算法之groupcache singleflight 防缓存崩溃算法

代码

一个很棒的代码，但是原理也很简单，主要是作者对互斥量和WaitGroup的妙用。

实现的主要功能就是防止大量的请求同时落库（ fn func() (interface{}, error)）， 在解决缓存击穿，缓存雪崩时有奇效。

package singleflight

import "sync"

// call is an in-flight or completed Do call
type call struct {
	wg  sync.WaitGroup
	val interface{}
	err error
}

// 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 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.
func (g *Group) Do(key string, fn func() (interface{}, error)) (interface{}, error) {
    // 加锁
	g.mu.Lock()
	if g.m == nil {
		g.m = make(map[string]*call)
	}
    // 如果已经有 call in flight, then wait
	if c, ok := g.m[key]; ok {
        // 解锁
		g.mu.Unlock()
        // 通过 wg 来等待， 因为另一个 g 已经对 wg.Add(1) 了
		c.wg.Wait()
		return c.val, c.err
	}
	c := new(call)
	c.wg.Add(1)
	g.m[key] = c
    // 这里不解锁，其他的 g 会先在 g.mu.Lock() 等待
	g.mu.Unlock()

	c.val, c.err = fn()
	c.wg.Done()

	g.mu.Lock()
	delete(g.m, key)
	g.mu.Unlock()

	return c.val, c.err
}