver
+		// doesn't support service configs, or doesn't provide a service config
+		// with the new addresses.
+		cc.maybeApplyDefaultServiceConfig(nil)
+
+		if cc.balancerWrapper != nil {
+			cc.balancerWrapper.resolverError(err)
+		}
+
+		// No addresses are valid with err set; return early.
+		cc.mu.Unlock()
+		return balancer.ErrBadResolverState
+	}
+
+	var ret error
+	if cc.dopts.disableServiceConfig || s.ServiceConfig == nil {
+		cc.maybeApplyDefaultServiceConfig(s.Addresses)
+		// TODO: do we need to apply a failing LB policy if there is no
+		// default, per the error handling design?
+	} else {
+		if sc, ok := s.ServiceConfig.Config.(*ServiceConfig); s.ServiceConfig.Err == nil && ok {
+			cc.applyServiceConfigAndBalancer(sc, s.Addresses)
+		} else {
+			ret = balancer.ErrBadResolverState
+			if cc.balancerWrapper == nil {
+				var err error
+				if s.ServiceConfig.Err != nil {
+					err = status.Errorf(codes.Unavailable, "error parsing service config: %v", s.ServiceConfig.Err)
+				} else {
+					err = status.Errorf(codes.Unavailable, "illegal service config type: %T", s.ServiceConfig.Config)
+				}
+				cc.blockingpicker.updatePicker(base.NewErrPicker(err))
+				cc.csMgr.updateState(connectivity.TransientFailure)
+				cc.mu.Unlock()
+				return ret
+			}
+		}
+	}
+
+	var balCfg serviceconfig.LoadBalancingConfig
+	if cc.dopts.balancerBuilder == nil && cc.sc != nil && cc.sc.lbConfig != nil {
+		balCfg = cc.sc.lbConfig.cfg
+	}
+
+	cbn := cc.curBalancerName
+	bw := cc.balancerWrapper
+	cc.mu.Unlock()
+	if cbn != grpclbName {
+		// Filter any grpclb addresses since we don't have the grpclb balancer.
+		for i := 0; i < len(s.Addresses); {
+			if s.Addresses[i].Type == resolver.GRPCLB {
+				copy(s.Addresses[i:], s.Addresses[i+1:])
+				s.Addresses = s.Addresses[:len(s.Addresses)-1]
+				continue
+			}
+			i++
+		}
+	}
+	uccsErr := bw.updateClientConnState(&balancer.ClientConnState{ResolverState: s, BalancerConfig: balCfg})
+	if ret == nil {
+		ret = uccsErr // prefer ErrBadResolver state since any other error is
+		// currently meaningless to the caller.
+	}
+	return ret
+}
+
+// switchBalancer starts the switching from current balancer to the balancer
+// with the given name.
+//
+// It will NOT send the current address list to the new balancer. If needed,
+// caller of this function should send address list to the new balancer after
+// this function returns.
+//
+// Caller must hold cc.mu.
+func (cc *ClientConn) switchBalancer(name string) {
+	if strings.EqualFold(cc.curBalancerName, name) {
+		return
+	}
+
+	grpclog.Infof("ClientConn switching balancer to %q", name)
+	if cc.dopts.balancerBuilder != nil {
+		grpclog.Infoln("ignoring balancer switching: Balancer DialOption used instead")
+		return
+	}
+	if cc.balancerWrapper != nil {
+		cc.balancerWrapper.close()
+	}
+
+	builder := balancer.Get(name)
+	if channelz.IsOn() {
+		if builder == nil {
+			channelz.AddTraceEvent(cc.channelzID, &channelz.TraceEventDesc{
+				Desc:     fmt.Sprintf("Channel switches to new LB policy %q due to fallback from invalid balancer name", PickFirstBalancerName),
+				Severity: channelz.CtWarning,
+			})
+		} else {
+			channelz.AddTraceEvent(cc.channelzID, &channelz.TraceEventDesc{
+				Desc:     fmt.Sprintf("Channel switches to new LB policy %q", name),
+				Severity: channelz.CtINFO,
+			})
+		}
+	}
+	if builder == nil {
+		grpclog.Infof("failed to get balancer builder for: %v, using pick_first instead", name)
+		builder = newPickfirstBuilder()
+	}
+
+	cc.curBalancerName = builder.Name()
+	cc.balancerWrapper = newCCBalancerWrapper(cc, builder, cc.balancerBuildOpts)
+}
+
+func (cc *ClientConn) handleSubConnStateChange(sc balancer.SubConn, s connectivity.State) {
+	cc.mu.Lock()
+	if cc.conns == nil {
+		cc.mu.Unlock()
+		return
+	}
+	// TODO(bar switching) send updates to all balancer wrappers when balancer
+	// gracefully switching is supported.
