vendor/google.golang.org/grpc/balancer/balancer.go - voltha-protos - Gitiles

 /*
  *
  * Copyright 2017 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 balancer defines APIs for load balancing in gRPC.
 // All APIs in this package are experimental.
 package balancer

 import (
 	"context"
 	"encoding/json"
 	"errors"
 	"net"
 	"strings"

 	"google.golang.org/grpc/connectivity"
 	"google.golang.org/grpc/credentials"
 	"google.golang.org/grpc/internal"
 	"google.golang.org/grpc/metadata"
 	"google.golang.org/grpc/resolver"
 	"google.golang.org/grpc/serviceconfig"
 )

 var (
 	// m is a map from name to balancer builder.
 	m = make(map[string]Builder)
 )

 // Register registers the balancer builder to the balancer map. b.Name
 // (lowercased) will be used as the name registered with this builder.  If the
 // Builder implements ConfigParser, ParseConfig will be called when new service
 // configs are received by the resolver, and the result will be provided to the
 // Balancer in UpdateClientConnState.
 //
 // NOTE: this function must only be called during initialization time (i.e. in
 // an init() function), and is not thread-safe. If multiple Balancers are
 // registered with the same name, the one registered last will take effect.
 func Register(b Builder) {
 	m[strings.ToLower(b.Name())] = b
 }

 // unregisterForTesting deletes the balancer with the given name from the
 // balancer map.
 //
 // This function is not thread-safe.
 func unregisterForTesting(name string) {
 	delete(m, name)
 }

 func init() {
 	internal.BalancerUnregister = unregisterForTesting
 }

 // Get returns the resolver builder registered with the given name.
 // Note that the compare is done in a case-insensitive fashion.
 // If no builder is register with the name, nil will be returned.
 func Get(name string) Builder {
 	if b, ok := m[strings.ToLower(name)]; ok {
 		return b
 	}
 	return nil
 }

 // A SubConn represents a single connection to a gRPC backend service.
 //
 // Each SubConn contains a list of addresses.
 //
 // All SubConns start in IDLE, and will not try to connect. To trigger the
 // connecting, Balancers must call Connect.  If a connection re-enters IDLE,
 // Balancers must call Connect again to trigger a new connection attempt.
 //
 // gRPC will try to connect to the addresses in sequence, and stop trying the
 // remainder once the first connection is successful. If an attempt to connect
 // to all addresses encounters an error, the SubConn will enter
 // TRANSIENT_FAILURE for a backoff period, and then transition to IDLE.
 //
 // Once established, if a connection is lost, the SubConn will transition
 // directly to IDLE.
 //
 // This interface is to be implemented by gRPC. Users should not need their own
 // implementation of this interface. For situations like testing, any
 // implementations should embed this interface. This allows gRPC to add new
 // methods to this interface.
 type SubConn interface {
 	// UpdateAddresses updates the addresses used in this SubConn.
 	// gRPC checks if currently-connected address is still in the new list.
 	// If it's in the list, the connection will be kept.
 	// If it's not in the list, the connection will gracefully closed, and
 	// a new connection will be created.
 	//
 	// This will trigger a state transition for the SubConn.
 	//
 	// Deprecated: This method is now part of the ClientConn interface and will
 	// eventually be removed from here.
 	UpdateAddresses([]resolver.Address)
 	// Connect starts the connecting for this SubConn.
 	Connect()
 }

 // NewSubConnOptions contains options to create new SubConn.
 type NewSubConnOptions struct {
 	// CredsBundle is the credentials bundle that will be used in the created
 	// SubConn. If it's nil, the original creds from grpc DialOptions will be
 	// used.
 	//
 	// Deprecated: Use the Attributes field in resolver.Address to pass
 	// arbitrary data to the credential handshaker.
 	CredsBundle credentials.Bundle
 	// HealthCheckEnabled indicates whether health check service should be
 	// enabled on this SubConn
 	HealthCheckEnabled bool
 }

