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Zoompf's Web Performance Blog

Note: Archived Content

This is the archived version of the Zoompf blog. Since our acquisition by Rigor, all our new research and posts on web performance are being published on The Rigor Blog

REDbot: Awesome HTTP Testing

 Billy Hoffman on December 22, 2011. Category: random

I am always on the lookout for new and cool web performance and quality tools. One of my favorite tools is REDbot. Every web performance advocate should be using REDbot regularly. Want to know why?

To start, REDbot was created by the awesome Australian Mark Nottingham. Mark writes some excellent technical-yet-easy-to-understand essays on the inner workings of HTTP, such as the definitive Caching tutorial for Web authors and web masters. With REDbot, Mark has taken his vast knowledge of all things HTTP and distilled that into a wonderful tool written in Python. So, Mark’s brain is Reason #1

Reason #2 is what the tool does. The best way to describe REDbot might be "HTTP Lint", which, funny enough, was the name of the first C# project of what became the Zoompf scanner. REDbot examines a server’s HTTP response headers and body for performance issues, quality issues, compatibility issues, adherence to the HTTP RFCs, and provides various ancillary info messages. Frankly the depth of issues it looks for is really quite amazing; currently over 150 different items can be detected and reported by REDbot.

screen of some of the issues that REDbot finds

As an example, here are just some the things that REDbot checks for with the Last-Modified header.

  • Is the date format valid? Invalid dates can’t get conditionally cached.
  • Is the date in the future? Resources in the future cannot be cached.
  • Does the web server correctly return a 304 if the resource has not been modified?
  • Are there duplicate Last-Modified headers? Do they have different values?

That is just scratching the surface of what REDbot can find out about your site. Is the Content-Length header right? Is chunked encoding working properly? How many inline caching proxies did the response go through? REDbot has helped me find and fix several issues with Zoompf’s own web infrastructure. In fact, many of the issues REDbot looks for were so helpful, we added them to list of issues that Zoompf tests for. While Zoompf does not include all of REDbot’s tests I don’t know of any other performance tools which look for these kinds of HTTP issues. For the reason of completeness alone, REDbot needs to be part of you toolset.

Of course, detecting some HTTP problems can get pretty involved. For example, REDbot and Zoompf will verify that a server properly responds to If-Not-Modified, If-None-Match, and Range requests. Additional REDbot and Zoompf can confirm that Vary and Accept-Encoding response headers are correctly operating. All of this involves sending multiple requests to the web server for each issue. Testing a single static resource can involve 5 to 6 requests and processing that many responses! While this isn’t so bad when testing a single URL, doing for multiple resources is time consuming. The public web instance of REDbot often times out when using the "check assets" feature to test multiple URLs at once. Zoompf website scans can take 2 times or 4 times longer to complete if you conduct these extended HTTP tests. We are playing with a few ways to be intelligent about when we send these extra test requests but it is still a work in progress. In the meantime, this extended HTTP testing capability is disabled by default for our customers and completely unavailable for free scans.

Reason #3: REDbot is open source and hosted on GitHub which makes it super easy to start using. It can run from the command line, but Mark “the Awesome Aussie” Nottingham (it’s his pro wrestling name, look it up) has setup a public instance of REDbot with a web interface where anyone can quickly test a resource. If you test an HTML page, you can click the "check assets" link underneath the response headers to recursively test all the referenced resources. This is a handy feature to rapidly test multiple URLs but as we said you will occasionally get timeouts.

Reason #4: It’s web UI is gorgeous. As someone who makes incredibly crappy web interfaces (and which I am convinced would somehow be better if I wrote them on a new Macbook Air), I drool over what Mark has done. Fades, transparency, context dialogs, this thing is sexy looking.

REDbot is an awesome tool which provides much needed HTTP insight and validation available nowhere else. I highly recommend it to anyone interested in the working of the web and I thank Mark Nottingham for his excellent contribution to our community.

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