singleflight使用场景

1. 缓存击穿：缓存在某个时间点过期的时候，恰好在这个时间点对这个Key有大量的并发请求过来，这些请求发现缓存过期一般都会从后端DB加载数据并回设到缓存，这个时候大并发的请求可能会瞬间把后端DB压垮。
   • 绝大多数公司都是这么用的
2. 请求资源去重复
   • 我们的用法，需要改动一行代码。

singleflight 简介

singleflight 在 golang.org/x/sync/singleflight 项目下，对外提供了以下几个方法

//Do方法，传入key，以及回调函数，如果key相同，fn方法只会执行一次，同步等待
//返回值v:表示fn执行结果
//返回值err:表示fn的返回的err
//返回值shared：表示是否是真实fn返回的还是从保存的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方法类似Do方法，只是返回的是一个chan
func (g *Group) DoChan(key string, fn func() (interface{}, error)) <-chan Result {
//设计Forget 控制key关联的值是否失效，默认以上两个方法只要fn方法执行完成后，内部维护的fn的值也删除（即并发结束后就失效了）
func (g *Group) Forget(key string) 

singleflight的使用

从singleflight的test探寻最简单用法

func TestDo(t *testing.T) {
    var g Group
    // key 可以理解资源的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)
    }
}

验证并发重复请求

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!")
    }
}

• 结果

singleflight的原理

call

call 用来表示一个正在执行或已完成的函数调用。

// 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用来记录fn发放执行的返回值
    val interface{}
    err error

    // forgotten indicates whether Forget was called with this call's key
    // while the call was still in flight.
    // 用来标识fn方法执行完成之后结果是否立马删除还是保留在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 用来记录fn方法执行的次数
    dups  int
    //用来记录DoChan中调用次数以及需要返回的数据
    chans []chan<- Result
}

Group

Group 可以看做是任务的分类。

// 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 函数

// 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)
    // 设置forgotten = true, doCall时 不再调用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方法中是通过waitgroup来控制的，主要流程如下：

1. 在Group中设置了一个map，如果key不存在，则实例化call(用来保存值信息)，并将key=>call的对应关系存入map中通过mutex保证了并发安全
2. 如果已经在调用中则key已经存在map，则wg.Wait
3. 在fn执行结束之后（在doCall方法中执行）执行wg.Done
4. 卡在第2步的方法得到执行，返回结果

其他的DoChan方法也是类似的逻辑，只是返回的是一个chan。