 // State contains the balancer's state relevant to the gRPC ClientConn.
 type State struct {
 	// State contains the connectivity state of the balancer, which is used to
 	// determine the state of the ClientConn.
 	ConnectivityState connectivity.State
 	// Picker is used to choose connections (SubConns) for RPCs.
 	Picker Picker
 }

 // ClientConn represents a gRPC ClientConn.
 //
 // This interface is to be implemented by gRPC. Users should not need a
 // brand new implementation of this interface. For the situations like
 // testing, the new implementation should embed this interface. This allows
 // gRPC to add new methods to this interface.
 type ClientConn interface {
 	// NewSubConn is called by balancer to create a new SubConn.
 	// It doesn't block and wait for the connections to be established.
 	// Behaviors of the SubConn can be controlled by options.
 	NewSubConn([]resolver.Address, NewSubConnOptions) (SubConn, error)
 	// RemoveSubConn removes the SubConn from ClientConn.
 	// The SubConn will be shutdown.
 	RemoveSubConn(SubConn)
 	// UpdateAddresses updates the addresses used in the passed in SubConn.
 	// gRPC checks if the currently connected address is still in the new list.
 	// If so, the connection will be kept. Else, the connection will be
 	// gracefully closed, and a new connection will be created.
 	//
 	// This will trigger a state transition for the SubConn.
 	UpdateAddresses(SubConn, []resolver.Address)

 	// UpdateState notifies gRPC that the balancer's internal state has
 	// changed.
 	//
 	// gRPC will update the connectivity state of the ClientConn, and will call
 	// Pick on the new Picker to pick new SubConns.
 	UpdateState(State)

 	// ResolveNow is called by balancer to notify gRPC to do a name resolving.
 	ResolveNow(resolver.ResolveNowOptions)

 	// Target returns the dial target for this ClientConn.
 	//
 	// Deprecated: Use the Target field in the BuildOptions instead.
 	Target() string
 }

 // BuildOptions contains additional information for Build.
 type BuildOptions struct {
 	// DialCreds is the transport credentials to use when communicating with a
 	// remote load balancer server. Balancer implementations which do not
 	// communicate with a remote load balancer server can ignore this field.
 	DialCreds credentials.TransportCredentials
 	// CredsBundle is the credentials bundle to use when communicating with a
 	// remote load balancer server. Balancer implementations which do not
 	// communicate with a remote load balancer server can ignore this field.
 	CredsBundle credentials.Bundle
 	// Dialer is the custom dialer to use when communicating with a remote load
 	// balancer server. Balancer implementations which do not communicate with a
 	// remote load balancer server can ignore this field.
 	Dialer func(context.Context, string) (net.Conn, error)
 	// Authority is the server name to use as part of the authentication
 	// handshake when communicating with a remote load balancer server. Balancer
 	// implementations which do not communicate with a remote load balancer
 	// server can ignore this field.
 	Authority string
 	// ChannelzParentID is the parent ClientConn's channelz ID.
 	ChannelzParentID int64
 	// CustomUserAgent is the custom user agent set on the parent ClientConn.
 	// The balancer should set the same custom user agent if it creates a
 	// ClientConn.
 	CustomUserAgent string
 	// Target contains the parsed address info of the dial target. It is the
 	// same resolver.Target as passed to the resolver. See the documentation for
 	// the resolver.Target type for details about what it contains.
 	Target resolver.Target
 }

 // Builder creates a balancer.
 type Builder interface {
 	// Build creates a new balancer with the ClientConn.
 	Build(cc ClientConn, opts BuildOptions) Balancer
 	// Name returns the name of balancers built by this builder.
 	// It will be used to pick balancers (for example in service config).
 	Name() string
 }

 // ConfigParser parses load balancer configs.
 type ConfigParser interface {
 	// ParseConfig parses the JSON load balancer config provided into an
 	// internal form or returns an error if the config is invalid.  For future
 	// compatibility reasons, unknown fields in the config should be ignored.
 	ParseConfig(LoadBalancingConfigJSON json.RawMessage) (serviceconfig.LoadBalancingConfig, error)
 }

