groupcache peer.go

peer定义了获取远程数据的框架？
先有两个interface，为peer必须实现的接口

```// 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.
//peer必须实现的接口
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(key string) (peer ProtoGetter, ok bool)
}

peerpicker接口的方法给定一个字符串，返回一个ProtoGetter，以及表示是否成功的ok，如果是单节点的话则用的是NoPeers，分布式情况下使用HTTPPool来代替NoPeers

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

根据groupName来返回peerpicker

func getPeers(groupName string) PeerPicker {
    if portPicker == nil {
        return NoPeers{}
    }
    pk := portPicker(groupName)
    if pk == nil {
        pk = NoPeers{}
    }
    return pk
}

函数类型,返回PeerPicker,根据上下文理解这个只会初始化一次.在Httppool中可以看到

var (
    portPicker func(groupName string) PeerPicker
)
func RegisterPeerPicker(fn func() PeerPicker) {
    if portPicker != nil {
        panic("RegisterPeerPicker called more than once")
    }
    portPicker = func(_ string) PeerPicker { return fn() }
}

func RegisterPerGroupPeerPicker(fn func(groupName string) PeerPicker) {
    if portPicker != nil {
        panic("RegisterPeerPicker called more than once")
    }
    portPicker = fn
}

总结下这个文件里的调用顺序应该就是protpicker->PeerPicker->ProtoGetter->Get