Web page composition sequence diagram


The Web Page composition sequence diagram illustrates the series of web client and web server-side events that occurs for a typical Http Request/Response sequence, ❶ beginning with a web browser client HTTP GET Request that references a URL. This is associated with a window.unload event inside the web browser.

The web browser client issues a GET Request for the URL that is passed to the TCP/IP networking stack inside the Windows OS. After looking up the IP address of the URL using the Domain Naming Service (DNS), a TCP connection is established with the web server that can be used to transmit the Request. The client machine then generates a TCP Send on the TCP Port assigned to the session.

This request traverses the network until it reaches the web server, where ❷ the TCP Receive request is processed. In Windows, the TCP/IP networking stack passes the HTTP request to the http.sys kernel-mode driver, which then routes the Request to the appropriate ASP.NET processing queue ❸. An ASP.NET worker process, w3wp.exe, takes the Get Request off its queue and processes it in User mode, generating the appropriate HTTP Response message. The HTTP Response message is passed back httpserver.sys kernel-mode driver ❹. Finally, the Http Server component of the OS passes the Http Response to TCP/IP, which then issues a TCP Send ❺, which is then packetized by the IP networking layer for transmission back to the original requestor ❻.

The web client continues to make GET Requests for any resources (image files, style sheets, JavaScript code, etc.) embedded in the original Response message. When all external references are resolved, the browser renders the page’s Document object. When the Page composition process is complete, the window.load event fires, which triggers the execution of any JavaScript code that is attached to the unload event Handler. At this point, if boomerang.js has been added to the web page, ❼ the web client will issue a GET Request for the boomerang beacon, which contains web page load time measurements obtained from the Nav/Timing API.

The various TCP and HttpServer ETW events associated with this sequence of events are shown in red. Correlating the server-side Http Request and Response events illustrated, the Web Application Trace Explorer calculates web application request rates and response times, information that is required for Service Level reporting. The Web Application Trace Explorer reports web application service levels summarized by URL..

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