 // PickInfo contains additional information for the Pick operation.
 type PickInfo struct {
 	// FullMethodName is the method name that NewClientStream() is called
 	// with. The canonical format is /service/Method.
 	FullMethodName string
 	// Ctx is the RPC's context, and may contain relevant RPC-level information
 	// like the outgoing header metadata.
 	Ctx context.Context
 }

 // DoneInfo contains additional information for done.
 type DoneInfo struct {
 	// Err is the rpc error the RPC finished with. It could be nil.
 	Err error
 	// Trailer contains the metadata from the RPC's trailer, if present.
 	Trailer metadata.MD
 	// BytesSent indicates if any bytes have been sent to the server.
 	BytesSent bool
 	// BytesReceived indicates if any byte has been received from the server.
 	BytesReceived bool
 	// ServerLoad is the load received from server. It's usually sent as part of
 	// trailing metadata.
 	//
 	// The only supported type now is *orca_v1.LoadReport.
 	ServerLoad interface{}
 }

 var (
 	// ErrNoSubConnAvailable indicates no SubConn is available for pick().
 	// gRPC will block the RPC until a new picker is available via UpdateState().
 	ErrNoSubConnAvailable = errors.New("no SubConn is available")
 	// ErrTransientFailure indicates all SubConns are in TransientFailure.
 	// WaitForReady RPCs will block, non-WaitForReady RPCs will fail.
 	//
 	// Deprecated: return an appropriate error based on the last resolution or
 	// connection attempt instead.  The behavior is the same for any non-gRPC
 	// status error.
 	ErrTransientFailure = errors.New("all SubConns are in TransientFailure")
 )

 // PickResult contains information related to a connection chosen for an RPC.
 type PickResult struct {
 	// SubConn is the connection to use for this pick, if its state is Ready.
 	// If the state is not Ready, gRPC will block the RPC until a new Picker is
 	// provided by the balancer (using ClientConn.UpdateState).  The SubConn
 	// must be one returned by ClientConn.NewSubConn.
 	SubConn SubConn

 	// Done is called when the RPC is completed.  If the SubConn is not ready,
 	// this will be called with a nil parameter.  If the SubConn is not a valid
 	// type, Done may not be called.  May be nil if the balancer does not wish
 	// to be notified when the RPC completes.
 	Done func(DoneInfo)
 }

 // TransientFailureError returns e.  It exists for backward compatibility and
 // will be deleted soon.
 //
 // Deprecated: no longer necessary, picker errors are treated this way by
 // default.
 func TransientFailureError(e error) error { return e }

 // Picker is used by gRPC to pick a SubConn to send an RPC.
 // Balancer is expected to generate a new picker from its snapshot every time its
 // internal state has changed.
 //
 // The pickers used by gRPC can be updated by ClientConn.UpdateState().
 type Picker interface {
 	// Pick returns the connection to use for this RPC and related information.
 	//
 	// Pick should not block.  If the balancer needs to do I/O or any blocking
 	// or time-consuming work to service this call, it should return
 	// ErrNoSubConnAvailable, and the Pick call will be repeated by gRPC when
 	// the Picker is updated (using ClientConn.UpdateState).
 	//
 	// If an error is returned:
 	//
 	// - If the error is ErrNoSubConnAvailable, gRPC will block until a new
 	//   Picker is provided by the balancer (using ClientConn.UpdateState).
 	//
 	// - If the error is a status error (implemented by the grpc/status
 	//   package), gRPC will terminate the RPC with the code and message
 	//   provided.
 	//
 	// - For all other errors, wait for ready RPCs will wait, but non-wait for
 	//   ready RPCs will be terminated with this error's Error() string and
 	//   status code Unavailable.
 	Pick(info PickInfo) (PickResult, error)
 }

