Refactor roundrobin to support custom picker (#1707)

Split round_robin balancer into a base balancer (which manages SubConn and connectivity states), and a round_robin picker.
A custom picker with other picking algorithms can be provided to build custom balancers.

balancer/base/balancer.go

@@ -0,0 +1,209 @@
+/*
+ *
+ * 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
+ *
+ *     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 base
+
+import (
+	"golang.org/x/net/context"
+	"google.golang.org/grpc/balancer"
+	"google.golang.org/grpc/connectivity"
+	"google.golang.org/grpc/grpclog"
+	"google.golang.org/grpc/resolver"
+)
+
+type baseBuilder struct {
+	name          string
+	pickerBuilder PickerBuilder
+}
+
+func (bb *baseBuilder) Build(cc balancer.ClientConn, opt balancer.BuildOptions) balancer.Balancer {
+	return &baseBalancer{
+		cc:            cc,
+		pickerBuilder: bb.pickerBuilder,
+
+		subConns: make(map[resolver.Address]balancer.SubConn),
+		scStates: make(map[balancer.SubConn]connectivity.State),
+		csEvltr:  &connectivityStateEvaluator{},
+		// Initialize picker to a picker that always return
+		// ErrNoSubConnAvailable, because when state of a SubConn changes, we
+		// may call UpdateBalancerState with this picker.
+		picker: NewErrPicker(balancer.ErrNoSubConnAvailable),
+	}
+}
+
+func (bb *baseBuilder) Name() string {
+	return bb.name
+}
+
+type baseBalancer struct {
+	cc            balancer.ClientConn
+	pickerBuilder PickerBuilder
+
+	csEvltr *connectivityStateEvaluator
+	state   connectivity.State
+
+	subConns map[resolver.Address]balancer.SubConn
+	scStates map[balancer.SubConn]connectivity.State
+	picker   balancer.Picker
+}
+
+func (b *baseBalancer) HandleResolvedAddrs(addrs []resolver.Address, err error) {
+	if err != nil {
+		grpclog.Infof("base.baseBalancer: HandleResolvedAddrs called with error %v", err)
+		return
+	}
+	grpclog.Infoln("base.baseBalancer: got new resolved addresses: ", addrs)
+	// addrsSet is the set converted from addrs, it's used for quick lookup of an address.
+	addrsSet := make(map[resolver.Address]struct{})
+	for _, a := range addrs {
+		addrsSet[a] = struct{}{}
+		if _, ok := b.subConns[a]; !ok {
+			// a is a new address (not existing in b.subConns).
+			sc, err := b.cc.NewSubConn([]resolver.Address{a}, balancer.NewSubConnOptions{})
+			if err != nil {
+				grpclog.Warningf("base.baseBalancer: failed to create new SubConn: %v", err)
+				continue
+			}
+			b.subConns[a] = sc
+			b.scStates[sc] = connectivity.Idle
+			sc.Connect()
+		}
+	}
+	for a, sc := range b.subConns {
+		// a was removed by resolver.
+		if _, ok := addrsSet[a]; !ok {
+			b.cc.RemoveSubConn(sc)
+			delete(b.subConns, a)
+			// Keep the state of this sc in b.scStates until sc's state becomes Shutdown.
+			// The entry will be deleted in HandleSubConnStateChange.
+		}
+	}
+}
+
+// regeneratePicker takes a snapshot of the balancer, and generates a picker
+// from it. The picker is
+//  - errPicker with ErrTransientFailure if the balancer is in TransientFailure,
+//  - built by the pickerBuilder with all READY SubConns otherwise.
+func (b *baseBalancer) regeneratePicker() {
+	if b.state == connectivity.TransientFailure {
+		b.picker = NewErrPicker(balancer.ErrTransientFailure)
+		return
+	}
+	readySCs := make(map[resolver.Address]balancer.SubConn)
+
+	// Filter out all ready SCs from full subConn map.
+	for addr, sc := range b.subConns {
+		if st, ok := b.scStates[sc]; ok && st == connectivity.Ready {
+			readySCs[addr] = sc
+		}
+	}
+	b.picker = b.pickerBuilder.Build(readySCs)
+}
+
+func (b *baseBalancer) HandleSubConnStateChange(sc balancer.SubConn, s connectivity.State) {
+	grpclog.Infof("base.baseBalancer: handle SubConn state change: %p, %v", sc, s)
+	oldS, ok := b.scStates[sc]
+	if !ok {
+		grpclog.Infof("base.baseBalancer: got state changes for an unknown SubConn: %p, %v", sc, s)
+		return
+	}
+	b.scStates[sc] = s
+	switch s {
+	case connectivity.Idle:
+		sc.Connect()
+	case connectivity.Shutdown:
+		// When an address was removed by resolver, b called RemoveSubConn but
+		// kept the sc's state in scStates. Remove state for this sc here.
+		delete(b.scStates, sc)
+	}
+
+	oldAggrState := b.state
+	b.state = b.csEvltr.recordTransition(oldS, s)
+
+	// Regenerate picker when one of the following happens:
+	//  - this sc became ready from not-ready
+	//  - this sc became not-ready from ready
+	//  - the aggregated state of balancer became TransientFailure from non-TransientFailure
+	//  - the aggregated state of balancer became non-TransientFailure from TransientFailure
+	if (s == connectivity.Ready) != (oldS == connectivity.Ready) ||
+		(b.state == connectivity.TransientFailure) != (oldAggrState == connectivity.TransientFailure) {
+		b.regeneratePicker()
+	}
+
+	b.cc.UpdateBalancerState(b.state, b.picker)
+	return
+}
+
+// Close is a nop because base balancer doesn't have internal state to clean up,
+// and it doesn't need to call RemoveSubConn for the SubConns.
+func (b *baseBalancer) Close() {
+}
+
+// NewErrPicker returns a picker that always returns err on Pick().
+func NewErrPicker(err error) balancer.Picker {
+	return &errPicker{err: err}
+}
+
+type errPicker struct {
+	err error // Pick() always returns this err.
+}
+
+func (p *errPicker) Pick(ctx context.Context, opts balancer.PickOptions) (balancer.SubConn, func(balancer.DoneInfo), error) {
+	return nil, nil, p.err
+}
+
+// connectivityStateEvaluator gets updated by addrConns when their
+// states transition, based on which it evaluates the state of
+// ClientConn.
+type connectivityStateEvaluator struct {
+	numReady            uint64 // Number of addrConns in ready state.
+	numConnecting       uint64 // Number of addrConns in connecting state.
+	numTransientFailure uint64 // Number of addrConns in transientFailure.
+}
+
+// recordTransition records state change happening in every subConn and based on
+// that it evaluates what aggregated state should be.
+// It can only transition between Ready, Connecting and TransientFailure. Other states,
+// Idle and Shutdown are transitioned into by ClientConn; in the beginning of the connection
+// before any subConn is created ClientConn is in idle state. In the end when ClientConn
+// closes it is in Shutdown state.
+//
+// recordTransition should only be called synchronously from the same goroutine.
+func (cse *connectivityStateEvaluator) recordTransition(oldState, newState connectivity.State) connectivity.State {
+	// Update counters.
+	for idx, state := range []connectivity.State{oldState, newState} {
+		updateVal := 2*uint64(idx) - 1 // -1 for oldState and +1 for new.
+		switch state {
+		case connectivity.Ready:
+			cse.numReady += updateVal
+		case connectivity.Connecting:
+			cse.numConnecting += updateVal
+		case connectivity.TransientFailure:
+			cse.numTransientFailure += updateVal
+		}
+	}
+
+	// Evaluate.
+	if cse.numReady > 0 {
+		return connectivity.Ready
+	}
+	if cse.numConnecting > 0 {
+		return connectivity.Connecting
+	}
+	return connectivity.TransientFailure
+}

