Understanding Tracing in ASP.NET 2.0
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Viewing Trace Data

Tracing can be viewed for multiple page requests at the application level by requesting a special page called trace.axd. We can enable tracing in Web.config as shown in code Listing 3.

Listing 3

<compilation debug="false" />
<authentication mode="Windows" />
<trace enabled ="true" pageOutput ="true" />        
protected void Page_Load(object sender, EventArgs e)
  System.Diagnostics.Trace.Write("This is Page_Load method!");

Trace.Write() method can be used in the Page_Load event as shown in the above code listing. We can also use the Trace.Warn method. When Trace.Warn method is used the trace output will display the given message in red. When we run the page, we can see (because we have set PageOutput=true) eleven different sections in the browser which provide a great deal of information as specified below.

Request Details: This section includes the ASP.NET Session ID, the character encoding of the request and response, the HTTP conversation's returned status code, the time of request and the type of web request.

TraceInformation: This section shows the different stages of processing that the page went through before being sent to the client. Each section has additional information about how long it took to complete. The timing information located here is valuable when profiling and searching for methods in the application that take too long to execute.

Figure 1

Control Tree: Control tree presents an HTML representation of the ASP.NET Control Tree. It shows each control's unique ID, run time type, the number of bytes it took to be rendered, and the bytes it requires in View State and Control State. This helps us determine whether the enabling control state could affect page transmission times.

Session State: This section displays every item in a particular user's session, their types and their values.

Application State: This section displays every item in the current application's Application object and their type and values.

Request Cookies: This lists all the cookies passed in when the page is requested.

Response Cookies: Lists all the cookies that were passed back during the page's response.

Headers Collection: Shows all the headers that might be passed in during the request from the browser, including Accept-Encoding, indicating whether the browser supports the compressed HTTP responses and Accept languages.

Form Collection: This section displays all of the Form Collection and all its keys and values.

QueryString Collection: This section displays the variables and values submitted in the Querystring collection in the form of keys and values.

Server Variables: This section lists all the server variables and their contents.

Trace.axd: Using page output of tracing we can get only the data collected for the current page request. However, if we want to collect detailed data for all the requests then we need to use Trace.axd. We can invoke Trace.axd tool for the application using http://machinename/applicationname/trace.axd, which is shown below.

Figure 2

In the above figure we can see that there are three request that have been made to this application and "Remaining:7" in the right side of the header suggests that there are seven more remaining requests before tracing stops for this application. The request limit can be set in Web.config which is already shown in code Listing 2.

Following are different options that we can set in web.config file for application tracing:

Enabled: This turns tracing on or off for all pages. It can be override on a page-by-page basis with the Page directive.

traceMode: Determines the sort order of trace messages. Values are SortByTime and SortByCategory.

localOnly: It determines whether tracing information will be visible only to the user of local computers or can be visible to the user of a remote computer. By default it is true.

pageOutput: It determines whether tracing information will be displayed on the page or not by setting values to true or false.

requestLimit: This is any integer number of page requests that are stored and displayed via trace.axd when we are using application level tracing. If we specify any value greater than 10000, ASP.NET treats it as 10000.

mostRecent: If true, ASP.NET keeps only the most recent trace messages. If it reaches the maximum number, the oldest request is removed whenever any new request comes. If it is false, ASP.NET stops collecting new trace messages and ignores subsequent requests. The default value is false.

writeToDiagonosticsTrace: If true, all calls to System.Web.UI.Page.Trace.Write are also forwarded to the System.Diagnostics.Trace.Write and enables us to use all the standard TraceListeners. The default is false

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