+	cc.balancerWrapper.handleSubConnStateChange(sc, s)
+	cc.mu.Unlock()
+}
+
+// newAddrConn creates an addrConn for addrs and adds it to cc.conns.
+//
+// Caller needs to make sure len(addrs) > 0.
+func (cc *ClientConn) newAddrConn(addrs []resolver.Address, opts balancer.NewSubConnOptions) (*addrConn, error) {
+	ac := &addrConn{
+		cc:           cc,
+		addrs:        addrs,
+		scopts:       opts,
+		dopts:        cc.dopts,
+		czData:       new(channelzData),
+		resetBackoff: make(chan struct{}),
+	}
+	ac.ctx, ac.cancel = context.WithCancel(cc.ctx)
+	// Track ac in cc. This needs to be done before any getTransport(...) is called.
+	cc.mu.Lock()
+	if cc.conns == nil {
+		cc.mu.Unlock()
+		return nil, ErrClientConnClosing
+	}
+	if channelz.IsOn() {
+		ac.channelzID = channelz.RegisterSubChannel(ac, cc.channelzID, "")
+		channelz.AddTraceEvent(ac.channelzID, &channelz.TraceEventDesc{
+			Desc:     "Subchannel Created",
+			Severity: channelz.CtINFO,
+			Parent: &channelz.TraceEventDesc{
+				Desc:     fmt.Sprintf("Subchannel(id:%d) created", ac.channelzID),
+				Severity: channelz.CtINFO,
+			},
+		})
+	}
+	cc.conns[ac] = struct{}{}
+	cc.mu.Unlock()
+	return ac, nil
+}
+
+// removeAddrConn removes the addrConn in the subConn from clientConn.
+// It also tears down the ac with the given error.
+func (cc *ClientConn) removeAddrConn(ac *addrConn, err error) {
+	cc.mu.Lock()
+	if cc.conns == nil {
+		cc.mu.Unlock()
+		return
+	}
+	delete(cc.conns, ac)
+	cc.mu.Unlock()
+	ac.tearDown(err)
+}
+
+func (cc *ClientConn) channelzMetric() *channelz.ChannelInternalMetric {
+	return &channelz.ChannelInternalMetric{
+		State:                    cc.GetState(),
+		Target:                   cc.target,
+		CallsStarted:             atomic.LoadInt64(&cc.czData.callsStarted),
+		CallsSucceeded:           atomic.LoadInt64(&cc.czData.callsSucceeded),
+		CallsFailed:              atomic.LoadInt64(&cc.czData.callsFailed),
+		LastCallStartedTimestamp: time.Unix(0, atomic.LoadInt64(&cc.czData.lastCallStartedTime)),
+	}
+}
+
+// Target returns the target string of the ClientConn.
+// This is an EXPERIMENTAL API.
+func (cc *ClientConn) Target() string {
+	return cc.target
+}
+
+func (cc *ClientConn) incrCallsStarted() {
+	atomic.AddInt64(&cc.czData.callsStarted, 1)
+	atomic.StoreInt64(&cc.czData.lastCallStartedTime, time.Now().UnixNano())
+}
+
+func (cc *ClientConn) incrCallsSucceeded() {
+	atomic.AddInt64(&cc.czData.callsSucceeded, 1)
+}
+
+func (cc *ClientConn) incrCallsFailed() {
+	atomic.AddInt64(&cc.czData.callsFailed, 1)
+}
+
+// connect starts creating a transport.
+// It does nothing if the ac is not IDLE.
+// TODO(bar) Move this to the addrConn section.
+func (ac *addrConn) connect() error {
+	ac.mu.Lock()
+	if ac.state == connectivity.Shutdown {
+		ac.mu.Unlock()
+		return errConnClosing
+	}
+	if ac.state != connectivity.Idle {
+		ac.mu.Unlock()
+		return nil
+	}
+	// Update connectivity state within the lock to prevent subsequent or
+	// concurrent calls from resetting the transport more than once.
+	ac.updateConnectivityState(connectivity.Connecting)
+	ac.mu.Unlock()
+
+	// Start a goroutine connecting to the server asynchronously.
+	go ac.resetTransport()
+	return nil
+}
+
+// tryUpdateAddrs tries to update ac.addrs with the new addresses list.
