groupcache源码(2) Group

Group对象

• Group对象结构以及方法

type Group struct {
    name       string
    getter     Getter
    peersOnce  sync.Once
    peers      PeerPicker
    cacheBytes int64 // limit for sum of mainCache and hotCache size

    mainCache cache
    hotCache cache

    loadGroup flightGroup
}

func (g *Group) initPeers()
func (g *Group) Get()
func (g *Group) load()
func (g *Group) getLocally()
func (g *Group) getFromPeer()
func (g *Group) lookupCache()
func (g *Group) populateCache()

NewGroup()

• 根据程序执行顺序，NewHTTPPool()->NewGroup()

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

// 调用过程
func main() {
    // groupcache.GetterFunc是一个实现了Getter.Get接口的函数类型，用于转换自定义函数的类型
    // 这样才能符合NewGroup()的Getter类型传入参数要求
    image_cache := groupcache.NewGroup("testGroup", 8<<30, groupcache.GetterFunc(
        func(ctx groupcache.Context, key string, dest groupcache.Sink) error {
            rows, _ := db.Query("SELECT key, value FROM tbl_cache_map where key = ?", key)
            for rows.Next() {
                p := new(TblCache)
                err := rows.Scan(&p.Key, &p.Value)
                if err != nil {
                    fmt.Println(err)
                }
                fmt.Printf("get %s of value from tbl_cache_map\n", key)
                return dest.SetString("tbl_cache_map.value : " + p.Value)
            }
            return nil
        }))
}

• 需要缓存数据方式需要用户自定义
• 因此groupcache.GetterFunc用于给自定义函数转换类型

newGroup()

func newGroup(name string, cacheBytes int64, getter Getter, peers PeerPicker) *Group {
    if getter == nil {
        panic("nil Getter")
    }
    mu.Lock()
    defer mu.Unlock()

    // initPeerServerOnce Group唯一标识
    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)
    }
    
    // 添加GroupMap
    groups[name] = g
    return g
}

• 初始化Group对象以及添加GroupMap