singleton.md

Singleton

The singleton pattern is used to ensure that there is only one instance of a resource shared by many consumers. For more information on the pattern, take a look at the singleton pattern page on Wikipedia.

This example is adapted from this blog post by Marcio Castilho

Usage

The typical implementation of a singleton usually involves the use of a GetInstance() method. Invoking this method will first check to see if an instance of the resource has already been created. If it has, that instance will be returned. If not, a new one will be created and returned.

The hard part here is to ensure that your GetInstance() method is thread-safe. The approach outlined by Marcio uses the Go sync.Once type to achieve a thread-safe implementation with very low overhead:

package singleton

import (
    "sync"
)

type singleton struct {
}

var instance *singleton
var once sync.Once

func GetInstance() *singleton {
    once.Do(func() {
        instance = &singleton{}
    })
    return instance
}

Here the once.Do() method wraps an anonymous function in which the singleton instance is created. Because sync.Once guarantees that the Do() method will only ever be called one time your instance value will never be allocated twice.