Singleton vs Instances

[Abhinandan NM]

I have two patterns in my mind. Can you guys tell me which one is better? and why?

Pattern 1.

package main

import "System"

func main() {
  System.boot()
}

package System

import (
  "github.com/codegangsta/cli"
  "github.com/gorilla/mux"

  "gopkg.in/mgo.v2"

)

type app struct {
  Cmd        *cli.App
  HTTPRouter *mux.Router

  Ses        *mgo.Session

}

// App is a singleton throughout the application
var App = new(app)

// Boot bootstraps the application
func Boot() {

  App.Cmd = cli.NewApp()

  App.HTTPRouter = mux.NewRouter()
  App.Ses = mgo.Dial("mongodb://user:pa...@server.compose.io/db_name")
}

Pattern 2.

package main

import "app"

func main() {
  a := app.NewApp()
  
}

package app

import (
  "github.com/codegangsta/cli"
  "github.com/gorilla/mux"
  "gopkg.in/mgo.v2"
)

type components struct {
  Cmd        *cli.App
  HTTPRouter *mux.Router
  Ses        *mgo.Session
}

// NewApp provides new app instance
func NewApp() *app {
  c := new(components)

  c.Cmd = cli.NewApp()

  c.HTTPRouter = mux.NewRouter()
  c.Ses = mgo.Dial("mongodb://user:pa...@server.compose.io/db_name")
a := new(app)
return c, a
}

In first pattern, the app servers like a singleton. In the second pattern, I make new instances of the app whenever I require it. Which one is better?

[Tamás Gulácsi]

Better for what? You haven't stated your goals. Why do you want a singleton? If you want a Singleton, can't you create it in init() ?

[Abhinandan NM]

Well, is it better to have single instances of things like mongo sessions throughout the app or is it better to create and close it whenever I want it?

[Henrik Johansson]

That depends on the things. Not all things work the same way. You may want to factor in other aspects such as how you want to test your code and general maintainability as well.

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[Jesse McNelis]

On Mon, May 11, 2015 at 3:39 PM, Abhinandan NM <nmabhi...@gmail.com> wrote:
> Well, is it better to have single instances of things like mongo sessions
> throughout the app or is it better to create and close it whenever I want
> it?

If you look at the standard library there are many example of packages
that do both.

The net/http package has a http.Client called DefaultClient which all
the package level functions use.
eg. http.Get() just calls http.DefaultClient.Get()

But the DefaultClient is just an instance of http.Client.


So, callers with simple needs can conveniently the DefaultClient, and
callers with complex needs can instantiate their own http.Client.