高并发的场景下，经常会出现并发重复请求资源的情况。
比如说，缓存失效时，我们去请求db获取最新的数据，如果这个key是一个热key，那么在缓存失效的瞬间，可能会有大量的并发请求访问到db，导致db访问量陡增，甚至是打崩db，这种场景也就是我们常说的缓存击穿。
并发请求资源
针对同一个key的并发请求，这些请求和响应实际上都是一样的。所以我们可以把这种并发请求优化为：只进行一次实际请求去访问资源，然后得到实际响应，所有的并发请求共享这个实际响应的结果
针对分布式场景，我们可以使用分布式锁来实现
针对单机场景，我们可以使用singleflight来实现
singleflight
singleflightsingleflight是golang内置的一个包，这个包提供了对重复函数调用的抑制功能，也就是保证并发请求只会有一个实际请求去访问资源，所有并发请求共享实际响应。
使用singleflight在golang sdk源码中的路径为：src/internal/singleflight
但是internal是golang sdk内部的包，所以我们不能直接去使用
使用步骤：
- 引入go mod
- 使用singleflight包
go get golang.org/x/sync
singleflight包主要提供了三个方法
// 方法作用：保证并发请求只会执行一次函数，并共享实际响应
// 请求参数
// key：请求的唯一标识，相同的key会被视为并发请求
// fn：实际需要执行的函数
// 响应参数
// v：实际执行函数的返回值
// err：实际执行函数的错误
// shared：返回值v是否被共享，若存在并发请求，则为true；若不存在并发请求则为false
func (g *Group) Do(key string, fn func() (any, error)) (v any, err error, shared bool)
// 方法作用：和Do类似，不过方法返回的是chan
func (g *Group) DoChan(key string, fn func() (any, error)) (<-chan Result, bool)
// 方法作用：删除key，一般来说不会直接使用这个方法
func (g *Group) ForgetUnshared(key string) bool
针对以上的三个方法，我们重点了解一下Do方法的使用即可
没有使用singleflight之前
package main
import (
"fmt"
"sync"
"Testing"
"time"
)
var (
mx sync.Mutex
wg sync.WaitGroup
cacheData = make(map[string]string, 0)
)
func TestSingleFlight(t *testing.T) {
// 添加10个任务，模拟并发请求
wg.Add(10)
for i := 0; i < 10; i {
go getData("demo")
}
// 等待所有任务完成
wg.Wait()
}
func getData(key string) {
data, _ := getDataFromCache(key)
if len(data) == 0 {
// 缓存没有找到，则进行回源
data, _ = getDataFromDB(key)
// 设置缓存
mx.Lock()
cacheData[key] = data
mx.Unlock()
}
fmt.Println(data)
// 任务完成
wg.Done()
}
func getDataFromCache(key string) (string, error) {
return cacheData[key], nil
}
func getDataFromDB(key string) (string, error) {
fmt.Println("getDataFromDB key: ", key)
// 模拟访问db的耗时
time.Sleep(10 * time.Millisecond)
return "db data", nil
}
执行TestSingleFlight函数后，会发现并发请求多次调用了getDataFromDB函数
使用singleflight之后
package main
import (
"fmt"
"golang.org/x/sync/singleflight"
"sync"
"testing"
"time"
)
var (
mx sync.Mutex
wg sync.WaitGroup
g singleflight.Group
cacheData = make(map[string]string, 0)
)
func TestSingleFlight(t *testing.T) {
// 添加10个任务
wg.Add(10)
for i := 0; i < 10; i {
go getDataSingleWarp("demo")
}
// 等待所有任务完成
wg.Wait()
}
func getDataSingleWarp(key string) {
data, _ := getDataFromCache(key)
if len(data) == 0 {
// 使用singleflight来避免并发请求，实际改动就这一行
d, _, shared := g.Do(key, func() (interface{}, Error) {
return getDataFromDB(key)
})
fmt.Println(shared)
data = d.(string)
// 设置缓存
mx.Lock()
cacheData[key] = data
mx.Unlock()
}
fmt.Println(data)
wg.Done()
}
func getDataFromCache(key string) (string, error) {
return cacheData[key], nil
}
func getDataFromDB(key string) (string, error) {
fmt.Println("getDataFromDB key: ", key)
// 模拟访问db的耗时
time.Sleep(10 * time.Millisecond)
return "db data", nil
}
执行TestSingleFlight函数后，会发现只调用了一次getDataFromDB函数
源码分析- Group struct：封装并发请求
- call struct：每一个需要执行的函数，都会被封装成一个call
- func Do：对并发请求进行控制的方法
// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package singleflight provides a duplicate function call suppression
// mechanism.
package singleflight // import "golang.org/x/sync/singleflight"
import (
"bytes"
"errors"
"fmt"
"runtime"
"runtime/debug"
"sync"
)
// errGoexit indicates the runtime.Goexit was called in
// the user given function.
var errGoexit = errors.New("runtime.Goexit was called")
// A panicError is an arbitrary value recovered from a panic
// with the stack trace during the execution of given function.