+//
+// If ac is Connecting, it returns false. The caller should tear down the ac and
+// create a new one. Note that the backoff will be reset when this happens.
+//
+// If ac is TransientFailure, it updates ac.addrs and returns true. The updated
+// addresses will be picked up by retry in the next iteration after backoff.
+//
+// If ac is Shutdown or Idle, it updates ac.addrs and returns true.
+//
+// If ac is Ready, it checks whether current connected address of ac is in the
+// new addrs list.
+//  - If true, it updates ac.addrs and returns true. The ac will keep using
+//    the existing connection.
+//  - If false, it does nothing and returns false.
+func (ac *addrConn) tryUpdateAddrs(addrs []resolver.Address) bool {
+	ac.mu.Lock()
+	defer ac.mu.Unlock()
+	grpclog.Infof("addrConn: tryUpdateAddrs curAddr: %v, addrs: %v", ac.curAddr, addrs)
+	if ac.state == connectivity.Shutdown ||
+		ac.state == connectivity.TransientFailure ||
+		ac.state == connectivity.Idle {
+		ac.addrs = addrs
+		return true
+	}
+
+	if ac.state == connectivity.Connecting {
+		return false
+	}
+
+	// ac.state is Ready, try to find the connected address.
+	var curAddrFound bool
+	for _, a := range addrs {
+		if reflect.DeepEqual(ac.curAddr, a) {
+			curAddrFound = true
+			break
+		}
+	}
+	grpclog.Infof("addrConn: tryUpdateAddrs curAddrFound: %v", curAddrFound)
+	if curAddrFound {
+		ac.addrs = addrs
+	}
+
+	return curAddrFound
+}
+
+// GetMethodConfig gets the method config of the input method.
+// If there's an exact match for input method (i.e. /service/method), we return
+// the corresponding MethodConfig.
+// If there isn't an exact match for the input method, we look for the default config
+// under the service (i.e /service/). If there is a default MethodConfig for
+// the service, we return it.
+// Otherwise, we return an empty MethodConfig.
+func (cc *ClientConn) GetMethodConfig(method string) MethodConfig {
+	// TODO: Avoid the locking here.
+	cc.mu.RLock()
+	defer cc.mu.RUnlock()
+	if cc.sc == nil {
+		return MethodConfig{}
+	}
+	m, ok := cc.sc.Methods[method]
+	if !ok {
+		i := strings.LastIndex(method, "/")
+		m = cc.sc.Methods[method[:i+1]]
+	}
+	return m
+}
+
+func (cc *ClientConn) healthCheckConfig() *healthCheckConfig {
+	cc.mu.RLock()
+	defer cc.mu.RUnlock()
+	if cc.sc == nil {
+		return nil
+	}
+	return cc.sc.healthCheckConfig
+}
+
+func (cc *ClientConn) getTransport(ctx context.Context, failfast bool, method string) (transport.ClientTransport, func(balancer.DoneInfo), error) {
+	t, done, err := cc.blockingpicker.pick(ctx, failfast, balancer.PickOptions{
+		FullMethodName: method,
+	})
+	if err != nil {
+		return nil, nil, toRPCErr(err)
+	}
+	return t, done, nil
+}
+
+func (cc *ClientConn) applyServiceConfigAndBalancer(sc *ServiceConfig, addrs []resolver.Address) {
+	if sc == nil {
+		// should never reach here.
+		return
+	}
+	cc.sc = sc
+
+	if cc.sc.retryThrottling != nil {
+		newThrottler := &retryThrottler{
+			tokens: cc.sc.retryThrottling.MaxTokens,
+			max:    cc.sc.retryThrottling.MaxTokens,
+			thresh: cc.sc.retryThrottling.MaxTokens / 2,
+			ratio:  cc.sc.retryThrottling.TokenRatio,
+		}
+		cc.retryThrottler.Store(newThrottler)
+	} else {
+		cc.retryThrottler.Store((*retryThrottler)(nil))
+	}
+
+	if cc.dopts.balancerBuilder == nil {
+		// Only look at balancer types and switch balancer if balancer dial
+		// option is not set.
+		var newBalancerName string
+		if cc.sc != nil && cc.sc.lbConfig != nil {
+			newBalancerName = cc.sc.lbConfig.name
+		} else {
+			var isGRPCLB bool
+			for _, a := range addrs {
+				if a.Type == resolver.GRPCLB {
+					isGRPCLB = true
+					break
+				}
+			}
+			if isGRPCLB {
+				newBalancerName = grpclbName
+			} else if cc.sc != nil && cc.sc.LB != nil {
+				newBalancerName = *cc.sc.LB
+			} else {
+				newBalancerName = PickFirstBalancerName
+			}
+		}
+		cc.switchBalancer(newBalancerName)
+	} else if cc.balancerWrapper == nil {
+		// Balancer dial option was set, and this is the first time handling
+		// resolved addresses. Build a balancer with dopts.balancerBuilder.