 // Balancer takes input from gRPC, manages SubConns, and collects and aggregates
 // the connectivity states.
 //
 // It also generates and updates the Picker used by gRPC to pick SubConns for RPCs.
 //
 // UpdateClientConnState, ResolverError, UpdateSubConnState, and Close are
 // guaranteed to be called synchronously from the same goroutine.  There's no
 // guarantee on picker.Pick, it may be called anytime.
 type Balancer interface {
 	// UpdateClientConnState is called by gRPC when the state of the ClientConn
 	// changes.  If the error returned is ErrBadResolverState, the ClientConn
 	// will begin calling ResolveNow on the active name resolver with
 	// exponential backoff until a subsequent call to UpdateClientConnState
 	// returns a nil error.  Any other errors are currently ignored.
 	UpdateClientConnState(ClientConnState) error
 	// ResolverError is called by gRPC when the name resolver reports an error.
 	ResolverError(error)
 	// UpdateSubConnState is called by gRPC when the state of a SubConn
 	// changes.
 	UpdateSubConnState(SubConn, SubConnState)
 	// Close closes the balancer. The balancer is not required to call
 	// ClientConn.RemoveSubConn for its existing SubConns.
 	Close()
 }

 // ExitIdler is an optional interface for balancers to implement.  If
 // implemented, ExitIdle will be called when ClientConn.Connect is called, if
 // the ClientConn is idle.  If unimplemented, ClientConn.Connect will cause
 // all SubConns to connect.
 //
 // Notice: it will be required for all balancers to implement this in a future
 // release.
 type ExitIdler interface {
 	// ExitIdle instructs the LB policy to reconnect to backends / exit the
 	// IDLE state, if appropriate and possible.  Note that SubConns that enter
 	// the IDLE state will not reconnect until SubConn.Connect is called.
 	ExitIdle()
 }

 // SubConnState describes the state of a SubConn.
 type SubConnState struct {
 	// ConnectivityState is the connectivity state of the SubConn.
 	ConnectivityState connectivity.State
 	// ConnectionError is set if the ConnectivityState is TransientFailure,
 	// describing the reason the SubConn failed.  Otherwise, it is nil.
 	ConnectionError error
 }

 // ClientConnState describes the state of a ClientConn relevant to the
 // balancer.
 type ClientConnState struct {
 	ResolverState resolver.State
 	// The parsed load balancing configuration returned by the builder's
 	// ParseConfig method, if implemented.
 	BalancerConfig serviceconfig.LoadBalancingConfig
 }

 // ErrBadResolverState may be returned by UpdateClientConnState to indicate a
 // problem with the provided name resolver data.
 var ErrBadResolverState = errors.New("bad resolver state")

 // ConnectivityStateEvaluator takes the connectivity states of multiple SubConns
 // and returns one aggregated connectivity state.
 //
 // It's not thread safe.
 type ConnectivityStateEvaluator struct {
 	numReady            uint64 // Number of addrConns in ready state.
 	numConnecting       uint64 // Number of addrConns in connecting state.
 	numTransientFailure uint64 // Number of addrConns in transient failure state.
 	numIdle             uint64 // Number of addrConns in idle state.
 }

 // RecordTransition records state change happening in subConn and based on that
 // it evaluates what aggregated state should be.
 //
 //   - If at least one SubConn in Ready, the aggregated state is Ready;
 //   - Else if at least one SubConn in Connecting, the aggregated state is Connecting;
 //   - Else if at least one SubConn is TransientFailure, the aggregated state is Transient Failure;
 //   - Else if at least one SubConn is Idle, the aggregated state is Idle;
 //   - Else there are no subconns and the aggregated state is Transient Failure
 //
 // Shutdown is not considered.
 func (cse *ConnectivityStateEvaluator) RecordTransition(oldState, newState connectivity.State) connectivity.State {
 	// Update counters.
 	for idx, state := range []connectivity.State{oldState, newState} {
 		updateVal := 2*uint64(idx) - 1 // -1 for oldState and +1 for new.
 		switch state {
 		case connectivity.Ready:
 			cse.numReady += updateVal
 		case connectivity.Connecting:
 			cse.numConnecting += updateVal
 		case connectivity.TransientFailure:
 			cse.numTransientFailure += updateVal
 		case connectivity.Idle:
 			cse.numIdle += updateVal
 		}
 	}