type panicError struct {
value interface{}
stack []byte
}
// Error implements error interface.
func (p *panicError) Error() string {
return fmt.Sprintf("%v\n\n%s", p.value, p.stack)
}
func newPanicError(v interface{}) error {
stack := debug.Stack()
// The first line of the stack trace is of the form "goroutine N [status]:"
// but by the time the panic reaches Do the goroutine may no longer exist
// and its status will have changed. Trim out the misleading line.
if line := bytes.IndexByte(stack[:], '\n'); line >= 0 {
stack = stack[line 1:]
}
return &panicError{value: v, stack: stack}
}
// call is an in-flight or completed singleflight.Do call
type call struct {
// 保证相同key，只会进行一次实际请求
// 相同key的并发请求会共享返回
wg sync.WaitGroup
// These fields are written once before the WaitGroup is done
// and are only read after the WaitGroup is done.
// 实际执行函数的返回值和错误
val interface{}
err error
// forgotten indicates whether Forget was called with this call's key
// while the call was still in flight.
// 是否已删除当前并发请求的key
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.
// 并发请求的次数
dups int
chans []chan<- Result
}
// 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
// key代表请求的唯一标识，相同的key会被视为并发请求
// value代表实际请求，每一个实际请求都会被封装为call
m map[string]*call // lazily initialized
}
// Result holds the results of Do, so they can be passed
// on a channel.
type Result struct {
Val interface{}
Err error
Shared bool
}
// 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) {
// 加锁
g.mu.Lock()
// 懒加载
if g.m == nil {
g.m = make(map[string]*call)
}
// 判断是否有并发请求，如果key已经存在，则说明存在并发请求
if c, ok := g.m[key]; ok {
// 并发请求次数 1
c.dups
// 解锁
g.mu.Unlock()
// 等待实际请求执行完
c.wg.Wait()
if e, ok := c.err.(*panicError); ok {
panic(e)
} else if c.err == errGoexit {
runtime.Goexit()
}
// 共享响应
return c.val, c.err, true
}
c := new(call)
c.wg.Add(1)
// 添加并发请求key
g.m[key] = c
// 解锁
g.mu.Unlock()
// 进行实际请求
g.doCall(c, key, fn)
return c.val, c.err, c.dups > 0
}
// DoChan is like Do but returns a channel that will receive the
// results when they are ready.
//
// The returned channel will not be closed.
func (g *Group) DoChan(key string, fn func() (interface{}, error)) <-chan Result {
ch := make(chan Result, 1)
g.mu.Lock()
if g.m == nil {
g.m = make(map[string]*call)
}
if c, ok := g.m[key]; ok {
c.dups
c.chans = append(c.chans, ch)
g.mu.Unlock()
return ch
}
c := &call{chans: []chan<- Result{ch}}
c.wg.Add(1)
g.m[key] = c
g.mu.Unlock()
go g.doCall(c, key, fn)
return ch
}
// doCall handles the single call for a key.
func (g *Group) doCall(c *call, key string, fn func() (interface{}, error)) {
// 正常返回标识
normalReturn := false
// 是否执行了recover标识
recovered := false
// use double-defer to distinguish panic from runtime.Goexit,
// more details see https://golang.org/cl/134395
defer func() {
// the given function invoked runtime.Goexit
if !normalReturn && !recovered {
c.err = errGoexit
}
// 实际请求执行完成
c.wg.Done()
// 加锁
g.mu.Lock()
defer g.mu.Unlock()
// 删除并发请求key
if !c.forgotten {
delete(g.m, key)
}
if e, ok := c.err.(*panicError); ok {
// In order to prevent the waiting channels from being blocked forever,
// needs to ensure that this panic cannot be recovered.
if len(c.chans) > 0 {
go panic(e)
select {} // Keep this goroutine around so that it will appear in the crash dump.
} else {
panic(e)
}
} else if c.err == errGoexit {
// Already in the process of goexit, no need to call again
} else {
// Normal return
for _, ch := range c.chans {
ch <- Result{c.val, c.err, c.dups > 0}
}
}
}()
// 匿名函数立即执行
func() {
defer func() {
if !normalReturn {
// Ideally, we would wait to take a stack trace until we've determined
// whether this is a panic or a runtime.Goexit.
//
// Unfortunately, the only way we can distinguish the two is to see
// whether the recover stopped the goroutine from terminating, and by
// the time we know that, the part of the stack trace relevant to the
// panic has been discarded.
if r := recover(); r != nil {
c.err = newPanicError(r)
}
}
}()
// 执行实际函数
c.val, c.err = fn()
// 正常返回
normalReturn = true
}()
if !normalReturn {
recovered = true
}
}
// Forget tells the singleflight to forget about a key. Future calls
// to Do for this key will call the function rather than waiting for
// an earlier call to complete.
func (g *Group) Forget(key string) {
g.mu.Lock()
if c, ok := g.m[key]; ok {
c.forgotten = true
}
delete(g.m, key)
g.mu.Unlock()
}
