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



This section is empty.


var (
	// ErrNoSubConnAvailable indicates no SubConn is available for pick().
	// gRPC will block the RPC until a new picker is available via UpdateBalancerState().
	ErrNoSubConnAvailable = errors.New("no SubConn is available")
	// ErrTransientFailure indicates all SubConns are in TransientFailure.
	// WaitForReady RPCs will block, non-WaitForReady RPCs will fail.
	ErrTransientFailure = errors.New("all SubConns are in TransientFailure")


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.

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.


type Balancer

type Balancer interface {
	// HandleSubConnStateChange is called by gRPC when the connectivity state
	// of sc has changed.
	// Balancer is expected to aggregate all the state of SubConn and report
	// that back to gRPC.
	// Balancer should also generate and update Pickers when its internal state has
	// been changed by the new state.
	HandleSubConnStateChange(sc SubConn, state connectivity.State)
	// HandleResolvedAddrs is called by gRPC to send updated resolved addresses to
	// balancers.
	// Balancer can create new SubConn or remove SubConn with the addresses.
	// An empty address slice and a non-nil error will be passed if the resolver returns
	// non-nil error to gRPC.
	HandleResolvedAddrs([]resolver.Address, error)
	// Close closes the balancer. The balancer is not required to call
	// ClientConn.RemoveSubConn for its existing SubConns.

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.

HandleSubConnectionStateChange, HandleResolvedAddrs 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 credential the Balancer implementation can
	// use to dial to a remote load balancer server. The Balancer implementations
	// can ignore this if it does not need to talk to another party securely.
	DialCreds credentials.TransportCredentials
	// Dialer is the custom dialer the Balancer implementation can use to dial
	// to a remote load balancer server. The Balancer implementations
	// can ignore this if it doesn't need to talk to remote balancer.
	Dialer func(context.Context, string) (net.Conn, error)
	// ChannelzParentID is the entity parent's channelz unique identification number.
	ChannelzParentID int64

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-insenstive 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.
	NewSubConn([]resolver.Address, NewSubConnOptions) (SubConn, error)
	// RemoveSubConn removes the SubConn from ClientConn.
	// The SubConn will be shutdown.

	// UpdateBalancerState is called by balancer to nofity gRPC that some internal
	// state in balancer has changed.
	// gRPC will update the connectivity state of the ClientConn, and will call pick
	// on the new picker to pick new SubConn.
	UpdateBalancerState(s connectivity.State, p Picker)

	// ResolveNow is called by balancer to notify gRPC to do a name resolving.

	// Target returns the dial target for this ClientConn.
	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 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) 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 the aggregated state is TransientFailure.

Idle and Shutdown are not considered.

type DoneInfo

type DoneInfo struct {
	// Err is the rpc error the RPC finished with. It could be nil.
	Err error
	// 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

DoneInfo contains additional information for done.

type NewSubConnOptions

type NewSubConnOptions struct{}

NewSubConnOptions contains options to create new SubConn.

type PickOptions

type PickOptions struct {
	// FullMethodName is the method name that NewClientStream() is called
	// with. The canonical format is /service/Method.
	FullMethodName string

PickOptions contains addition information for the Pick operation.

type Picker

type Picker interface {
	// Pick returns the SubConn to be used to send the RPC.
	// The returned SubConn must be one returned by NewSubConn().
	// This functions is expected to return:
	// - a SubConn that is known to be READY;
	// - ErrNoSubConnAvailable if no SubConn is available, but progress is being
	//   made (for example, some SubConn is in CONNECTING mode);
	// - other errors if no active connecting is happening (for example, all SubConn
	//   are in TRANSIENT_FAILURE mode).
	// If a SubConn is returned:
	// - If it is READY, gRPC will send the RPC on it;
	// - If it is not ready, or becomes not ready after it's returned, gRPC will block
	//   until UpdateBalancerState() is called and will call pick on the new picker.
	// If the returned error is not nil:
	// - If the error is ErrNoSubConnAvailable, gRPC will block until UpdateBalancerState()
	// - If the error is ErrTransientFailure:
	//   - If the RPC is wait-for-ready, gRPC will block until UpdateBalancerState()
	//     is called to pick again;
	//   - Otherwise, RPC will fail with unavailable error.
	// - Else (error is other non-nil error):
	//   - The RPC will fail with unavailable error.
	// The returned done() function will be called once the rpc has finished, with the
	// final status of that RPC.
	// done may be nil if balancer doesn't care about the RPC status.
	Pick(ctx context.Context, opts PickOptions) (conn SubConn, done func(DoneInfo), err 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.UpdateBalancerState().

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.
	// Connect starts the connecting for this SubConn.

SubConn represents a gRPC sub connection. Each sub connection contains a list of addresses. gRPC will try to connect to them (in sequence), and stop trying the remainder once one connection is successful.

The reconnect backoff will be applied on the list, not a single address. For example, try_on_all_addresses -> backoff -> try_on_all_addresses.

All SubConns start in IDLE, and will not try to connect. To trigger the connecting, Balancers must call Connect. When the connection encounters an error, it will reconnect immediately. When the connection becomes IDLE, it will not reconnect unless Connect is called.

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.

Source Files


Path Synopsis
base Package base defines a balancer base that can be used to build balancers with different picking algorithms.
grpclb Package grpclb defines a grpclb balancer.
roundrobin Package roundrobin defines a roundrobin balancer.