geecache.go

package geecache

import (
	"fmt"
	"geecache/singleflight"
	"log"
	"sync"
)

//A Getter loads data for a key.
type Getter interface {
	Get(key string) ([]byte, error)
}

//A GetterFunc implements Getter with a function
type GetterFunc func(key string) ([]byte, error)

//Get implements Getter interface function
func (f GetterFunc) Get(key string) ([]byte, error) {
	return f(key)
}

//A Group is a cache namespace and associated data load spread over
type Group struct {
	name      string
	getter    Getter
	mainCache cache
	peers     PeerPicker
	// use singleflight.Group to make sure that
	// each key is only fetched once
	loader  *singleflight.Group
}

var (
	mu     sync.RWMutex
	groups = make(map[string]*Group)
)

//NewGroup creat a new instance of group
func NewGroup(name string, cacheBytes int64, getter Getter) *Group {
	if getter == nil {
		panic("nil Getter")
	}
	mu.Lock()
	defer mu.Unlock()
	g := &Group{
		name:      name,
		getter:    getter,
		mainCache: cache{cacheBytes: cacheBytes},
		loader: &singleflight.Group{},
	}
	groups[name] = g
	return g
}

//GetGroup returns the named group previously creat with NewGroup,or
//nil if there's no such group
func GetGroup(name string) *Group {
	mu.RLock()
	g := groups[name]
	mu.Unlock()
	return g
}

//Get value for  a key from cache
func (g *Group) Get(key string) (ByteView, error) {
	if key == "" {
		return ByteView{}, fmt.Errorf("key is required")
	}
	if v, ok := g.mainCache.get(key); ok {
		log.Println("[GeeCache] hit")
		return v, nil
	}
	return g.load(key)
}

func (g *Group) getLocally(key string) (ByteView, error) {
	bytes, err := g.getter.Get(key)
	if err != nil {
		return ByteView{}, err
	}
	value := ByteView{b: cloneBytes(bytes)}
	g.populateCache(key, value)
	return value, nil
}

func (g *Group) populateCache(key string, value ByteView) {
	g.mainCache.add(key, value)
}

// RegisterPeers registers a PeerPicker for choosing remote peer
func (g *Group) RegisterPeers(peers PeerPicker) {
	if g.peers != nil {
		panic("RegisterPeerPicker called more than once")
	}
	g.peers = peers
}

func (g *Group) load(key string) (value ByteView, err error) {
	// each key is only fetched once (either locally or remotely)
	// regardless of the number of concurrent callers.
	viewi,err := g.loader.Do(key, func() (interface{}, error) {
		if g.peers != nil {
			if peer,ok:=g.peers.PickPeer(key);ok {
				if value,err = g.getFromPeer(peer,key);err == nil {
					return value,nil
				}
				log.Println("[GeeCache] Failed to get from peer", err)
			}
		}
		return g.getLocally(key)
	})
	if err != nil {
		return viewi.(ByteView),nil
	}
	return
}

func (g *Group) getFromPeer(peer PeerGetter, key string) (ByteView, error) {
	bytes, err := peer.Get(g.name, key)
	if err != nil {
		return ByteView{}, err
	}
	return ByteView{b: bytes}, nil
}