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* 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
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* 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 (
var (
// m is a map from name to balancer builder.
m = make(map[string]Builder)
// defaultBuilder is the default balancer to use.
defaultBuilder Builder // TODO(bar) install pickfirst as default.
// Register registers the balancer builder to the balancer map.
// b.Name will be used as the name registered with this builder.
func Register(b Builder) {
m[b.Name()] = b
// Get returns the resolver builder registered with the given name.
// If no builder is register with the name, the default pickfirst will
// be used.
func Get(name string) Builder {
if b, ok := m[name]; ok {
return b
return defaultBuilder
// 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.
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.
// NewSubConnOptions contains options to create new SubConn.
type NewSubConnOptions struct{}
// ClientConn represents a gRPC 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)
// Target returns the dial target for this ClientConn.
Target() string
// BuildOptions contains additional information for Build.
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)
// 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
// PickOptions contains addition information for the Pick operation.
type PickOptions struct{}
// DoneInfo contains additional information for done.
type DoneInfo struct {
// Err is the rpc error the RPC finished with. It could be nil.
Err error
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")
// Picker is used by gRPC to pick a SubConn to send an RPC.
// Balancer is expected to generate a new picker from its snapshot everytime its
// internal state has changed.
// The pickers used by gRPC can be updated by ClientConn.UpdateBalancerState().
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
// this call until a new picker is updated 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)
// 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.
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.