examples: Add custom load balancer example

balancer/endpointsharding/endpointsharding.go

@@ -0,0 +1,293 @@
+/*
+ *
+ * Copyright 2024 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 endpointsharding implements a load balancing policy that manages
+// homogenous child policies each owning a single endpoint.
+//
+// # Experimental
+//
+// Notice: This package is EXPERIMENTAL and may be changed or removed in a
+// later release.
+package endpointsharding
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"sync"
+	"sync/atomic"
+
+	"google.golang.org/grpc/balancer"
+	"google.golang.org/grpc/balancer/base"
+	"google.golang.org/grpc/connectivity"
+	"google.golang.org/grpc/internal/balancer/gracefulswitch"
+	"google.golang.org/grpc/internal/grpcrand"
+	"google.golang.org/grpc/resolver"
+	"google.golang.org/grpc/serviceconfig"
+)
+
+// ChildState is the balancer state of a child along with the endpoint which
+// identifies the child balancer.
+type ChildState struct {
+	Endpoint resolver.Endpoint
+	State    balancer.State
+}
+
+// NewBalancer returns a load balancing policy that manages homogenous child
+// policies each owning a single endpoint.
+func NewBalancer(cc balancer.ClientConn, opts balancer.BuildOptions) balancer.Balancer {
+	es := &endpointSharding{
+		cc:    cc,
+		bOpts: opts,
+	}
+	es.children.Store(resolver.NewEndpointMap())
+	return es
+}
+
+// endpointSharding is a balancer that wraps child balancers. It creates a child
+// balancer with child config for every unique Endpoint received. It updates the
+// child states on any update from parent or child.
+type endpointSharding struct {
+	cc    balancer.ClientConn
+	bOpts balancer.BuildOptions
+
+	children atomic.Pointer[resolver.EndpointMap]
+
+	// inhibitChildUpdates is set during UpdateClientConnState/ResolverError
+	// calls (calls to children will each produce an update, only want one
+	// update).
+	inhibitChildUpdates atomic.Bool
+
+	mu sync.Mutex // Sync updateState callouts and childState recent state updates
+}
+
+// UpdateClientConnState creates a child for new endpoints and deletes children
+// for endpoints that are no longer present. It also updates all the children,
+// and sends a single synchronous update of the childrens' aggregated state at
+// the end of the UpdateClientConnState operation. If any endpoint has no
+// addresses, returns error without forwarding any updates. Otherwise returns
+// first error found from a child, but fully processes the new update.
+func (es *endpointSharding) UpdateClientConnState(state balancer.ClientConnState) error {
+	if len(state.ResolverState.Endpoints) == 0 {
+		return errors.New("endpoints list is empty")
+	}
+	// Check/return early if any endpoints have no addresses.
+	// TODO: make this configurable if needed.
+	for i, endpoint := range state.ResolverState.Endpoints {
+		if len(endpoint.Addresses) == 0 {
+			return fmt.Errorf("endpoint %d has empty addresses", i)
+		}
+	}
+
+	es.inhibitChildUpdates.Store(true)
+	defer func() {
+		es.inhibitChildUpdates.Store(false)
+		es.updateState()
+	}()
+	var ret error
+
+	children := es.children.Load()
+	newChildren := resolver.NewEndpointMap()
+
+	// Update/Create new children.
+	for _, endpoint := range state.ResolverState.Endpoints {
+		if _, ok := newChildren.Get(endpoint); ok {
+			// Endpoint child was already created, continue to avoid duplicate
+			// update.
+			continue
+		}
+		var bal *balancerWrapper
+		if child, ok := children.Get(endpoint); ok {
+			bal = child.(*balancerWrapper)
+		} else {
+			bal = &balancerWrapper{
+				childState: ChildState{Endpoint: endpoint},
+				ClientConn: es.cc,
+				es:         es,
+			}
+			bal.Balancer = gracefulswitch.NewBalancer(bal, es.bOpts)
+		}
+		newChildren.Set(endpoint, bal)
+		if err := bal.UpdateClientConnState(balancer.ClientConnState{
+			BalancerConfig: state.BalancerConfig,
+			ResolverState: resolver.State{
+				Endpoints:  []resolver.Endpoint{endpoint},
+				Attributes: state.ResolverState.Attributes,
+			},
+		}); err != nil && ret == nil {
+			// Return first error found, and always commit full processing of
+			// updating children. If desired to process more specific errors
+			// across all endpoints, caller should make these specific
+			// validations, this is a current limitation for simplicities sake.
+			ret = err
+		}
+	}
+	// Delete old children that are no longer present.
+	for _, e := range children.Keys() {
+		child, _ := children.Get(e)
+		bal := child.(balancer.Balancer)
+		if _, ok := newChildren.Get(e); !ok {
+			bal.Close()
+		}
+	}
+	es.children.Store(newChildren)
+	return ret
+}
+
+// ResolverError forwards the resolver error to all of the endpointSharding's
