# 8.4 Redis缓存实战分析

## 8.4.1 缓存不存在的键,以防穿透

代码已经实现.见[8.3.4 模拟缓存穿透现象](https://github.com/rayallen20/StudyRedisBaseOnDocker/blob/master/note/%E7%AC%AC8%E7%AB%A0%20GO%E6%95%B4%E5%90%88MySQL%E4%B8%8ERedis/8.3%20Redis%E4%B8%8EMySQL%E7%9A%84%E6%95%B4%E5%90%88.md#834-%E6%A8%A1%E6%8B%9F%E7%BC%93%E5%AD%98%E7%A9%BF%E9%80%8F%E7%8E%B0%E8%B1%A1)

## 8.4.2 合理设置超时时间,以防内存溢出

代码已经实现.见[8.3.5 模拟内存使用不当的场景](https://github.com/rayallen20/StudyRedisBaseOnDocker/blob/master/note/%E7%AC%AC8%E7%AB%A0%20GO%E6%95%B4%E5%90%88MySQL%E4%B8%8ERedis/8.3%20Redis%E4%B8%8EMySQL%E7%9A%84%E6%95%B4%E5%90%88.md#835-%E6%A8%A1%E6%8B%9F%E5%86%85%E5%AD%98%E4%BD%BF%E7%94%A8%E4%B8%8D%E5%BD%93%E7%9A%84%E5%9C%BA%E6%99%AF)

## 8.4.3 超时时间外加随机数,以防穿透

我们在之前设置的所有超时时间均为1小时(3600秒).假设我们在某一时刻批量添加了几千个缓存数据,那么按照之前程序设置的超时时间,在1小时之后,这几千个key会同时失效.那么对这批数据的请求会被同时发送到MySQL(当然我们是假定会有对这几千个key的请求),那么MySQL同样有可能崩溃.

解决方法:**设置超时时间的数值采用`整数 + 随机数`的方式**

在8.3小节的代码基础上修改:

工程结构如下:

```
(base) yanglei@yuanhong mysqlAndRedis % tree ./
./
├── biz
│   └── student.go
├── cache
│   ├── conf.go
│   ├── conn.go
│   ├── genExpireSecond.go
│   └── student.go
├── controller
│   └── student.go
├── db
│   ├── conf.go
│   ├── conn.go
│   └── student.go
├── go.mod
├── go.sum
├── lib
│   └── randInt.go
├── main.go
├── request
│   └── student
│       └── getStudentById.go
└── resp
    └── response.go

8 directories, 15 files
```

`lib/randInt.go`:

```go
package lib

import (
	"math/rand"
	"time"
)

func GenRandInt(ceiling int) int {
	rand.Seed(time.Now().UnixNano())
	return rand.Intn(ceiling)
}
```

`cache/genExpireSecond.go`:

```go
package cache

import "mysqlAndRedis/lib"

const ExpireSecond = 3600

const CeilSecond = 60

func genExpireSecond() int {
	return ExpireSecond + lib.GenRandInt(CeilSecond)
}
```

`cache/student.go`:

```go
package cache

import (
	"github.com/gomodule/redigo/redis"
	"strconv"
)

const StudentKeyPrefix = "Stu"

type Student struct {
	Id    int
	Name  string
	Age   int
	Score float64
	Exist bool
}

func (s *Student) FindById(id int) (err error) {
	// 判断键是否存在
	s.Exist, err = s.exists(id)
	if err != nil {
		return err
	}

	// 键不存在则直接返回
	if !s.Exist {
		return nil
	}

	// 确认键存在,则从redis中读取(有可能读到的是一个0值)
	idStr := strconv.Itoa(id)
	key := StudentKeyPrefix + idStr
	reply, err := redis.Values(Conn.Do("LRANGE", key, 0, -1))
	if err != nil {
		return err
	}

	s.Id, err = redis.Int(reply[0], nil)
	if err != nil {
		return err
	}

	s.Name, err = redis.String(reply[1], nil)
	if err != nil {
		return err
	}

	s.Age, err = redis.Int(reply[2], nil)
	if err != nil {
		return err
	}

	s.Score, err = redis.Float64(reply[3], nil)
	if err != nil {
		return err
	}

	// 若读取了该key 则重置过期时间
	// 此处忽略错误
	_ = s.setExpireTime(id)

	return nil
}

func (s *Student) SaveById(id int) error {
	idStr := strconv.Itoa(id)
	key := StudentKeyPrefix + idStr
	_, err := Conn.Do("RPUSH", key, s.Id, s.Name, s.Age, s.Score)
	if err != nil {
		return err
	}

	err = s.setExpireTime(id)
	if err != nil {
		return err
	}

	return nil
}

func (s *Student) exists(id int) (bool, error) {
	idStr := strconv.Itoa(id)
	key := StudentKeyPrefix + idStr
	reply, err := redis.Int(Conn.Do("EXISTS", key))
	if err != nil {
		return false, err
	}

	return reply == 1, nil
}

func (s *Student) setExpireTime(id int) error {
	idStr := strconv.Itoa(id)
	key := StudentKeyPrefix + idStr
	_, err := Conn.Do("EXPIRE", key, genExpireSecond())
	return err
}
```

运行后查询`id = 1`和`id = 2`的数据:

```
(base) yanglei@yuanhong ~ % redis-cli
127.0.0.1:6379> AUTH redis_user redis_password
OK
127.0.0.1:6379> TTL Stu1
(integer) 3610
127.0.0.1:6379> TTL Stu2
(integer) 3643
```

可以看到,各个键的生存时间都有所区别了


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