GO—web程序中的请求缓存设置

背景

假设用户数据存在数据库，现在需要一个函数，通过用户名称返回用户信息。

期望：在一次web请求中，不过调用多少次这个函数，只请求一次数据库。

基本信息

type User struct {
	Name string
	Age  int
}

func GetALLUserFromDB() []*User {
	return []*User{
		{
			Name: "Tom",
			Age:  18,
		},
		{
			Name: "Jerry",
			Age:  20,
		},
	}
}

重点

go的赋值都是值copy

实现

函数调用数据库

// GetUser 每次从数据库获取数据，根据名称返回user
func GetUser(_ context.Context, name string) *User {
	fmt.Println("GetALLUserFromDB")
	allUser := GetALLUserFromDB()
	allUserMap := make(map[string]*User)
	for _, user := range allUser {
		allUserMap[user.Name] = user
	}
	return allUserMap[name]
}

func testGetUser(names []string) {
	for _, name := range names {
		fmt.Println(GetUser(context.Background(), name))
	}
}

• 每次函数调用都会走到数据库
• 不满足期望

使用一个map存储用户名和用户信息的关系

// GetUserWithParamCache 传入缓存map，如果缓存map为空，从数据库获取数据，根据名称返回user
func GetUserWithParamCache(_ context.Context, name string, cache map[string]*User) *User {
	if cache == nil {
		fmt.Println("GetALLUserFromDB")
		allUser := GetALLUserFromDB()
		allUserMap := make(map[string]*User)
		for _, user := range allUser {
			allUserMap[user.Name] = user
		}
		cache = allUserMap
	}
	return cache[name]
}

func testGetUserWithParamCache(names []string) {
	var cache map[string]*User
	for _, name := range names {
		fmt.Println(GetUserWithParamCache(context.Background(), name, cache))
	}
}

• 参数传递是值copy，这里的形参不会影响实参。所以下次请求该函数的时候，cache总是为空
• 注意，这里map的值copy是map的结构，其指向数据如果发生了变化，还是能影响实参的指向的数据的
  • 参考附录
• 不满足期望

使用一个map存储用户名和用户信息的关系，map参数是指针类型

// GetUserWithParamCacheWithPointer 传入缓存map指针，如果缓存map为空，从数据库获取数据，根据名称返回user
func GetUserWithParamCacheWithPointer(_ context.Context, name string, cache *map[string]*User) *User {
	if cache == nil || *cache == nil {
		fmt.Println("GetALLUserFromDB")
		allUser := GetALLUserFromDB()
		allUserMap := make(map[string]*User)
		for _, user := range allUser {
			allUserMap[user.Name] = user
		}
		*cache = allUserMap
	}
	return (*cache)[name]
}

func testGetUserWithParamCacheWithPointer(names []string) {
	var cache map[string]*User
	for _, name := range names {
		fmt.Println(GetUserWithParamCacheWithPointer(context.Background(), name, &cache))
	}
}

• 实参和形参都指向同一个map，该map变更时，实参指向的map也会变
• 满足期望

使用context存储缓存

// GetUserWithCtxCache 从context中获取缓存map，如果缓存map为空，从数据库获取数据，根据名称返回user
func GetUserWithCtxCache(ctx context.Context, name string) *User {
	cache, ok := ctx.Value("cache").(map[string]*User)
	if !ok {
		fmt.Println("GetALLUserFromDB")
		allUser := GetALLUserFromDB()
		allUserMap := make(map[string]*User)
		for _, user := range allUser {
			allUserMap[user.Name] = user
		}
		cache = allUserMap
		ctx = context.WithValue(ctx, "cache", allUserMap)
	}

	return cache[name]
}

func testGetUserWithCtxCache(names []string) {
	ctx := context.Background()
	for _, name := range names {
		fmt.Println(GetUserWithCtxCache(ctx, name))
	}
}

• context也是值copy，实参不受影响
• 不满足期望

context参数是指针，存储缓存

// GetUserWithCtxCacheWithPointer context为指针。从context中获取缓存map指针，如果缓存map为空，从数据库获取数据，根据名称返回user
func GetUserWithCtxCacheWithPointer(ctx *context.Context, name string) *User {
	cache, ok := (*ctx).Value("cache").(map[string]*User)
	if !ok {
		fmt.Println("GetALLUserFromDB")
		allUser := GetALLUserFromDB()
		allUserMap := make(map[string]*User)
		for _, user := range allUser {
			allUserMap[user.Name] = user
		}
		cache = allUserMap
		*ctx = context.WithValue(*ctx, "cache", allUserMap)
	}

	return cache[name]
}

func testGetUserWithCtxCacheWithPointer(names []string) {
	ctx := context.Background()
	for _, name := range names {
		fmt.Println(GetUserWithCtxCacheWithPointer(&ctx, name))
	}
}

• 满足期望

附录

context设置value的情景

	t := context.Background().Value("cache")
	x,ok := t.(map[string]*User)
	fmt.Println(x,ok)

	ct := context.WithValue(context.Background(), "cache", nil)
	t = ct.Value("cache")
	x,ok = t.(map[string]*User)
	fmt.Println(x,ok)

	ct = context.WithValue(context.Background(), "cache", make(map[string]*User))
	t = ct.Value("cache")
	x,ok = t.(map[string]*User)
	fmt.Println(x,ok)

	ct = context.WithValue(context.Background(), "cache", map[string]*User{"Tom":&User{Name:"Tom",Age:18},"Jerry":&User{Name:"Jerry",Age:20}})
	t = ct.Value("cache")
	x,ok = t.(map[string]*User)
	fmt.Println(x,ok)

map作为参数

package main

func ChangeMap(t map[string]string) {
	t["a"] = "b"

