Fiddler's Streaming Mode

Introduction

By default, Fiddler operates in Buffering Mode.  In Buffering Mode, Fiddler buffers HTTP(S) responses completely before returning them to the client.  This gives the Fiddler user the opportunity to modify the response before the client receives it.

What is streaming mode?

When in streaming mode, Fiddler will return response bytes to the client immediately, as it reads them from the remote server.  This is primarily important for low-level network timing scenarios, because most web browsers (Internet Explorer, Firefox, etc) will parse partially-downloaded HTML pages and begin downloading needed resources (e.g. CSS, JS, etc) in parallel, even before the remote server has finished delivering the HTML.  In some cases, this leads to a measurable performance improvement.

Streaming mode is also useful in some other cases, particularly when visiting Internet Radio sites that deliver a never-ending stream of MP3 data.  An audio stream, by definition, is never complete, so Fiddler's attempt to buffer it completely will always fail.

Enabling Streaming Mode

In the Fiddler Timeline tab, you can simply right-click the transfer timeline and check the "Enable Fiddler Streaming" option.

Alternatively, you can enable streaming on a per-session basis by setting the bBufferResponse property to false on the Session object.  Note that you must set this property in FiddlerScript's OnBeforeRequest handler or IFiddlerExtension's IAutoTamper.AutoTamperRequestBefore or IAutoTamper.AutoTamperRequestAfter handler.

Streaming mode is available in Fiddler v2.1 and later.

Session handling in Streaming Mode

For sessions in streaming mode, response breakpoints will not be triggered, and changes to the response headers or body will not be seen by the client.

Note to Developers: IAutoTamper::AutoTamperResponseBefore is the last event that fires on a streamed session, after the response data has already been returned to the client. 

Stated another way, this means that AutoTamperResponseAfter and FiddlerScript's OnBeforeResponse are never fired for streamed bodies.

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