+// children and sends a single synchronous update of the childStates at the end
+// of the ResolverError operation.
+func (es *endpointSharding) ResolverError(err error) {
+	es.inhibitChildUpdates.Store(true)
+	defer func() {
+		es.inhibitChildUpdates.Store(false)
+		es.updateState()
+	}()
+	children := es.children.Load()
+	for _, child := range children.Values() {
+		bal := child.(balancer.Balancer)
+		bal.ResolverError(err)
+	}
+}
+
+func (es *endpointSharding) UpdateSubConnState(sc balancer.SubConn, state balancer.SubConnState) {
+	// UpdateSubConnState is deprecated.
+}
+
+func (es *endpointSharding) Close() {
+	children := es.children.Load()
+	for _, child := range children.Values() {
+		bal := child.(balancer.Balancer)
+		bal.Close()
+	}
+}
+
+// updateState updates this component's state. It sends the aggregated state,
+// and a picker with round robin behavior with all the child states present if
+// needed.
+func (es *endpointSharding) updateState() {
+	if es.inhibitChildUpdates.Load() {
+		return
+	}
+	var readyPickers, connectingPickers, idlePickers, transientFailurePickers []balancer.Picker
+
+	es.mu.Lock()
+	defer es.mu.Unlock()
+
+	children := es.children.Load()
+	childStates := make([]ChildState, 0, children.Len())
+
+	for _, child := range children.Values() {
+		bw := child.(*balancerWrapper)
+		childState := bw.childState
+		childStates = append(childStates, childState)
+		childPicker := childState.State.Picker
+		switch childState.State.ConnectivityState {
+		case connectivity.Ready:
+			readyPickers = append(readyPickers, childPicker)
+		case connectivity.Connecting:
+			connectingPickers = append(connectingPickers, childPicker)
+		case connectivity.Idle:
+			idlePickers = append(idlePickers, childPicker)
+		case connectivity.TransientFailure:
+			transientFailurePickers = append(transientFailurePickers, childPicker)
+			// connectivity.Shutdown shouldn't appear.
+		}
+	}
+
+	// Construct the round robin picker based off the aggregated state. Whatever
+	// the aggregated state, use the pickers present that are currently in that
+	// state only.
+	var aggState connectivity.State
+	var pickers []balancer.Picker
+	if len(readyPickers) >= 1 {
+		aggState = connectivity.Ready
+		pickers = readyPickers
+	} else if len(connectingPickers) >= 1 {
+		aggState = connectivity.Connecting
+		pickers = connectingPickers
+	} else if len(idlePickers) >= 1 {
+		aggState = connectivity.Idle
+		pickers = idlePickers
+	} else if len(transientFailurePickers) >= 1 {
+		aggState = connectivity.TransientFailure
+		pickers = transientFailurePickers
+	} else {
+		aggState = connectivity.TransientFailure
+		pickers = []balancer.Picker{base.NewErrPicker(errors.New("no children to pick from"))}
+	} // No children (resolver error before valid update).
+	p := &pickerWithChildStates{
+		pickers:     pickers,
+		childStates: childStates,
+		next:        uint32(grpcrand.Intn(len(pickers))),
+	}
+	es.cc.UpdateState(balancer.State{
+		ConnectivityState: aggState,
+		Picker:            p,
+	})
+}
+
+// pickerWithChildStates delegates to the pickers it holds in a round robin
+// fashion. It also contains the childStates of all the endpointSharding's
+// children.
+type pickerWithChildStates struct {
+	pickers     []balancer.Picker
+	childStates []ChildState
+	next        uint32
+}
+
+func (p *pickerWithChildStates) Pick(info balancer.PickInfo) (balancer.PickResult, error) {
+	nextIndex := atomic.AddUint32(&p.next, 1)
+	picker := p.pickers[nextIndex%uint32(len(p.pickers))]
+	return picker.Pick(info)
+}
+
+// ChildStatesFromPicker returns the state of all the children managed by the
+// endpoint sharding balancer that created this picker.
+func ChildStatesFromPicker(picker balancer.Picker) []ChildState {
+	p, ok := picker.(*pickerWithChildStates)
+	if !ok {
+		return nil
+	}
+	return p.childStates
+}
+
+// balancerWrapper is a wrapper of a balancer. It ID's a child balancer by
+// endpoint, and persists recent child balancer state.
+type balancerWrapper struct {
+	balancer.Balancer   // Simply forward balancer.Balancer operations.
+	balancer.ClientConn // embed to intercept UpdateState, doesn't deal with SubConns
+
+	es *endpointSharding
+
+	childState ChildState
+}
+
+func (bw *balancerWrapper) UpdateState(state balancer.State) {
+	bw.es.mu.Lock()
+	bw.childState.State = state
+	bw.es.mu.Unlock()
+	bw.es.updateState()
+}
+
+func ParseConfig(cfg json.RawMessage) (serviceconfig.LoadBalancingConfig, error) {
+	return gracefulswitch.ParseConfig(cfg)
+}
+
+// PickFirstConfig is a pick first config without shuffling enabled.
+const PickFirstConfig = "[{\"pick_first\": {}}]"