}

func main() {
	x := map[string]string{"a": "a"}
	ChangeMap(x)
	println(x["a"])
}

完整代码

package main

import (
	"context"
	"fmt"
)

type User struct {
	Name string
	Age  int
}

func GetALLUserFromDB() []*User {
	return []*User{
		{
			Name: "Tom",
			Age:  18,
		},
		{
			Name: "Jerry",
			Age:  20,
		},
	}
}

// GetUser 每次从数据库获取数据，根据名称返回user
func GetUser(_ context.Context, name string) *User {
	fmt.Println("GetALLUserFromDB")
	allUser := GetALLUserFromDB()
	allUserMap := make(map[string]*User)
	for _, user := range allUser {
		allUserMap[user.Name] = user
	}
	return allUserMap[name]
}

func testGetUser(names []string) {
	for _, name := range names {
		fmt.Println(GetUser(context.Background(), name))
	}
}

// GetUserWithParamCache 传入缓存map，如果缓存map为空，从数据库获取数据，根据名称返回user
func GetUserWithParamCache(_ context.Context, name string, cache map[string]*User) *User {
	if cache == nil {
		fmt.Println("GetALLUserFromDB")
		allUser := GetALLUserFromDB()
		allUserMap := make(map[string]*User)
		for _, user := range allUser {
			allUserMap[user.Name] = user
		}
		cache = allUserMap
	}
	return cache[name]
}

func testGetUserWithParamCache(names []string) {
	var cache map[string]*User
	for _, name := range names {
		fmt.Println(GetUserWithParamCache(context.Background(), name, cache))
	}
}

// GetUserWithParamCacheWithPointer 传入缓存map指针，如果缓存map为空，从数据库获取数据，根据名称返回user
func GetUserWithParamCacheWithPointer(_ context.Context, name string, cache *map[string]*User) *User {
	if cache == nil || *cache == nil {
		fmt.Println("GetALLUserFromDB")
		allUser := GetALLUserFromDB()
		allUserMap := make(map[string]*User)
		for _, user := range allUser {
			allUserMap[user.Name] = user
		}
		*cache = allUserMap
	}
	return (*cache)[name]
}

func testGetUserWithParamCacheWithPointer(names []string) {
	var cache map[string]*User
	for _, name := range names {
		fmt.Println(GetUserWithParamCacheWithPointer(context.Background(), name, &cache))
	}
}

// GetUserWithCtxCache 从context中获取缓存map，如果缓存map为空，从数据库获取数据，根据名称返回user
func GetUserWithCtxCache(ctx context.Context, name string) *User {
	cache, ok := ctx.Value("cache").(map[string]*User)
	if !ok {
		fmt.Println("GetALLUserFromDB")
		allUser := GetALLUserFromDB()
		allUserMap := make(map[string]*User)
		for _, user := range allUser {
			allUserMap[user.Name] = user
		}
		cache = allUserMap
		ctx = context.WithValue(ctx, "cache", allUserMap)
	}

	return cache[name]
}

func testGetUserWithCtxCache(names []string) {
	ctx := context.Background()
	for _, name := range names {
		fmt.Println(GetUserWithCtxCache(ctx, name))
	}
}

// GetUserWithCtxCacheWithPointer context为指针。从context中获取缓存map指针，如果缓存map为空，从数据库获取数据，根据名称返回user
func GetUserWithCtxCacheWithPointer(ctx *context.Context, name string) *User {
	cache, ok := (*ctx).Value("cache").(map[string]*User)
	if !ok {
		fmt.Println("GetALLUserFromDB")
		allUser := GetALLUserFromDB()
		allUserMap := make(map[string]*User)
		for _, user := range allUser {
			allUserMap[user.Name] = user
		}
		cache = allUserMap
		*ctx = context.WithValue(*ctx, "cache", allUserMap)
	}

	return cache[name]
}

func testGetUserWithCtxCacheWithPointer(names []string) {
	ctx := context.Background()
	for _, name := range names {
		fmt.Println(GetUserWithCtxCacheWithPointer(&ctx, name))
	}
}



func main() {
	names := []string{"Tom", "Jerry", "Lucy"}

	fmt.Println("=====================================", "testGetUser")
	testGetUser(names)
	fmt.Println("=====================================", "testGetUserWithParamCache")
	testGetUserWithParamCache(names)
	fmt.Println("=====================================", "testGetUserWithParamCacheWithPointer")
	testGetUserWithParamCacheWithPointer(names)
	fmt.Println("=====================================", "testGetUserWithCtxCache")
	testGetUserWithCtxCache(names)
	fmt.Println("=====================================", "testGetUserWithCtxCacheWithPointer")
	testGetUserWithCtxCacheWithPointer(names)

	t := context.Background().Value("cache")
	x,ok := t.(map[string]*User)
	fmt.Println(x,ok)

	ct := context.WithValue(context.Background(), "cache", nil)
	t = ct.Value("cache")
	x,ok = t.(map[string]*User)
	fmt.Println(x,ok)

	ct = context.WithValue(context.Background(), "cache", make(map[string]*User))
	t = ct.Value("cache")
	x,ok = t.(map[string]*User)
	fmt.Println(x,ok)

	ct = context.WithValue(context.Background(), "cache", map[string]*User{"Tom":&User{Name:"Tom",Age:18},"Jerry":&User{Name:"Jerry",Age:20}})
	t = ct.Value("cache")
	x,ok = t.(map[string]*User)
	fmt.Println(x,ok)

}