 	// Evaluate.
 	if cse.numReady > 0 {
 		return connectivity.Ready
 	}
 	if cse.numConnecting > 0 {
 		return connectivity.Connecting
 	}
 	if cse.numTransientFailure > 0 {
 		return connectivity.TransientFailure
 	}
 	if cse.numIdle > 0 {
 		return connectivity.Idle
 	}
 	return connectivity.TransientFailure
 }
	/*
	*
	* Copyright 2017 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 balancer defines APIs for load balancing in gRPC.
	// All APIs in this package are experimental.
	package balancer

	import (
	"context"
	"encoding/json"
	"errors"
	"net"
	"strings"

	"google.golang.org/grpc/connectivity"
	"google.golang.org/grpc/credentials"
	"google.golang.org/grpc/internal"
	"google.golang.org/grpc/metadata"
	"google.golang.org/grpc/resolver"
	"google.golang.org/grpc/serviceconfig"
	)

	var (
	// m is a map from name to balancer builder.
	m = make(map[string]Builder)
	)

	// Register registers the balancer builder to the balancer map. b.Name
	// (lowercased) will be used as the name registered with this builder. If the
	// Builder implements ConfigParser, ParseConfig will be called when new service
	// configs are received by the resolver, and the result will be provided to the
	// Balancer in UpdateClientConnState.
	//
	// NOTE: this function must only be called during initialization time (i.e. in
	// an init() function), and is not thread-safe. If multiple Balancers are
	// registered with the same name, the one registered last will take effect.
	func Register(b Builder) {
	m[strings.ToLower(b.Name())] = b
	}

	// unregisterForTesting deletes the balancer with the given name from the
	// balancer map.
	//
	// This function is not thread-safe.
	func unregisterForTesting(name string) {
	delete(m, name)
	}

	func init() {
	internal.BalancerUnregister = unregisterForTesting
	}

	// Get returns the resolver builder registered with the given name.
	// Note that the compare is done in a case-insensitive fashion.
	// If no builder is register with the name, nil will be returned.
	func Get(name string) Builder {
	if b, ok := m[strings.ToLower(name)]; ok {
	return b
	}
	return nil
	}

	// A SubConn represents a single connection to a gRPC backend service.
	//
	// Each SubConn contains a list of addresses.
	//
	// All SubConns start in IDLE, and will not try to connect. To trigger the
	// connecting, Balancers must call Connect. If a connection re-enters IDLE,
	// Balancers must call Connect again to trigger a new connection attempt.
	//
	// gRPC will try to connect to the addresses in sequence, and stop trying the
	// remainder once the first connection is successful. If an attempt to connect
	// to all addresses encounters an error, the SubConn will enter
	// TRANSIENT_FAILURE for a backoff period, and then transition to IDLE.
	//
	// Once established, if a connection is lost, the SubConn will transition
	// directly to IDLE.
	//
	// This interface is to be implemented by gRPC. Users should not need their own
	// implementation of this interface. For situations like testing, any
	// implementations should embed this interface. This allows gRPC to add new
	// methods to this interface.
	type SubConn interface {
	// UpdateAddresses updates the addresses used in this SubConn.
	// gRPC checks if currently-connected address is still in the new list.
	// If it's in the list, the connection will be kept.
	// If it's not in the list, the connection will gracefully closed, and
	// a new connection will be created.
	//
	// This will trigger a state transition for the SubConn.
	//
	// Deprecated: This method is now part of the ClientConn interface and will
	// eventually be removed from here.
	UpdateAddresses([]resolver.Address)
	// Connect starts the connecting for this SubConn.
	Connect()
	}

	// NewSubConnOptions contains options to create new SubConn.
	type NewSubConnOptions struct {
	// CredsBundle is the credentials bundle that will be used in the created
	// SubConn. If it's nil, the original creds from grpc DialOptions will be
	// used.
	//
	// Deprecated: Use the Attributes field in resolver.Address to pass
	// arbitrary data to the credential handshaker.
	CredsBundle credentials.Bundle
	// HealthCheckEnabled indicates whether health check service should be
	// enabled on this SubConn
	HealthCheckEnabled bool
	}

