peers.go

/*
Copyright 2012 Google Inc.

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.
*/

// peers.go defines how processes find and communicate with their peers.

package groupcache

import (
	"context"

	pb "github.com/golang/groupcache/groupcachepb"
)

// ProtoGetter is the interface that must be implemented by a peer.
type ProtoGetter interface {
	Get(ctx context.Context, in *pb.GetRequest, out *pb.GetResponse) error
}

// PeerPicker is the interface that must be implemented to locate
// the peer that owns a specific key.
type PeerPicker interface {
	// PickPeer returns the peer that owns the specific key
	// and true to indicate that a remote peer was nominated.
	// It returns nil, false if the key owner is the current peer.
	// PickPeer 根据 key 返回应该处理这个 key 的节点
	// ok 为 true 代表找到了节点
	// nil, false 代表当前节点就是 key 的处理器
	PickPeer(key string) (peer ProtoGetter, ok bool)
}

// NoPeers is an implementation of PeerPicker that never finds a peer.
type NoPeers struct{}

// 所有的peer结构都要实现PeerPicker接口，即给定一个字符串，返回一个ProtoGetter,ok bool。
// 在后面还会看到，如果是有peer的情况，用的是HTTPPool来代替NoPeers。
func (NoPeers) PickPeer(key string) (peer ProtoGetter, ok bool) { return }

var (
	portPicker func(groupName string) PeerPicker
)

// RegisterPeerPicker registers the peer initialization function.
// It is called once, when the first group is created.
// Either RegisterPeerPicker or RegisterPerGroupPeerPicker should be
// called exactly once, but not both.
func RegisterPeerPicker(fn func() PeerPicker) {
	if portPicker != nil {
		panic("RegisterPeerPicker called more than once")
	}
	portPicker = func(_ string) PeerPicker { return fn() }
}

// RegisterPerGroupPeerPicker registers the peer initialization function,
// which takes the groupName, to be used in choosing a PeerPicker.
// It is called once, when the first group is created.
// Either RegisterPeerPicker or RegisterPerGroupPeerPicker should be
// called exactly once, but not both.
func RegisterPerGroupPeerPicker(fn func(groupName string) PeerPicker) {
	if portPicker != nil {
		panic("RegisterPeerPicker called more than once")
	}
	portPicker = fn
}

// 会根据groupName返回这种peer结构，当然它们的共同点是一样的，也就是返回值类型为PeerPicker接口
func getPeers(groupName string) PeerPicker {
	if portPicker == nil {
		return NoPeers{}
	}
	pk := portPicker(groupName)
	if pk == nil {
		pk = NoPeers{}
	}
	return pk
}