Go by Example: Pattern: Singleton

3 ways of singleton pattern

 
package main

import (
    "fmt"
    "sync"
)

type singleton struct {
    a int
}

var instance *singleton
var mu sync.Mutex

1) singleton pattern
not thread safe

 
func GetInstance() *singleton {
    if instance == nil {
        instance = new(singleton)
    }
    return instance
}

2) thread safe with lock
check -> lock -> check to reduce the lock access

 
func GetInstanceThreadSafe() *singleton {
    if instance == nil {
        mu.Lock()
        defer mu.Unlock()
        if instance == nil {
            instance = new(singleton)
        }
    }
    return instance
}

Once is an object that will perform exactly one action.

 
var once sync.Once

3) singleton with once.Do

 
func GetInstanceThreadSafe2() *singleton {
    once.Do(func() {
        instance = new(singleton)
    })
    return instance
}

func main() {
    i1 := GetInstance()
    i2 := GetInstance()
    fmt.Printf("%p, %p, equal ? %v\n", i1, i2, i1 == i2)

    fmt.Println("GetInstanceThreadSafe")
    var wg sync.WaitGroup
    for i := 0; i < 5; i++ {
        wg.Add(1)
        go func() {
            p := GetInstanceThreadSafe()
            fmt.Printf("%p ", p)
            wg.Done()
        }()
    }
    wg.Wait()

    instance = nil
    fmt.Println("")
    fmt.Println("GetInstanceThreadSafe2")
    for i := 0; i < 5; i++ {
        wg.Add(1)
        go func() {
            p := GetInstanceThreadSafe2()
            fmt.Printf("%p ", p)
            wg.Done()
        }()
    }
    wg.Wait()
}

ref: https://blog.kennycoder.io/2021/08/22/Golang-Singleton-%E5%AF%A6%E7%8F%BE%E6%96%B9%E5%BC%8F%E6%8E%A2%E8%A8%8E/

 
$ go run pattern-singleton.go
 
0x140000a4008, 0x140000a4008, equal ? true
GetInstanceThreadSafe
0x140000a4008 0x140000a4008 0x140000a4008 0x140000a4008 0x140000a4008 
GetInstanceThreadSafe2
0x14000104010 0x14000104010 0x14000104010 0x14000104010 0x14000104010

Next example: Pattern: Fan-In.

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