	// State contains the balancer's state relevant to the gRPC ClientConn.
	type State struct {
	// State contains the connectivity state of the balancer, which is used to
	// determine the state of the ClientConn.
	ConnectivityState connectivity.State
	// Picker is used to choose connections (SubConns) for RPCs.
	Picker Picker
	}

	// ClientConn represents a gRPC ClientConn.
	//
	// This interface is to be implemented by gRPC. Users should not need a
	// brand new implementation of this interface. For the situations like
	// testing, the new implementation should embed this interface. This allows
	// gRPC to add new methods to this interface.
	type ClientConn interface {
	// NewSubConn is called by balancer to create a new SubConn.
	// It doesn't block and wait for the connections to be established.
	// Behaviors of the SubConn can be controlled by options.
	NewSubConn([]resolver.Address, NewSubConnOptions) (SubConn, error)
	// RemoveSubConn removes the SubConn from ClientConn.
	// The SubConn will be shutdown.
	RemoveSubConn(SubConn)
	// UpdateAddresses updates the addresses used in the passed in SubConn.
	// gRPC checks if the currently connected address is still in the new list.
	// If so, the connection will be kept. Else, the connection will be
	// gracefully closed, and a new connection will be created.
	//
	// This will trigger a state transition for the SubConn.
	UpdateAddresses(SubConn, []resolver.Address)

	// UpdateState notifies gRPC that the balancer's internal state has
	// changed.
	//
	// gRPC will update the connectivity state of the ClientConn, and will call
	// Pick on the new Picker to pick new SubConns.
	UpdateState(State)

	// ResolveNow is called by balancer to notify gRPC to do a name resolving.
	ResolveNow(resolver.ResolveNowOptions)

	// Target returns the dial target for this ClientConn.
	//
	// Deprecated: Use the Target field in the BuildOptions instead.
	Target() string
	}

	// BuildOptions contains additional information for Build.
	type BuildOptions struct {
	// DialCreds is the transport credentials to use when communicating with a
	// remote load balancer server. Balancer implementations which do not
	// communicate with a remote load balancer server can ignore this field.
	DialCreds credentials.TransportCredentials
	// CredsBundle is the credentials bundle to use when communicating with a
	// remote load balancer server. Balancer implementations which do not
	// communicate with a remote load balancer server can ignore this field.
	CredsBundle credentials.Bundle
	// Dialer is the custom dialer to use when communicating with a remote load
	// balancer server. Balancer implementations which do not communicate with a
	// remote load balancer server can ignore this field.
	Dialer func(context.Context, string) (net.Conn, error)
	// Authority is the server name to use as part of the authentication
	// handshake when communicating with a remote load balancer server. Balancer
	// implementations which do not communicate with a remote load balancer
	// server can ignore this field.
	Authority string
	// ChannelzParentID is the parent ClientConn's channelz ID.
	ChannelzParentID int64
	// CustomUserAgent is the custom user agent set on the parent ClientConn.
	// The balancer should set the same custom user agent if it creates a
	// ClientConn.
	CustomUserAgent string
	// Target contains the parsed address info of the dial target. It is the
	// same resolver.Target as passed to the resolver. See the documentation for
	// the resolver.Target type for details about what it contains.
	Target resolver.Target
	}

	// Builder creates a balancer.
	type Builder interface {
	// Build creates a new balancer with the ClientConn.
	Build(cc ClientConn, opts BuildOptions) Balancer
	// Name returns the name of balancers built by this builder.
	// It will be used to pick balancers (for example in service config).
	Name() string
	}

	// ConfigParser parses load balancer configs.
	type ConfigParser interface {
	// ParseConfig parses the JSON load balancer config provided into an
	// internal form or returns an error if the config is invalid. For future
	// compatibility reasons, unknown fields in the config should be ignored.
	ParseConfig(LoadBalancingConfigJSON json.RawMessage) (serviceconfig.LoadBalancingConfig, error)
	}