examples/examples_test.sh

@@ -55,6 +55,7 @@ EXAMPLES=(
     "features/authz"
     "features/cancellation"
     "features/compression"
+    "features/customloadbalancer"
     "features/deadline"
     "features/encryption/TLS"
     "features/error_details"
@@ -109,6 +110,7 @@ declare -A EXPECTED_SERVER_OUTPUT=(
     ["features/authz"]="unary echoing message \"hello world\""
     ["features/cancellation"]="server: error receiving from stream: rpc error: code = Canceled desc = context canceled"
     ["features/compression"]="UnaryEcho called with message \"compress\""
+    ["features/customloadbalancer"]="serving on localhost:50051"
     ["features/deadline"]=""
     ["features/encryption/TLS"]=""
     ["features/error_details"]=""
@@ -132,6 +134,7 @@ declare -A EXPECTED_CLIENT_OUTPUT=(
     ["features/authz"]="UnaryEcho:  hello world"
     ["features/cancellation"]="cancelling context"
     ["features/compression"]="UnaryEcho call returned \"compress\", <nil>"
+    ["features/customloadbalancer"]="Successful multiple iterations of 1:2 ratio"
     ["features/deadline"]="wanted = DeadlineExceeded, got = DeadlineExceeded"
     ["features/encryption/TLS"]="UnaryEcho:  hello world"
     ["features/error_details"]="Greeting: Hello world"

examples/features/customloadbalancer/README.md

@@ -0,0 +1,52 @@
+# Custom Load Balancer
+
+This example shows how to deploy a custom load balancer in a `ClientConn`.
+
+## Try it
+
+```
+go run server/main.go
+```
+
+```
+go run client/main.go
+```
+
+## Explanation
+
+Two echo servers are serving on "localhost:20000" and "localhost:20001". They
+will include their serving address in the response. So the server on
+"localhost:20001" will reply to the RPC with `this is
+examples/customloadbalancing (from localhost:20001)`.
+
+A client is created, to connect to both of these servers (they get both server
+addresses from the name resolver in two separate endpoints). The client is
+configured with the load balancer specified in the service config, which in this
+case is custom_round_robin.
+
+### custom_round_robin
+
+The client is configured to use `custom_round_robin`. `custom_round_robin`
+creates a pick first child for every endpoint it receives. It waits until both
+pick first children become ready, then defers to the first pick first child's
+picker, choosing the connection to localhost:20000, except every chooseSecond
+times, where it defers to second pick first child's picker, choosing the
+connection to localhost:20001 (or vice versa).
+
+`custom_round_robin` is written as a delegating policy wrapping `pick_first`
+load balancers, one for every endpoint received. This is the intended way a user
+written custom lb should be specified, as pick first will contain a lot of
+useful functionality, such as Sticky Transient Failure, Happy Eyeballs, and
+Health Checking.
+
+```
+this is examples/customloadbalancing (from localhost:50050)
+this is examples/customloadbalancing (from localhost:50050)
+this is examples/customloadbalancing (from localhost:50051)
+this is examples/customloadbalancing (from localhost:50050)
+this is examples/customloadbalancing (from localhost:50050)
+this is examples/customloadbalancing (from localhost:50051)
+this is examples/customloadbalancing (from localhost:50050)
+this is examples/customloadbalancing (from localhost:50050)
+this is examples/customloadbalancing (from localhost:50051)
+```