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)
• type Balancer
• type BuildOptions
• type Builder
  • func Get(name string) Builder
• type ClientConn
• type DoneInfo
• type NewSubConnOptions
• type PickOptions
• type Picker
• type SubConn

Variables

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")
)

Functions

func Register

func Register(b Builder)

Register registers the balancer builder to the balancer map. b.Name will be used as the name registered with this builder.

Types

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.
	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.

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)
}

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. If no builder is register with the name, the default pickfirst will be used.

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.
	RemoveSubConn(SubConn)

	// 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
}

ClientConn represents a gRPC ClientConn.

type DoneInfo

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

DoneInfo contains additional information for done.

type NewSubConnOptions

type NewSubConnOptions struct{}

NewSubConnOptions contains options to create new SubConn.

type PickOptions

type PickOptions struct{}

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
	//   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)
}

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 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.
	UpdateAddresses([]resolver.Address)
	// Connect starts the connecting for this SubConn.
	Connect()
}

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.