	// PickInfo contains additional information for the Pick operation.
	type PickInfo struct {
	// FullMethodName is the method name that NewClientStream() is called
	// with. The canonical format is /service/Method.
	FullMethodName string
	// Ctx is the RPC's context, and may contain relevant RPC-level information
	// like the outgoing header metadata.
	Ctx context.Context
	}

	// DoneInfo contains additional information for done.
	type DoneInfo struct {
	// Err is the rpc error the RPC finished with. It could be nil.
	Err error
	// Trailer contains the metadata from the RPC's trailer, if present.
	Trailer metadata.MD
	// BytesSent indicates if any bytes have been sent to the server.
	BytesSent bool
	// BytesReceived indicates if any byte has been received from the server.
	BytesReceived bool
	// ServerLoad is the load received from server. It's usually sent as part of
	// trailing metadata.
	//
	// The only supported type now is *orca_v1.LoadReport.
	ServerLoad interface{}
	}

	var (
	// ErrNoSubConnAvailable indicates no SubConn is available for pick().
	// gRPC will block the RPC until a new picker is available via UpdateState().
	ErrNoSubConnAvailable = errors.New("no SubConn is available")
	// ErrTransientFailure indicates all SubConns are in TransientFailure.
	// WaitForReady RPCs will block, non-WaitForReady RPCs will fail.
	//
	// Deprecated: return an appropriate error based on the last resolution or
	// connection attempt instead. The behavior is the same for any non-gRPC
	// status error.
	ErrTransientFailure = errors.New("all SubConns are in TransientFailure")
	)

	// PickResult contains information related to a connection chosen for an RPC.
	type PickResult struct {
	// SubConn is the connection to use for this pick, if its state is Ready.
	// If the state is not Ready, gRPC will block the RPC until a new Picker is
	// provided by the balancer (using ClientConn.UpdateState). The SubConn
	// must be one returned by ClientConn.NewSubConn.
	SubConn SubConn

	// Done is called when the RPC is completed. If the SubConn is not ready,
	// this will be called with a nil parameter. If the SubConn is not a valid
	// type, Done may not be called. May be nil if the balancer does not wish
	// to be notified when the RPC completes.
	Done func(DoneInfo)
	}

	// TransientFailureError returns e. It exists for backward compatibility and
	// will be deleted soon.
	//
	// Deprecated: no longer necessary, picker errors are treated this way by
	// default.
	func TransientFailureError(e error) error { return e }

	// Picker is used by gRPC to pick a SubConn to send an RPC.
	// Balancer is expected to generate a new picker from its snapshot every time its
	// internal state has changed.
	//
	// The pickers used by gRPC can be updated by ClientConn.UpdateState().
	type Picker interface {
	// Pick returns the connection to use for this RPC and related information.
	//
	// Pick should not block. If the balancer needs to do I/O or any blocking
	// or time-consuming work to service this call, it should return
	// ErrNoSubConnAvailable, and the Pick call will be repeated by gRPC when
	// the Picker is updated (using ClientConn.UpdateState).
	//
	// If an error is returned:
	//
	// - If the error is ErrNoSubConnAvailable, gRPC will block until a new
	// Picker is provided by the balancer (using ClientConn.UpdateState).
	//
	// - If the error is a status error (implemented by the grpc/status
	// package), gRPC will terminate the RPC with the code and message
	// provided.
	//
	// - For all other errors, wait for ready RPCs will wait, but non-wait for
	// ready RPCs will be terminated with this error's Error() string and
	// status code Unavailable.
	Pick(info PickInfo) (PickResult, error)
	}

