ASP .NET Application Life Cycle + Singleton Instance Life Time

Question

Please considerer the following scenario :

1. I have created a full-web application by using the ASP .NET MVC 3 framework. Now my application is managed by a web server.
2. An HTTP request is received on the server-side of my application.
3. A class implementing the singleton design pattern is instanciated on server-side.
4. A response is sent to the browser.
5. Another HTTP request is received on the server-side of my application. Is the singleton instance used at step 2 still available on server-side ?

I read some information about the life cycle of an ASP .NET application on this page : http://msdn.microsoft.com/en-us/library/ms178473.aspx

But I am still not able to answer my question.

Thanks in advance for your future help

---

I have just made some tests under VS2010.

Here is the list of the main components of my project :

• The Home controller containing an Index HttpGet action method.
• The view resulting from the Index action method.
• The SingletonTest class which implements the singleton design pattern.

Here is the code of the SingletonTest class :

public class SingletonTest
{
    private int counter;
    private static SingletonTest instance = null;

    public int Counter
    {
        get
        {
            return counter;
        }
    }

    public static SingletonTest Instance 
    {
        get 
        {
            if (instance == null)
                instance = new SingletonTest();

            return instance;
        }
    }

    private SingletonTest() 
    {
        counter = 0;
    }

    public void IncrementCounter()
    {
        counter++;
    }
}

Here is the code of the Index action method :

public ActionResult Index()
{
    SingletonTest st = SingletonTest.Instance;
    st.IncrementCounter();

    return View();
}

Here is the code of the view :

@SingletonTest.Instance.Counter

Here the test scenario I have followed :

1. The IIS server has been automatically launched by VS2010.
2. I have requested the /Home/Index/ URL then the value 1 has been displayed.
3. I have requested the /Home/Index/ URL then the value 2 has been displayed.
4. ...

This test shows that the SingletonTest instance made at Step 1 is available when processing the next requests. I guess that a memory space is allocated to my web application on the server.

Then I have stopped the IIS server and I have followed my test scenario again. I have got the same results as before : 1, 2, ....

Answer 1

Even though the singleton may persist across multiple requests you need to be careful for exactly the reasons of your second test - when IIS is restarted or the app pool is recycled everything will be lost.

Are you sure that you need a singleton instance?

If you're looking to persist some state across all requests it would be better to use an external storage such as a database.

Answer 2

What about multi-instances of the same application that IIS Will Create to process the concurrent requests?

I thinks singleton Object will not be the same if IIS Create multiple instances of the same application in high traffic situation