golang中cache组件的使用之groupcache（二）

mapgroups

初始化

callInitPeerServer

mu.Lock()
defer mu.Unlock()
initPeerServerOnce.Do(callInitPeerServer)

singleflight.GroupnamegetterpeerscacheBytes

	g := &Group{
		name:       name,
		getter:     getter,
		peers:      peers,
		cacheBytes: cacheBytes,
		loadGroup:  &singleflight.Group{},
	}

mainCachehotCache_Stats

type Group struct {
	name       string
	getter     Getter
	peersOnce  sync.Once
	peers      PeerPicker
	cacheBytes int64 
	mainCache cache
	hotCache cache
	loadGroup flightGroup

	_ int32 
	Stats Stats
}

源码

func NewGroup(name string, cacheBytes int64, getter Getter) *Group {
	return newGroup(name, cacheBytes, getter, nil)
}

// If peers is nil, the peerPicker is called via a sync.Once to initialize it.
func newGroup(name string, cacheBytes int64, getter Getter, peers PeerPicker) *Group {
	if getter == nil {
		panic("nil Getter")
	}
	mu.Lock()
	defer mu.Unlock()
	initPeerServerOnce.Do(callInitPeerServer)
	if _, dup := groups[name]; dup {
		panic("duplicate registration of group " + name)
	}
	g := &Group{
		name:       name,
		getter:     getter,
		peers:      peers,
		cacheBytes: cacheBytes,
		loadGroup:  &singleflight.Group{},
	}
	if fn := newGroupHook; fn != nil {
		fn(g)
	}
	groups[name] = g
	return g
}

get value

获取时会首先初始化一次所有的peers（仅初始化一次）,然后更新计数

g.peersOnce.Do(g.initPeers)
g.Stats.Gets.Add(1)

先从主缓存中获取数据，然后是热缓存。如果命中直接返回

value, cacheHit := g.lookupCache(key)

if cacheHit {
	g.Stats.CacheHits.Add(1)
	return setSinkView(dest, value)
}

loadGroupgetter

func (g *Group) load(ctx context.Context, key string, dest Sink) (value ByteView, destPopulated bool, err error) {
	g.Stats.Loads.Add(1)
	viewi, err := g.loadGroup.Do(key, func() (interface{}, error) {
		// Check the cache again because singleflight can only dedup calls
		// that overlap concurrently.  It's possible for 2 concurrent
		// requests to miss the cache, resulting in 2 load() calls.  An
		// unfortunate goroutine scheduling would result in this callback
		// being run twice, serially.  If we don't check the cache again,
		// cache.nbytes would be incremented below even though there will
		// be only one entry for this key.
		//
		// Consider the following serialized event ordering for two
		// goroutines in which this callback gets called twice for the
		// same key:
		// 1: Get("key")
		// 2: Get("key")
		// 1: lookupCache("key")
		// 2: lookupCache("key")
		// 1: load("key")
		// 2: load("key")
		// 1: loadGroup.Do("key", fn)
		// 1: fn()
		// 2: loadGroup.Do("key", fn)
		// 2: fn()
		if value, cacheHit := g.lookupCache(key); cacheHit {
			g.Stats.CacheHits.Add(1)
			return value, nil
		}
		g.Stats.LoadsDeduped.Add(1)
		var value ByteView
		var err error
		if peer, ok := g.peers.PickPeer(key); ok {
			value, err = g.getFromPeer(ctx, peer, key)
			if err == nil {
				g.Stats.PeerLoads.Add(1)
				return value, nil
			}
			g.Stats.PeerErrors.Add(1)
			// TODO(bradfitz): log the peer's error? keep
			// log of the past few for /groupcachez?  It's
			// probably boring (normal task movement), so not
			// worth logging I imagine.
		}
		value, err = g.getLocally(ctx, key, dest)
		if err != nil {
			g.Stats.LocalLoadErrs.Add(1)
			return nil, err
		}
		g.Stats.LocalLoads.Add(1)
		destPopulated = true // only one caller of load gets this return value
		g.populateCache(key, value, &g.mainCache)
		return value, nil
	})
	if err == nil {
		value = viewi.(ByteView)
	}
	return
}

源码

func (g *Group) Get(ctx context.Context, key string, dest Sink) error {
	g.peersOnce.Do(g.initPeers)
	g.Stats.Gets.Add(1)
	if dest == nil {
		return errors.New("groupcache: nil dest Sink")
	}
	value, cacheHit := g.lookupCache(key)

	if cacheHit {
		g.Stats.CacheHits.Add(1)
		return setSinkView(dest, value)
	}

	// Optimization to avoid double unmarshalling or copying: keep
	// track of whether the dest was already populated. One caller
	// (if local) will set this; the losers will not. The common
	// case will likely be one caller.
	destPopulated := false
	value, destPopulated, err := g.load(ctx, key, dest)
	if err != nil {
		return err
	}
	if destPopulated {
		return nil
	}
	return setSinkView(dest, value)
}