ASP Page Life Cycle

Introduction

Many of us know that IIS is a web server and we use it in our .Net application since we need a web server to run a web application. But I wonder as many of us don't know the internal architecture of IIS. This article is written for beginners to  know the architecture of IIS.

How the simple web page execution happens?

As all of us know a request comes from Client (Browser) and sends to Server (we call it as Web server) in turn server process the request and sends response back to the client in according to the client request.

But internally in the web server there is quite interesting process that happens. To get aware of that process we should first of all know about the architecture of the IIS

It mainly consists of 3 Parts/Files

1. Inetinfo.exec
2. ISAPI Filer (Container for Internet Server Application Interface dlls),
3. Worker Process (aspnet_wp.exe)

When ever a request comes from the client:

Inetinfo.exe is the ASP.Net request handler that handles the requests from the client .If it's for static resources like HTML files or image files inetinfo.exe process the request and sent to client. If the request is with extension aspx/asp, inetinfo.exe processes the request to API filter. ISAPI filter will have several runtime modules called as ISAPI extensions. To process the request ISAPI filter takes the help of these runtime modules. The runtime module loaded for ASP page is asp.dll. And for ASP.NET page it's ASPNET_ISAPI.dll. From here the request is processed to the "worker process". Worker Process will have several application domains.

Application Domain

The purpose of the application domain is in order to isolate one application from another. When ever we create a new application, application domains are created automatically by the CLRHost. Worker process will create a block of memory related to particular application. Application domains provide a more secure and versatile unit of processing that the common language runtime can use to provide isolation between applications. Application domains are normally created by runtime hosts. Runtime host is responsible for bootstrapping the common language runtime before an application is run.

Worker process sends the request to HTTPPIPE line.(HTTP Pipeline is nonetheless collection of .net framework classes). HTTP Pipeline compiles the request into a library and makes a call to HTTP runtime and runtime creates an instance of page class

Public Class File

    Inherits System.Web.UI.Page

End Class 'File

ASP.Net web page is a class derived from page class, this page class resides in system.web.dll

After creating instance pf page class HTTP Runtime immediately invokes process request method of page class

Dim Req As New Page

Req.ProcessRequest()

Process Request Method does following things:

1. Intialize the memory
2. Load the view state
3. Page execution and post back events
4. Rendering HTML content
5. Releasing the memory

Process Request Method executes set of events for page class .These are called as Page life cycle events.

Page Life Cycle Events

• Page_Init
  The server controls are loaded and initialized from the Web form's view state. This is the first step in a Web form's life cycle.
• Page_Load
  The server controls are loaded in the page object. View state information is available at this point, so this is where you put code to change control settings or display text on the page.
• Page_PreRender
  The application is about to render the page object.
• Page_Unload
  The page is unloaded from memory.
• Page_Disposed
  The page object is released from memory. This is the last event in the life of a page object.
• Page_Error
  An unhandled exception occurs.
• Page_AbortTransaction
  A transaction is aborted.
• Page_CommitTransaction
  A transaction is accepted.
• Page_DataBinding
  A server control on the page binds to a data source. 
• Process Request Method finally renders HTML Page

Dependencies:

When the request comes to ASP.net worker process, it will be forwarded to HTTP Application factory. This "Application Factory" will maintain address of the application domains which are currently executing under worker process. If the required virtual directory application domain is unavailable it will create a new application domain. If the application domain is already existing, the request will be forwarded to corresponding AppDomain.

Application Domain maintains page handler factory class. This will contain all libraries addresses corresponding to webpage. If the requested webpage library is available the instance of the page class is created, if the library is unavailable the request will be forwarded to HTTP pipeline.