+		cc.curBalancerName = cc.dopts.balancerBuilder.Name()
+		cc.balancerWrapper = newCCBalancerWrapper(cc, cc.dopts.balancerBuilder, cc.balancerBuildOpts)
+	}
+}
+
+func (cc *ClientConn) resolveNow(o resolver.ResolveNowOption) {
+	cc.mu.RLock()
+	r := cc.resolverWrapper
+	cc.mu.RUnlock()
+	if r == nil {
+		return
+	}
+	go r.resolveNow(o)
+}
+
+// ResetConnectBackoff wakes up all subchannels in transient failure and causes
+// them to attempt another connection immediately.  It also resets the backoff
+// times used for subsequent attempts regardless of the current state.
+//
+// In general, this function should not be used.  Typical service or network
+// outages result in a reasonable client reconnection strategy by default.
+// However, if a previously unavailable network becomes available, this may be
+// used to trigger an immediate reconnect.
+//
+// This API is EXPERIMENTAL.
+func (cc *ClientConn) ResetConnectBackoff() {
+	cc.mu.Lock()
+	conns := cc.conns
+	cc.mu.Unlock()
+	for ac := range conns {
+		ac.resetConnectBackoff()
+	}
+}
+
+// Close tears down the ClientConn and all underlying connections.
+func (cc *ClientConn) Close() error {
+	defer cc.cancel()
+
+	cc.mu.Lock()
+	if cc.conns == nil {
+		cc.mu.Unlock()
+		return ErrClientConnClosing
+	}
+	conns := cc.conns
+	cc.conns = nil
+	cc.csMgr.updateState(connectivity.Shutdown)
+
+	rWrapper := cc.resolverWrapper
+	cc.resolverWrapper = nil
+	bWrapper := cc.balancerWrapper
+	cc.balancerWrapper = nil
+	cc.mu.Unlock()
+
+	cc.blockingpicker.close()
+
+	if rWrapper != nil {
+		rWrapper.close()
+	}
+	if bWrapper != nil {
+		bWrapper.close()
+	}
+
+	for ac := range conns {
+		ac.tearDown(ErrClientConnClosing)
+	}
+	if channelz.IsOn() {
+		ted := &channelz.TraceEventDesc{
+			Desc:     "Channel Deleted",
+			Severity: channelz.CtINFO,
+		}
+		if cc.dopts.channelzParentID != 0 {
+			ted.Parent = &channelz.TraceEventDesc{
+				Desc:     fmt.Sprintf("Nested channel(id:%d) deleted", cc.channelzID),
+				Severity: channelz.CtINFO,
+			}
+		}
+		channelz.AddTraceEvent(cc.channelzID, ted)
+		// TraceEvent needs to be called before RemoveEntry, as TraceEvent may add trace reference to
+		// the entity being deleted, and thus prevent it from being deleted right away.
+		channelz.RemoveEntry(cc.channelzID)
+	}
+	return nil
+}
+
+// addrConn is a network connection to a given address.
+type addrConn struct {
+	ctx    context.Context
+	cancel context.CancelFunc
+
+	cc     *ClientConn
+	dopts  dialOptions
+	acbw   balancer.SubConn
+	scopts balancer.NewSubConnOptions
+
+	// transport is set when there's a viable transport (note: ac state may not be READY as LB channel
+	// health checking may require server to report healthy to set ac to READY), and is reset
+	// to nil when the current transport should no longer be used to create a stream (e.g. after GoAway
+	// is received, transport is closed, ac has been torn down).
+	transport transport.ClientTransport // The current transport.
+
+	mu      sync.Mutex
+	curAddr resolver.Address   // The current address.
+	addrs   []resolver.Address // All addresses that the resolver resolved to.
+
+	// Use updateConnectivityState for updating addrConn's connectivity state.
+	state connectivity.State
+
+	backoffIdx   int // Needs to be stateful for resetConnectBackoff.
+	resetBackoff chan struct{}
+
+	channelzID int64 // channelz unique identification number.
+	czData     *channelzData
+}
+
+// Note: this requires a lock on ac.mu.
