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内存缓存-设计

 2 years ago
source link: https://forrestsu.github.io/posts/architecture/cache/memory-cache-design/
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内存缓存-设计

2021年12月8日
| 字数 1226

在业务开发场景中，主要用到两个开发能力：接口粘合+缓存。

一个好的缓存设计，能够降低服务的时延，抵抗流量洪峰。 比如最近的西安健康码事件，明显缓存设计上是存在明显问题的 :-)。

2 什么场景需要加缓存？

  • 提高服务质量，降低时延；
  • 减少对下游的请求量，节省资源

3 缓存常见问题

缓存问题 解决办法 (1) 缓存失效的策略 惰性过期（读取时判断如果过期了，就返回且异步更新） (2) 缓存集中失效 设置随机过期时间 (3) 缓存击穿(大量穿透导致雪崩) singleflight 合并多个相同的请求

3.1 50行代码，实现一个可用的 singleflight？

groupCache 库实现了一个可用的 singleflight，借助于 sync.WaitGroup 和 sync.Mutex， 代码非常简洁。

// 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) {
	g.mu.Lock()
	if g.m == nil {
		g.m = make(map[string]*call)
	}
	if c, ok := g.m[key]; ok {
		g.mu.Unlock()
		c.wg.Wait()
		return c.val, c.err
	}
	c := new(call)
	c.wg.Add(1)
	g.m[key] = c
	g.mu.Unlock()
	// execute fn
	c.val, c.err = fn()
	c.wg.Done()
	// remove key
	g.mu.Lock()
	delete(g.m, key)
	g.mu.Unlock()
	return c.val, c.err
}

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