Saturday, April 17, 2010

ASP.NET application and page life cycle
Introduction
In this article we will try to understand what are the different events which takes place right from the time the user sends a request, until the time request is rendered on the browser. So we will first try to understand the two broader steps of an ASP.NET request and then we will move in to different events emitted from ‘HttpHandler’, ‘HttpModule’ and ASP.NET page object. As we move in this event journey we will try to understand what kind of
logic should go in each every of these events.

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The Two step process

From 30,000 feet level ASP.NET request processing is a 2 step process as shown below. User sends a request to the IIS:-
• ASP.NET creates an environment which can process the request. In other words it creates the application object, request, response and context objects to process the request.

• Once the environment is created the request is processed through series of events which is processed by using modules, handlers and page objects. To keep it short lets name this step as MHPM (Module, handler, page and Module event), we will come to details later.



In the coming sections we will understand both these main steps in more details.Creation of ASP.NET environment
Step 1:- The user sends a request to IIS. IIS first checks which ISAPI extension can serve this request. Depending on file extension the request is processed. For instance if the page is an ‘.ASPX page’ then it will be passed to ‘aspnet_isapi.dll’ for processing.
Step 2:- If this the first request to the website then a class called as ‘ApplicationManager’ creates an application domain where the website can run. As we all know application domain creates isolation between two web applications hosted on the same IIS. So in case there is issue in one app domain it does not affect the other app domain.
Step 3:- The newly created application domain creates hosting environment i.e. the ‘HttpRuntime’ object. Once the hosting environment is created necessary core ASP.NET objects like ‘HttpContext’ , ‘HttpRequest’ and ‘HttpResponse’ objects are created.
Step 4:- Once all the core ASP.NET objects are created ‘HttpApplication’ object is created to serve the request. In case you have a ‘global.asax’ file in your system then object of the ‘global.asax’ file will be created. Please note
‘global.asax’ file inherits from ‘HttpApplication’ class.
Note: The first time an ASP.NET page is attached to an application, a new instance of ‘HttpApplication’ is created. Said and done to maximize performance, ‘HttpApplication’ instances might be reused for multiple requests.Step 5:- The ‘HttpApplication’ object is then assigned to the core ASP.NET objects to process the page.
Step 6:- ‘HttpApplication’ then starts processing the request by http module events , handlers and page events. It fires the MHPM event for request processing.
Note: - For more details



http://msdn.microsoft.com/en-us/library/ms178473.aspx




Below image explains how the internal object model looks like for an ASP.NET request. At the top level is the ASP.NET runtime which has creates an ‘Appdomain’ which in turn has ‘HttpRuntime’ with ‘request’, ‘response’ and ‘context’ objects.



Process request using MHPM events fired


Once ‘HttpApplication’ is created it starts processing request it goes through 3 different sections ‘HttpModule’ , ‘Page’ and ‘HttpHandler’. As it moves through these sections it invokes different events which the developer can extend and add customize logic to the same.Before we move ahead lets understand what are ‘HttpModule’ and ‘HttpHandlers’. They help us to inject custom logic before and after the ASP.NET page is processed. The main differences between both of them are:-
• If you want to inject logic based in file extensions like ‘.ASPX’ , ‘.HTML’ then you use ‘HttpHandler’. In other words ‘HttpHandler’ is an extension based processor.



• If you want to inject logic in the events of ASP.NET pipleline then you use ‘HttpModule’. ASP.NET . In other word ‘HttpModule’ is an event based processor.




You can read more about the differences from http://tinyurl.com/yyfuc9r
Below is the logical flow of how the request is processed. There are 4 important steps MHPM as explained below :-
Step 1(M à HttpModule):- Client request processing starts. Before the ASP.NET engine goes and creates the ASP.NET ‘HttpModule’ emits events which can be used to inject customized logic. There are 6 important events which you can utilize before your page object is created ‘BeginRequest’,’AuthenticateRequest’,’AuthorizeRequest’,’ResolveRequestCache’,’AcquireRequestState’
and ‘PreRequestHandlerExecute’.
Step 2 (H à ‘HttpHandler’ ) :- Once the above 6 events are fired , ASP.NET engine will invoke ‘ProcessRequest’ event if you have implemented ‘HttpHandler’ in your project.
Step 3 (P – ASP.NET page):- Once the ‘HttpHandler’ logic executes the ASP.NET page object is created. While the ASP.NET page object is created many events are fired which can help us to write our custom logic inside those page events. There are 6 important events which provides us placeholder to write
logic inside ASP.NET pages ‘Init’ , ‘Load’ , ‘validate’ , ‘event’ , ‘render’ and ‘unload’. You can remember the word ‘SILVER’ to remember the events S – Start ( does not signify anything as such just forms the word ) , I – (Init) , L ( Load) , V ( Validate) , E ( Event) and R ( Render).
Step4 (M à HttpModule):- Once the page object is executed and unloaded from memory ‘HttpModule’ provides post page execution events which can be used to inject custom post-processing logic. There are 4 important post-processing events ‘PostRequestHandlerExecute’, ‘ReleaserequestState’, ‘UpdateRequestCache’ and ‘EndRequest’.
Below figure shows the same in a pictorial format.