+func (ac *addrConn) updateConnectivityState(s connectivity.State) {
+	if ac.state == s {
+		return
+	}
+
+	updateMsg := fmt.Sprintf("Subchannel Connectivity change to %v", s)
+	ac.state = s
+	if channelz.IsOn() {
+		channelz.AddTraceEvent(ac.channelzID, &channelz.TraceEventDesc{
+			Desc:     updateMsg,
+			Severity: channelz.CtINFO,
+		})
+	}
+	ac.cc.handleSubConnStateChange(ac.acbw, s)
+}
+
+// adjustParams updates parameters used to create transports upon
+// receiving a GoAway.
+func (ac *addrConn) adjustParams(r transport.GoAwayReason) {
+	switch r {
+	case transport.GoAwayTooManyPings:
+		v := 2 * ac.dopts.copts.KeepaliveParams.Time
+		ac.cc.mu.Lock()
+		if v > ac.cc.mkp.Time {
+			ac.cc.mkp.Time = v
+		}
+		ac.cc.mu.Unlock()
+	}
+}
+
+func (ac *addrConn) resetTransport() {
+	for i := 0; ; i++ {
+		if i > 0 {
+			ac.cc.resolveNow(resolver.ResolveNowOption{})
+		}
+
+		ac.mu.Lock()
+		if ac.state == connectivity.Shutdown {
+			ac.mu.Unlock()
+			return
+		}
+
+		addrs := ac.addrs
+		backoffFor := ac.dopts.bs.Backoff(ac.backoffIdx)
+		// This will be the duration that dial gets to finish.
+		dialDuration := minConnectTimeout
+		if ac.dopts.minConnectTimeout != nil {
+			dialDuration = ac.dopts.minConnectTimeout()
+		}
+
+		if dialDuration < backoffFor {
+			// Give dial more time as we keep failing to connect.
+			dialDuration = backoffFor
+		}
+		// We can potentially spend all the time trying the first address, and
+		// if the server accepts the connection and then hangs, the following
+		// addresses will never be tried.
+		//
+		// The spec doesn't mention what should be done for multiple addresses.
+		// https://github.com/grpc/grpc/blob/master/doc/connection-backoff.md#proposed-backoff-algorithm
+		connectDeadline := time.Now().Add(dialDuration)
+
+		ac.updateConnectivityState(connectivity.Connecting)
+		ac.transport = nil
+		ac.mu.Unlock()
+
+		newTr, addr, reconnect, err := ac.tryAllAddrs(addrs, connectDeadline)
+		if err != nil {
+			// After exhausting all addresses, the addrConn enters
+			// TRANSIENT_FAILURE.
+			ac.mu.Lock()
+			if ac.state == connectivity.Shutdown {
+				ac.mu.Unlock()
+				return
+			}
+			ac.updateConnectivityState(connectivity.TransientFailure)
+
+			// Backoff.
+			b := ac.resetBackoff
+			ac.mu.Unlock()
+
+			timer := time.NewTimer(backoffFor)
+			select {
+			case <-timer.C:
+				ac.mu.Lock()
+				ac.backoffIdx++
+				ac.mu.Unlock()
+			case <-b:
+				timer.Stop()
+			case <-ac.ctx.Done():
+				timer.Stop()
+				return
+			}
+			continue
+		}
+
+		ac.mu.Lock()
+		if ac.state == connectivity.Shutdown {
+			ac.mu.Unlock()
+			newTr.Close()
+			return
+		}
+		ac.curAddr = addr
+		ac.transport = newTr
+		ac.backoffIdx = 0
+
+		hctx, hcancel := context.WithCancel(ac.ctx)
+		ac.startHealthCheck(hctx)
+		ac.mu.Unlock()
+
+		// Block until the created transport is down. And when this happens,
+		// we restart from the top of the addr list.
+		<-reconnect.Done()
+		hcancel()
+		// restart connecting - the top of the loop will set state to
+		// CONNECTING.  This is against the current connectivity semantics doc,
+		// however it allows for graceful behavior for RPCs not yet dispatched
+		// - unfortunate timing would otherwise lead to the RPC failing even
+		// though the TRANSIENT_FAILURE state (called for by the doc) would be
+		// instantaneous.
+		//
+		// Ideally we should transition to Idle here and block until there is
+		// RPC activity that leads to the balancer requesting a reconnect of
+		// the associated SubConn.
+	}
+}
+
+// tryAllAddrs tries to creates a connection to the addresses, and stop when at the
+// first successful one. It returns the transport, the address and a Event in
+// the successful case. The Event fires when the returned transport disconnects.
