balancer

Documentation

Overview

Package balancer defines APIs for load balancing in gRPC. All APIs in this package are experimental.

Index

• Variables
• func Register(b Builder)
• func TransientFailureError(e error) error
• type Balancer
• type BuildOptions
• type Builder
  • func Get(name string) Builder
• type ClientConn
• type ClientConnState
• type ConfigParser
• type ConnectivityStateEvaluator
  • func (cse *ConnectivityStateEvaluator) CurrentState() connectivity.State
  • func (cse *ConnectivityStateEvaluator) RecordTransition(oldState, newState connectivity.State) connectivity.State
• type DoneInfo
• type ExitIdler
• type NewSubConnOptions
• type PickInfo
• type PickResult
• type Picker
• type Producer
• type ProducerBuilder
• type State
• type SubConn
• type SubConnState

Variables

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")
)

var ErrBadResolverState = errors.New("bad resolver state")

ErrBadResolverState may be returned by UpdateClientConnState to indicate a problem with the provided name resolver data.

Functions

func Register

func Register(b 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 TransientFailureError

func TransientFailureError(e error) error

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.

Types

type Balancer

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.
	//
	// Deprecated: Use NewSubConnOptions.StateListener when creating the
	// SubConn instead.
	UpdateSubConnState(SubConn, SubConnState)
	// Close closes the balancer. The balancer is not currently required to
	// call SubConn.Shutdown for its existing SubConns; however, this will be
	// required in a future release, so it is recommended.
	Close()
}

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 BuildOptions

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 *channelz.Identifier
	// 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
}

BuildOptions contains additional information for Build.

type Builder

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
}

Builder creates a balancer.

func Get

func Get(name string) Builder

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.

type ClientConn

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.
	//
	// Deprecated: please be aware that in a future version, SubConns will only
	// support one address per SubConn.
	NewSubConn([]resolver.Address, NewSubConnOptions) (SubConn, error)
	// RemoveSubConn removes the SubConn from ClientConn.
	// The SubConn will be shutdown.
	//
	// Deprecated: use SubConn.Shutdown instead.
	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 may trigger a state transition for the SubConn.
	//
	// Deprecated: this method will be removed.  Create new SubConns for new
	// addresses instead.
	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
}

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 ClientConnState

type ClientConnState struct {
	ResolverState resolver.State
	// The parsed load balancing configuration returned by the builder's
	// ParseConfig method, if implemented.
	BalancerConfig serviceconfig.LoadBalancingConfig
}

ClientConnState describes the state of a ClientConn relevant to the balancer.

type ConfigParser

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)
}

ConfigParser parses load balancer configs.

type ConnectivityStateEvaluator

type ConnectivityStateEvaluator struct {
	// contains filtered or unexported fields
}

ConnectivityStateEvaluator takes the connectivity states of multiple SubConns and returns one aggregated connectivity state.

It's not thread safe.

func (*ConnectivityStateEvaluator) CurrentState

func (cse *ConnectivityStateEvaluator) CurrentState() connectivity.State

CurrentState returns the current aggregate conn state by evaluating the counters

func (*ConnectivityStateEvaluator) RecordTransition

func (cse *ConnectivityStateEvaluator) RecordTransition(oldState, newState connectivity.State) connectivity.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 Idle, the aggregated state is Idle;
• Else if at least one SubConn is TransientFailure (or there are no SubConns), the aggregated state is Transient Failure.

Shutdown is not considered.

type DoneInfo

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_v3.LoadReport.
	ServerLoad any
}

DoneInfo contains additional information for done.

type ExitIdler

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()
}

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 NewSubConnOptions

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
	// StateListener is called when the state of the subconn changes.  If nil,
	// Balancer.UpdateSubConnState will be called instead.  Will never be
	// invoked until after Connect() is called on the SubConn created with
	// these options.
	StateListener func(SubConnState)
}

NewSubConnOptions contains options to create new SubConn.

type PickInfo

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
}

PickInfo contains additional information for the Pick operation.

type PickResult

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)

	// Metadata provides a way for LB policies to inject arbitrary per-call
	// metadata. Any metadata returned here will be merged with existing
	// metadata added by the client application.
	//
	// LB policies with child policies are responsible for propagating metadata
	// injected by their children to the ClientConn, as part of Pick().
	Metadata metadata.MD
}

PickResult contains information related to a connection chosen for an RPC.

type Picker

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)
}

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 Producer

type Producer any

A Producer is a type shared among potentially many consumers. It is associated with a SubConn, and an implementation will typically contain other methods to provide additional functionality, e.g. configuration or subscription registration.

type ProducerBuilder

type ProducerBuilder interface {
	// Build creates a Producer.  The first parameter is always a
	// grpc.ClientConnInterface (a type to allow creating RPCs/streams on the
	// associated SubConn), but is declared as `any` to avoid a dependency
	// cycle.  Should also return a close function that will be called when all
	// references to the Producer have been given up.
	Build(grpcClientConnInterface any) (p Producer, close func())
}

A ProducerBuilder is a simple constructor for a Producer. It is used by the SubConn to create producers when needed.

type State

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
}

State contains the balancer's state relevant to the gRPC ClientConn.

type SubConn

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 will be removed.  Create new SubConns for new
	// addresses instead.
	UpdateAddresses([]resolver.Address)
	// Connect starts the connecting for this SubConn.
	Connect()
	// GetOrBuildProducer returns a reference to the existing Producer for this
	// ProducerBuilder in this SubConn, or, if one does not currently exist,
	// creates a new one and returns it.  Returns a close function which must
	// be called when the Producer is no longer needed.
	GetOrBuildProducer(ProducerBuilder) (p Producer, close func())
	// Shutdown shuts down the SubConn gracefully.  Any started RPCs will be
	// allowed to complete.  No future calls should be made on the SubConn.
	// One final state update will be delivered to the StateListener (or
	// UpdateSubConnState; deprecated) with ConnectivityState of Shutdown to
	// indicate the shutdown operation.  This may be delivered before
	// in-progress RPCs are complete and the actual connection is closed.
	Shutdown()
}

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 SubConnState

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
}

SubConnState describes the state of a SubConn.