In What event we should do what?
The million dollar question is in which events we should do what? . Below is the table which shows in which event what kind of logic or code can go.




A sample code for demonstration
With this article we have attached a sample code which shows how the events actually fire. In this code we have created a ‘HttpModule’ and ‘Httphandler’ in this project and we have displayed a simple response write in all events , below is how the output looks like.Below is the class for ‘HttpModule’ which tracks all event s and adds it to a
global collection.



public class clsHttpModule : IHttpModule

{

......

void OnUpdateRequestCache(object sender, EventArgs a)

{

objArrayList.Add("httpModule:OnUpdateRequestCache");

}

void OnReleaseRequestState(object sender, EventArgs a)

{

objArrayList.Add("httpModule:OnReleaseRequestState");

}

void OnPostRequestHandlerExecute(object sender, EventArgs a)

{

objArrayList.Add("httpModule:OnPostRequestHandlerExecute");

}

void OnPreRequestHandlerExecute(object sender, EventArgs a)

{

objArrayList.Add("httpModule:OnPreRequestHandlerExecute");

}

void OnAcquireRequestState(object sender, EventArgs a)

{

objArrayList.Add("httpModule:OnAcquireRequestState");

}

void OnResolveRequestCache(object sender, EventArgs a)

{

objArrayList.Add("httpModule:OnResolveRequestCache");

}

void OnAuthorization(object sender, EventArgs a)

{
objArrayList.Add("httpModule:OnAuthorization");

}

void OnAuthentication(object sender, EventArgs a)

{


objArrayList.Add("httpModule:AuthenticateRequest");
}

void OnBeginrequest(object sender, EventArgs a)

{


objArrayList.Add("httpModule:BeginRequest");

}

void OnEndRequest(object sender, EventArgs a)

{

objArrayList.Add("httpModule:EndRequest");

objArrayList.Add("<hr>");

foreach (string str in objArrayList)

{
httpApp.Context.Response.Write(str + "<br>") ;

}

}

} 

Below is the code snippet for ‘HttpHandler’ which tracks ‘ProcessRequest’ event.
public class clsHttpHandler : IHttpHandler

{

public void ProcessRequest(HttpContext context)

{

clsHttpModule.objArrayList.Add("HttpHandler:ProcessRequest");

context.Response.Redirect("Default.aspx");

}
}


We are also tracking all the events from the ASP.NET page.
public partial class _Default : System.Web.UI.Page 
{
protected void Page_init(object sender, EventArgs e)
{

clsHttpModule.objArrayList.Add("Page:Init");
}
protected void Page_Load(object sender, EventArgs e)
{
clsHttpModule.objArrayList.Add("Page:Load");
}
public override void Validate()
{
clsHttpModule.objArrayList.Add("Page:Validate");
}
protected void Button1_Click(object sender, EventArgs e)
{
clsHttpModule.objArrayList.Add("Page:Event");
}
protected override void Render(HtmlTextWriter output)
{
clsHttpModule.objArrayList.Add("Page:Render");
base.Render(output);
}
protected void Page_Unload(object sender, EventArgs e)
{
clsHttpModule.objArrayList.Add("Page:UnLoad");
}}
Below is how the display looks like with all events as per the sequence discussed in the previous section.

Zooming ASP.NET page events

In the above section we have seen the overall flow of events for an ASP.NET page request. One of the most important section is the ASP.NET page, we have not discussed the same in detail. So let’s take some luxury to describe the ASP.NET page events in more detail in this section. Any ASP.NET page has 2 parts one is the page which is displayed on the browser which has HTML tags , hidden values in form of viewstate and data on the HTML inputs. When the page is posted these HTML tags are created in to ASP.NET controls with viewstate and form data tied up together on the server. Once you get these full server controls on the behind code you can execute and write your own login on the same and render the page back to the browser.
Now between these HTML controls coming live on the server as ASP.NET controls, the ASP.NET page emits out lot of events which can be consumed to inject logic. Depending on what task / logic you want to performwe need to put these logics appropriately in those events.
Note: - Most of the developers directly use the ‘page_load’ method for everything, which is not a good thought. So it’s either populating the controls, setting view state, applying themes etc everything happens on the page load. So if we can put logic in proper events as per the nature of the logic that would really make your code clean.

References

I am not so smart to write this article by myself ;-) , lot of things I have plugged from the below articles. Intercepting filters:-http://msdn.microsoft.com/en-us/library/ms998536.aspx Explains how to implement Httphandlers and modules:- http://msdn.microsoft.com/en-us/library/system.web.httpapplication.aspx Httphandlers and Httpmodules :-http://www.15seconds.com/Issue/020417.htm Implementing security using modules and handlers :-http://joel.net/articles/asp.net2_security.aspx Difference between Httpapplication and global.asax :-http://codebetter.com/blogs/karlseguin/archive/2006/06/12/146356.aspx

2 comments:

Anonymous said...

Its very nice to read form every body--- Thanks shiv sir.

Ramanji said...

Excellent Shiv...You are great in explaining the concepts...you are born talented. you have very good dedication and helping nature ..u r my fan ..and moral for my life