+func (ac *addrConn) tryAllAddrs(addrs []resolver.Address, connectDeadline time.Time) (transport.ClientTransport, resolver.Address, *grpcsync.Event, error) {
+	for _, addr := range addrs {
+		ac.mu.Lock()
+		if ac.state == connectivity.Shutdown {
+			ac.mu.Unlock()
+			return nil, resolver.Address{}, nil, errConnClosing
+		}
+
+		ac.cc.mu.RLock()
+		ac.dopts.copts.KeepaliveParams = ac.cc.mkp
+		ac.cc.mu.RUnlock()
+
+		copts := ac.dopts.copts
+		if ac.scopts.CredsBundle != nil {
+			copts.CredsBundle = ac.scopts.CredsBundle
+		}
+		ac.mu.Unlock()
+
+		if channelz.IsOn() {
+			channelz.AddTraceEvent(ac.channelzID, &channelz.TraceEventDesc{
+				Desc:     fmt.Sprintf("Subchannel picks a new address %q to connect", addr.Addr),
+				Severity: channelz.CtINFO,
+			})
+		}
+
+		newTr, reconnect, err := ac.createTransport(addr, copts, connectDeadline)
+		if err == nil {
+			return newTr, addr, reconnect, nil
+		}
+		ac.cc.blockingpicker.updateConnectionError(err)
+	}
+
+	// Couldn't connect to any address.
+	return nil, resolver.Address{}, nil, fmt.Errorf("couldn't connect to any address")
+}
+
+// createTransport creates a connection to addr. It returns the transport and a
+// Event in the successful case. The Event fires when the returned transport
+// disconnects.
+func (ac *addrConn) createTransport(addr resolver.Address, copts transport.ConnectOptions, connectDeadline time.Time) (transport.ClientTransport, *grpcsync.Event, error) {
+	prefaceReceived := make(chan struct{})
+	onCloseCalled := make(chan struct{})
+	reconnect := grpcsync.NewEvent()
+
+	authority := ac.cc.authority
+	// addr.ServerName takes precedent over ClientConn authority, if present.
+	if addr.ServerName != "" {
+		authority = addr.ServerName
+	}
+
+	target := transport.TargetInfo{
+		Addr:      addr.Addr,
+		Metadata:  addr.Metadata,
+		Authority: authority,
+	}
+
+	once := sync.Once{}
+	onGoAway := func(r transport.GoAwayReason) {
+		ac.mu.Lock()
+		ac.adjustParams(r)
+		once.Do(func() {
+			if ac.state == connectivity.Ready {
+				// Prevent this SubConn from being used for new RPCs by setting its
+				// state to Connecting.
+				//
+				// TODO: this should be Idle when grpc-go properly supports it.
+				ac.updateConnectivityState(connectivity.Connecting)
+			}
+		})
+		ac.mu.Unlock()
+		reconnect.Fire()
+	}
+
+	onClose := func() {
+		ac.mu.Lock()
+		once.Do(func() {
+			if ac.state == connectivity.Ready {
+				// Prevent this SubConn from being used for new RPCs by setting its
+				// state to Connecting.
+				//
+				// TODO: this should be Idle when grpc-go properly supports it.
+				ac.updateConnectivityState(connectivity.Connecting)
+			}
+		})
+		ac.mu.Unlock()
+		close(onCloseCalled)
+		reconnect.Fire()
+	}
+
+	onPrefaceReceipt := func() {
+		close(prefaceReceived)
+	}
+
+	connectCtx, cancel := context.WithDeadline(ac.ctx, connectDeadline)
+	defer cancel()
+	if channelz.IsOn() {
+		copts.ChannelzParentID = ac.channelzID
+	}
+
+	newTr, err := transport.NewClientTransport(connectCtx, ac.cc.ctx, target, copts, onPrefaceReceipt, onGoAway, onClose)
+	if err != nil {
+		// newTr is either nil, or closed.
+		grpclog.Warningf("grpc: addrConn.createTransport failed to connect to %v. Err :%v. Reconnecting...", addr, err)
+		return nil, nil, err
+	}
+
+	select {
+	case <-time.After(connectDeadline.Sub(time.Now())):
+		// We didn't get the preface in time.
+		newTr.Close()
+		grpclog.Warningf("grpc: addrConn.createTransport failed to connect to %v: didn't receive server preface in time. Reconnecting...", addr)
+		return nil, nil, errors.New("timed out waiting for server handshake")
+	case <-prefaceReceived:
+		// We got the preface - huzzah! things are good.
+	case <-onCloseCalled:
+		// The transport has already closed - noop.