	// Balancer takes input from gRPC, manages SubConns, and collects and aggregates
	// the connectivity states.
	//
	// It also generates and updates the Picker used by gRPC to pick SubConns for RPCs.
	//
	// UpdateClientConnState, ResolverError, UpdateSubConnState, and Close are
	// guaranteed to be called synchronously from the same goroutine. There's no
	// guarantee on picker.Pick, it may be called anytime.
	type Balancer interface {
	// UpdateClientConnState is called by gRPC when the state of the ClientConn
	// changes. If the error returned is ErrBadResolverState, the ClientConn
	// will begin calling ResolveNow on the active name resolver with
	// exponential backoff until a subsequent call to UpdateClientConnState
	// returns a nil error. Any other errors are currently ignored.
	UpdateClientConnState(ClientConnState) error
	// ResolverError is called by gRPC when the name resolver reports an error.
	ResolverError(error)
	// UpdateSubConnState is called by gRPC when the state of a SubConn
	// changes.
	UpdateSubConnState(SubConn, SubConnState)
	// Close closes the balancer. The balancer is not required to call
	// ClientConn.RemoveSubConn for its existing SubConns.
	Close()
	}

	// ExitIdler is an optional interface for balancers to implement. If
	// implemented, ExitIdle will be called when ClientConn.Connect is called, if
	// the ClientConn is idle. If unimplemented, ClientConn.Connect will cause
	// all SubConns to connect.
	//
	// Notice: it will be required for all balancers to implement this in a future
	// release.
	type ExitIdler interface {
	// ExitIdle instructs the LB policy to reconnect to backends / exit the
	// IDLE state, if appropriate and possible. Note that SubConns that enter
	// the IDLE state will not reconnect until SubConn.Connect is called.
	ExitIdle()
	}

	// SubConnState describes the state of a SubConn.
	type SubConnState struct {
	// ConnectivityState is the connectivity state of the SubConn.
	ConnectivityState connectivity.State
	// ConnectionError is set if the ConnectivityState is TransientFailure,
	// describing the reason the SubConn failed. Otherwise, it is nil.
	ConnectionError error
	}

	// ClientConnState describes the state of a ClientConn relevant to the
	// balancer.
	type ClientConnState struct {
	ResolverState resolver.State
	// The parsed load balancing configuration returned by the builder's
	// ParseConfig method, if implemented.
	BalancerConfig serviceconfig.LoadBalancingConfig
	}

	// ErrBadResolverState may be returned by UpdateClientConnState to indicate a
	// problem with the provided name resolver data.
	var ErrBadResolverState = errors.New("bad resolver state")

	// ConnectivityStateEvaluator takes the connectivity states of multiple SubConns
	// and returns one aggregated connectivity state.
	//
	// It's not thread safe.
	type ConnectivityStateEvaluator struct {
	numReady uint64 // Number of addrConns in ready state.
	numConnecting uint64 // Number of addrConns in connecting state.
	numTransientFailure uint64 // Number of addrConns in transient failure state.
	numIdle uint64 // Number of addrConns in idle state.
	}

	// RecordTransition records state change happening in subConn and based on that
	// it evaluates what aggregated state should be.
	//
	// - If at least one SubConn in Ready, the aggregated state is Ready;
	// - Else if at least one SubConn in Connecting, the aggregated state is Connecting;
	// - Else if at least one SubConn is TransientFailure, the aggregated state is Transient Failure;
	// - Else if at least one SubConn is Idle, the aggregated state is Idle;
	// - Else there are no subconns and the aggregated state is Transient Failure
	//
	// Shutdown is not considered.
	func (cse *ConnectivityStateEvaluator) RecordTransition(oldState, newState connectivity.State) connectivity.State {
	// Update counters.
	for idx, state := range []connectivity.State{oldState, newState} {
	updateVal := 2*uint64(idx) - 1 // -1 for oldState and +1 for new.
	switch state {
	case connectivity.Ready:
	cse.numReady += updateVal
	case connectivity.Connecting:
	cse.numConnecting += updateVal
	case connectivity.TransientFailure:
	cse.numTransientFailure += updateVal
	case connectivity.Idle:
	cse.numIdle += updateVal
	}
	}

	// Evaluate.
	if cse.numReady > 0 {
	return connectivity.Ready
	}
	if cse.numConnecting > 0 {
	return connectivity.Connecting
	}
	if cse.numTransientFailure > 0 {
	return connectivity.TransientFailure
	}
	if cse.numIdle > 0 {
	return connectivity.Idle
	}
	return connectivity.TransientFailure
	}