balancer/base/base.go

@@ -0,0 +1,52 @@
+/*
+ *
+ * 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
+ *
+ *     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 base defines a balancer base that can be used to build balancers with
+// different picking algorithms.
+//
+// The base balancer creates a new SubConn for each resolved address. The
+// provided picker will only be notified about READY SubConns.
+//
+// This package is the base of round_robin balancer, its purpose is to be used
+// to build round_robin like balancers with complex picking algorithms.
+// Balancers with more complicated logic should try to implement a balancer
+// builder from scratch.
+//
+// All APIs in this package are experimental.
+package base
+
+import (
+	"google.golang.org/grpc/balancer"
+	"google.golang.org/grpc/resolver"
+)
+
+// PickerBuilder creates balancer.Picker.
+type PickerBuilder interface {
+	// Build takes a slice of ready SubConns, and returns a picker that will be
+	// used by gRPC to pick a SubConn.
+	Build(readySCs map[resolver.Address]balancer.SubConn) balancer.Picker
+}
+
+// NewBalancerBuilder returns a balancer builder. The balancers
+// built by this builder will use the picker builder to build pickers.
+func NewBalancerBuilder(name string, pb PickerBuilder) balancer.Builder {
+	return &baseBuilder{
+		name:          name,
+		pickerBuilder: pb,
+	}
+}