+		return nil, nil, errors.New("connection closed")
+		// TODO(deklerk) this should bail on ac.ctx.Done(). Add a test and fix.
+	}
+	return newTr, reconnect, nil
+}
+
+// startHealthCheck starts the health checking stream (RPC) to watch the health
+// stats of this connection if health checking is requested and configured.
+//
+// LB channel health checking is enabled when all requirements below are met:
+// 1. it is not disabled by the user with the WithDisableHealthCheck DialOption
+// 2. internal.HealthCheckFunc is set by importing the grpc/healthcheck package
+// 3. a service config with non-empty healthCheckConfig field is provided
+// 4. the load balancer requests it
+//
+// It sets addrConn to READY if the health checking stream is not started.
+//
+// Caller must hold ac.mu.
+func (ac *addrConn) startHealthCheck(ctx context.Context) {
+	var healthcheckManagingState bool
+	defer func() {
+		if !healthcheckManagingState {
+			ac.updateConnectivityState(connectivity.Ready)
+		}
+	}()
+
+	if ac.cc.dopts.disableHealthCheck {
+		return
+	}
+	healthCheckConfig := ac.cc.healthCheckConfig()
+	if healthCheckConfig == nil {
+		return
+	}
+	if !ac.scopts.HealthCheckEnabled {
+		return
+	}
+	healthCheckFunc := ac.cc.dopts.healthCheckFunc
+	if healthCheckFunc == nil {
+		// The health package is not imported to set health check function.
+		//
+		// TODO: add a link to the health check doc in the error message.
+		grpclog.Error("Health check is requested but health check function is not set.")
+		return
+	}
+
+	healthcheckManagingState = true
+
+	// Set up the health check helper functions.
+	currentTr := ac.transport
+	newStream := func(method string) (interface{}, error) {
+		ac.mu.Lock()
+		if ac.transport != currentTr {
+			ac.mu.Unlock()
+			return nil, status.Error(codes.Canceled, "the provided transport is no longer valid to use")
+		}
+		ac.mu.Unlock()
+		return newNonRetryClientStream(ctx, &StreamDesc{ServerStreams: true}, method, currentTr, ac)
+	}
+	setConnectivityState := func(s connectivity.State) {
+		ac.mu.Lock()
+		defer ac.mu.Unlock()
+		if ac.transport != currentTr {
+			return
+		}
+		ac.updateConnectivityState(s)
+	}
+	// Start the health checking stream.
+	go func() {
+		err := ac.cc.dopts.healthCheckFunc(ctx, newStream, setConnectivityState, healthCheckConfig.ServiceName)
+		if err != nil {
+			if status.Code(err) == codes.Unimplemented {
+				if channelz.IsOn() {
+					channelz.AddTraceEvent(ac.channelzID, &channelz.TraceEventDesc{
+						Desc:     "Subchannel health check is unimplemented at server side, thus health check is disabled",
+						Severity: channelz.CtError,
+					})
+				}
+				grpclog.Error("Subchannel health check is unimplemented at server side, thus health check is disabled")
+			} else {
+				grpclog.Errorf("HealthCheckFunc exits with unexpected error %v", err)
+			}
+		}
+	}()
+}
+
+func (ac *addrConn) resetConnectBackoff() {
+	ac.mu.Lock()
+	close(ac.resetBackoff)
+	ac.backoffIdx = 0
+	ac.resetBackoff = make(chan struct{})
+	ac.mu.Unlock()
+}
+
+// getReadyTransport returns the transport if ac's state is READY.
+// Otherwise it returns nil, false.
+// If ac's state is IDLE, it will trigger ac to connect.
+func (ac *addrConn) getReadyTransport() (transport.ClientTransport, bool) {
+	ac.mu.Lock()
+	if ac.state == connectivity.Ready && ac.transport != nil {
+		t := ac.transport
+		ac.mu.Unlock()
+		return t, true
+	}
+	var idle bool
+	if ac.state == connectivity.Idle {
+		idle = true
+	}
+	ac.mu.Unlock()
+	// Trigger idle ac to connect.
+	if idle {
+		ac.connect()
+	}
+	return nil, false
+}
+
+// tearDown starts to tear down the addrConn.
+// TODO(zhaoq): Make this synchronous to avoid unbounded memory consumption in
+// some edge cases (e.g., the caller opens and closes many addrConn's in a
+// tight loop.
+// tearDown doesn't remove ac from ac.cc.conns.
