缓存击穿：热key过期，同一时间大量请求进入到数据库
例子1：
package main
import (
"fmt"
"sync"
"time"
"golang.org/x/sync/singleflight"
)
func main() {
g := singleflight.Group{}
wg := sync.WaitGroup{}
for i := 0; i < 20; i++ {
wg.Add(1)
go func(j int) {
defer wg.Done()
// The return value shared indicates whether v was given to multiple callers, shared 表示返回 val 是否被多个调用者共享了https://segmentfault.com/q/1010000022916754?_ea=49700336
val, err, shared := g.Do("userId", queryMysql)
if err != nil {
fmt.Println(err)
return
}
fmt.Printf("index: %d, val: %d, shared: %t\n", j, val, shared)
}(i)
}
wg.Wait()
}
// 模拟数据库查询方法
func queryMysql() (interface{}, error) {
time.Sleep(time.Millisecond * 500)
fmt.Println("mysql query")
return 1, nil
}例子2：
package main
import (
"errors"
"log"
"sync"
"time"
"golang.org/x/sync/singleflight"
)
var errorNotExist = errors.New("not exist")
var g singleflight.Group
func main() {
var wg sync.WaitGroup
cpu_amount := 8
wg.Add(cpu_amount)
for i := 0; i < cpu_amount; i++ {
go func() {
defer wg.Done()
// sleep的作用是确保8个协程同时启动，需要多核cpu
time.Sleep(1 * time.Second)
data, err := getData("key")
if err != nil {
log.Print(err)
return
}
log.Println(data)
}()
}
wg.Wait()
}
// 获取数据
func getData(key string) (string, error) {
// 获取缓存
data, err := getDataFromCache(key)
if err == errorNotExist {
v, err, _ := g.Do(key, func() (interface{}, error) {
// 从数据库获取数据,只有第一个请求会进入数据库，其他请求会复用返回值）
return getDataFromDB(key)
})
if err != nil {
log.Println(err)
return "", err
}
data = v.(string)
} else if err != nil {
return "", err
}
return data, nil
}
func getDataFromCache(key string) (string, error) {
return "", errorNotExist
}
func getDataFromDB(key string) (string, error) {
log.Printf("get %s from database", key)
return "data", nil
}
来自anson博客
