If your application is slow and users who mainly use IE complain about issues that you never faced during development, you should try Fiddler, the HTTP debugging tool, to investigate the reasons. There is more to Fiddler than what meets the eye. If you have just got started with Fiddler, here are a few not-so-obvious things that you can explore -

1. Keyboard shortcuts - These are the ones that I use frequently:
CTRL+A - Select all sessions
CTRL+X - Clear Session List
CTRL+H - View Header Inspector
CTRL+T - View TextView Inspector
Alt+Q - Activate the QuickExec command line
CTRL+F - Find sessions

2. QuickExec box - The QuickExec box is a thin black box just above the Fiddler status bar that lets you type commands instead of clicking on the menu options. Just type CLS when you wish to clear the Web Sessions, use ?word to look for a specific word among the Session URLs or filter URLs that are of a specific size using the commands > size and < size.

3. Extensions - The extensible nature of Fiddler allows independent developers to build Extensions on top of Fiddler through script and .NET code. neXpert Performance Tool is a helpful Fiddler Extension from Microsoft that helps analyze the captured Sessions. It can provide a report on possible ways to improve performance of web pages. This is especially helpful for ASP.NET developers. It is comparable to the YSlow add-on for Firebug on Firefox. The current version of neXpert, 1.1, allows you to type additional commands in the QuickExec box that the pre-defined ones.

If you have neXpert 1.0, you can remove it from Add or Remove Programs applet.

4. Interpret Icons - Allowing static files like images, external CSS & JavaScript files to be cached can prevent browsers from fetching it on subsequent page visits thus improving page loading time. If you have applied caching for static files like images, external CSS & JavaScript files, how do you verify it is indeed cached? The floppy icon against a Session will indicate that the Response was HTTP/304 and thus cached. This list from the Fiddler Documentation shows what the different icons shown with the HTTP codes & tracked URLs represent:

5. Session Archives - As a web developer, if you want to study the steps that led to issues (like say, a redirection loop) within a set of web pages, you can run the application with Fiddler ON and then save that set of HTTP Sessions. You can likewise ask a Tester who has spotted an issue to share the Session Archive involving the faulty web pages. There are at least 3 ways in which you can save a Session Archive.
a. Select all (View > Select All or CTRL+A) Sessions or specific Sessions (click on required Sessions with Ctrl key pressed) and then from the menu choose File > Save > Sessions > in ArchiveZip... option. You can then save the ArchiveZip which has a .SAZ extension at a file path that you can choose.
b. Use the dump command in the QuickExec box
c. In the neXpert tab in Fiddler, click on the checkbox Save Sessions \w Report to Zip and click on the Create Report button.

Alternatively when the neXpert Addon is installed, use the command savesaz filename in the the QuickExec box Depending on the number of Sessions that you want to archive, it may take some time for the operation to complete.

In order to analyze the Session Archive, use File > Load Archive... menu option to load the list of Sessions

Biography - mv ark

M.V. 'Anil' Radhakrishna is a seasoned developer and a Microsoft MVP (ASP/ASP.NET). He blogs his little discoveries and Web development tips, tricks and trivia quite regularly. You can find some of his unusual code samples & snippets at his Code Gallery.

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Article Discussion: Fiddler Tips & Tricks
mv ark posted at Thursday, May 28, 2009 12:25 PM
Original Article