目錄
- Posted by 微博@Yangsc_o
- 原創文章,版權宣告:自由轉載-非商用-非衍生-保持署名 | Creative Commons BY-NC-ND 3.0
前言
最近從java轉到go,來公司第一個開發工作就是對一個資源請求去重複,最終發現這個singleflight這個好東西,分享一下。
singleflight使用場景
- 快取擊穿:快取在某個時間點過期的時候,恰好在這個時間點對這個Key有大量的併發請求過來,這些請求發現快取過期一般都會從後端DB載入資料並回設到快取,這個時候大併發的請求可能會瞬間把後端DB壓垮。
- 絕大多數公司都是這麼用的
- 請求資源去重複
- 我們的用法,需要改動一行程式碼。
singleflight 簡介
singleflight
在 golang.org/x/sync/singleflight
專案下,對外提供了以下幾個方法
//Do方法,傳入key,以及回撥函式,如果key相同,fn方法只會執行一次,同步等待
//返回值v:表示fn執行結果
//返回值err:表示fn的返回的err
//返回值shared:表示是否是真實fn返回的還是從儲存的map[key]返回的,也就是共享的
func (g *Group) Do(key string, fn func() (interface{}, error)) (v interface{}, err error, shared bool) {
//DoChan方法類似Do方法,只是返回的是一個chan
func (g *Group) DoChan(key string, fn func() (interface{}, error)) <-chan Result {
//設計Forget 控制key關聯的值是否失效,預設以上兩個方法只要fn方法執行完成後,內部維護的fn的值也刪除(即併發結束後就失效了)
func (g *Group) Forget(key string)
singleflight的使用
從singleflight的test探尋最簡單用法
func TestDo(t *testing.T) {
var g Group
// key 可以理解資源的id
v, err, _ := g.Do("key", func() (interface{}, error) {
// do what you want
return "bar", nil
})
if got, want := fmt.Sprintf("%v (%T)", v, v), "bar (string)"; got != want {
t.Errorf("Do = %v; want %v", got, want)
}
if err != nil {
t.Errorf("Do error = %v", err)
}
}
驗證併發重複請求
func process(g *Group, t *testing.T, ch chan int, key string) {
for count := 0; count < 10; count++ {
v, err, shared := g.Do(key, func() (interface{}, error) {
time.Sleep(1000 * time.Millisecond)
return "bar", nil
})
t.Log("v = ", v, " err = ", err, " shared =", shared, " ch :", ch, "g ", len(g.m))
if got, want := fmt.Sprintf("%v (%T)", v, v), "bar (string)"; got != want {
t.Errorf("Do = %v; want %v", got, want)
}
if err != nil {
t.Errorf("Do error = %v", err)
}
}
ch <- 1
}
func TestDo1(t *testing.T) {
var g Group
channels := make([]chan int, 10)
key := "key"
for i := 0; i < 10; i++ {
channels[i] = make(chan int)
go process(&g, t, channels[i], key)
}
for i, ch := range channels {
<-ch
fmt.Println("routine ", i, "quit!")
}
}
- 結果
singleflight的原理
call
call 用來表示一個正在執行或已完成的函式呼叫。
// call is an in-flight or completed singleflight.Do call
type call struct {
wg sync.WaitGroup
// These fields are written once before the WaitGroup is done
// and are only read after the WaitGroup is done.
//val和err用來記錄fn發放執行的返回值
val interface{}
err error
// forgotten indicates whether Forget was called with this call's key
// while the call was still in flight.
// 用來標識fn方法執行完成之後結果是否立馬刪除還是保留在singleflight中
forgotten bool
// These fields are read and written with the singleflight
// mutex held before the WaitGroup is done, and are read but
// not written after the WaitGroup is done.
//dups 用來記錄fn方法執行的次數
dups int
//用來記錄DoChan中呼叫次數以及需要返回的資料
chans []chan<- Result
}
Group
Group 可以看做是任務的分類。
// Group represents a class of work and forms a namespace in which
// units of work can be executed with duplicate suppression.
type Group struct {
mu sync.Mutex // protects m
m map[string]*call // lazily initialized
}
Do 函式
// Do executes and returns the results of the given function, making
// sure that only one execution is in-flight for a given key at a
// time. If a duplicate comes in, the duplicate caller waits for the
// original to complete and receives the same results.
// The return value shared indicates whether v was given to multiple callers.
func (g *Group) Do(key string, fn func() (interface{}, error)) (v interface{}, err error, shared bool) {
g.mu.Lock()
if g.m == nil {
g.m = make(map[string]*call)
}
if c, ok := g.m[key]; ok {
c.dups++
g.mu.Unlock()
c.wg.Wait()
return c.val, c.err, true
}
c := new(call)
// 設定forgotten = true, doCall時 不再呼叫delete(g.m, key)
// c.forgotten = true
c.wg.Add(1)
g.m[key] = c
g.mu.Unlock()
g.doCall(c, key, fn)
return c.val, c.err, c.dups > 0
}
// doCall handles the single call for a key.
func (g *Group) doCall(c *call, key string, fn func() (interface{}, error)) {
c.val, c.err = fn()
c.wg.Done()
g.mu.Lock()
if !c.forgotten {
delete(g.m, key)
}
for _, ch := range c.chans {
ch <- Result{c.val, c.err, c.dups > 0}
}
g.mu.Unlock()
}
在Do方法中是通過waitgroup來控制的,主要流程如下:
- 在Group中設定了一個map,如果key不存在,則例項化call(用來儲存值資訊),並將key=>call的對應關係存入map中通過mutex保證了併發安全
- 如果已經在呼叫中則key已經存在map,則wg.Wait
- 在fn執行結束之後(在doCall方法中執行)執行wg.Done
- 卡在第2步的方法得到執行,返回結果
其他的DoChan方法也是類似的邏輯,只是返回的是一個chan。