+func (ac *addrConn) tearDown(err error) {
+	ac.mu.Lock()
+	if ac.state == connectivity.Shutdown {
+		ac.mu.Unlock()
+		return
+	}
+	curTr := ac.transport
+	ac.transport = nil
+	// We have to set the state to Shutdown before anything else to prevent races
+	// between setting the state and logic that waits on context cancellation / etc.
+	ac.updateConnectivityState(connectivity.Shutdown)
+	ac.cancel()
+	ac.curAddr = resolver.Address{}
+	if err == errConnDrain && curTr != nil {
+		// GracefulClose(...) may be executed multiple times when
+		// i) receiving multiple GoAway frames from the server; or
+		// ii) there are concurrent name resolver/Balancer triggered
+		// address removal and GoAway.
+		// We have to unlock and re-lock here because GracefulClose => Close => onClose, which requires locking ac.mu.
+		ac.mu.Unlock()
+		curTr.GracefulClose()
+		ac.mu.Lock()
+	}
+	if channelz.IsOn() {
+		channelz.AddTraceEvent(ac.channelzID, &channelz.TraceEventDesc{
+			Desc:     "Subchannel Deleted",
+			Severity: channelz.CtINFO,
+			Parent: &channelz.TraceEventDesc{
+				Desc:     fmt.Sprintf("Subchanel(id:%d) deleted", ac.channelzID),
+				Severity: channelz.CtINFO,
+			},
+		})
+		// TraceEvent needs to be called before RemoveEntry, as TraceEvent may add trace reference to
+		// the entity being deleted, and thus prevent it from being deleted right away.
+		channelz.RemoveEntry(ac.channelzID)
+	}
+	ac.mu.Unlock()
+}
+
+func (ac *addrConn) getState() connectivity.State {
+	ac.mu.Lock()
+	defer ac.mu.Unlock()
+	return ac.state
+}
+
+func (ac *addrConn) ChannelzMetric() *channelz.ChannelInternalMetric {
+	ac.mu.Lock()
+	addr := ac.curAddr.Addr
+	ac.mu.Unlock()
+	return &channelz.ChannelInternalMetric{
+		State:                    ac.getState(),
+		Target:                   addr,
+		CallsStarted:             atomic.LoadInt64(&ac.czData.callsStarted),
+		CallsSucceeded:           atomic.LoadInt64(&ac.czData.callsSucceeded),
+		CallsFailed:              atomic.LoadInt64(&ac.czData.callsFailed),
+		LastCallStartedTimestamp: time.Unix(0, atomic.LoadInt64(&ac.czData.lastCallStartedTime)),
+	}
+}
+
+func (ac *addrConn) incrCallsStarted() {
+	atomic.AddInt64(&ac.czData.callsStarted, 1)
+	atomic.StoreInt64(&ac.czData.lastCallStartedTime, time.Now().UnixNano())
+}
+
+func (ac *addrConn) incrCallsSucceeded() {
+	atomic.AddInt64(&ac.czData.callsSucceeded, 1)
+}
+
+func (ac *addrConn) incrCallsFailed() {
+	atomic.AddInt64(&ac.czData.callsFailed, 1)
+}
+
+type retryThrottler struct {
+	max    float64
+	thresh float64
+	ratio  float64
+
+	mu     sync.Mutex
+	tokens float64 // TODO(dfawley): replace with atomic and remove lock.
+}
+
+// throttle subtracts a retry token from the pool and returns whether a retry
+// should be throttled (disallowed) based upon the retry throttling policy in
+// the service config.
+func (rt *retryThrottler) throttle() bool {
+	if rt == nil {
+		return false
+	}
+	rt.mu.Lock()
+	defer rt.mu.Unlock()
+	rt.tokens--
+	if rt.tokens < 0 {
+		rt.tokens = 0
+	}
+	return rt.tokens <= rt.thresh
+}
+
+func (rt *retryThrottler) successfulRPC() {
+	if rt == nil {
+		return
+	}
+	rt.mu.Lock()
+	defer rt.mu.Unlock()
+	rt.tokens += rt.ratio
+	if rt.tokens > rt.max {
+		rt.tokens = rt.max
+	}
+}
+
+type channelzChannel struct {
+	cc *ClientConn
+}
+
+func (c *channelzChannel) ChannelzMetric() *channelz.ChannelInternalMetric {
+	return c.cc.channelzMetric()
+}
+
+// ErrClientConnTimeout indicates that the ClientConn cannot establish the
+// underlying connections within the specified timeout.
+//
+// Deprecated: This error is never returned by grpc and should not be
+// referenced by users.
+var ErrClientConnTimeout = errors.New("grpc: timed out when dialing")