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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

Useless Duplicate Cookies

 Billy Hoffman on March 8, 2010. Category: Optimization

In our last post where we described the 300 issues Zoompf checks your website for during its web performance asessment we said that the #1 way we discover new web performance issues is simply looking at web responses. This story is a perfect example of how that actually happens. Today (in fact, about 2 hours ago) we were helping a client optimize their site when we noticed a rather long HTTP Set-Cookie header. This is what we saw:

Now that is rather difficult to look at. So we cleaned up the code, trimmed out the expires and path information for each cookie declaration, and aligned each cookie name/value pair on its own line. This is the clean version:

Set-Cookie:
cisession=a%3A4%3A%7Bs%3A10%3A%22session_id... [snip],
cisession=a%3A4%3A%7Bs%3A10%3A%22session_id... [snip],
cisession=a%3A4%3A%7Bs%3A10%3A%22session_id... [snip],
cisession=a%3A4%3A%7Bs%3A10%3A%22session_id... [snip],
cisession=a%3A4%3A%7Bs%3A10%3A%22session_id... [snip],
cisession=a%3A4%3A%7Bs%3A10%3A%22session_id... [snip],
cisession=a%3A4%3A%7Bs%3A10%3A%22session_id... [snip],
cisession=a%3A4%3A%7Bs%3A10%3A%22session_id... [snip],
cisession=a%3A4%3A%7Bs%3A10%3A%22session_id... [snip],

As you can see, the web application is setting the cisession cookie 9 separate times! And every time it gets set to the very same value. Now each distinct cookie name can only have one value. The web browser will use the last declaration. So this response needlessly sets the cookie 8 times. The original Set-Cookie header’s value was 3681 bytes long. But when you remove the first 8 cisession cookie declarations and instead only have 1 cisession cookie that size is reduce to 409 bytes, a reduction of 89%.

Well that’s a nice find. But then things got worse. This site used rotating cookie values where the value of the cookie is changes on each and every page (this is often done in banking and e-commerce applications to mitigate session hijacking). In this case that meant every page generated by PHP hadthese 9 cookie declarations. By identifying and resolving this problem we helped the client take 3 kilobytes of every HTML response! Now that’s a really nice performance optimization!

Cause of the Issue

This client had an online store. To uniquely identify each visitor and provide them with a shopping cart the application code had to set a session identifier for the visitor. They had a single function which would verify the client had a session identifier and set the new appropriate value. This function was called 9 separate times in different parts of the code during page generation. However the function did not check to see if the session identifier had already been set for this cycle. It just appended on a new cookie declaration. So every time a page was generated, 9 cookie declarations would be added on to the HTTP response.

This issue was hard to detect. Since the browser only uses the last declaration, HTTP requests back to the server only contain 1 cookie, not 9. For the same reason if you use a browser add-on to examine the stored cookies you will only see 1 cookie and not 9. In fact, we had to modify Zoompf’s code to detect this. The System.Net classes in Microsoft .NET were automatically collapsing the 9 redundant cookies into a single cookie. This means our code only saw one cookie as well.

One-off Issue or Plague?

We wanted to see how prevalent the issue of Duplicate Cookies is. So we wrote some quick code and we then re-analyzed approximately 700 web performance scans we have already performed on other websites to see who else had the issue. We found 16 other websites, or around 2.5% of websites we had assessed had this issue. While it is by no means as common an issue as say Images without any caching information (Check #172) we were surprised at how common the issue is. Spot checking those 16 website shows the same fundamental issue: the same cookie getting set to the same value multiple times in a single HTTP response. Again, this is most likely caused by repeated execution of the same function or code path which sets the cookie value.

Since it is a fairly easy mistake to make and is not a one-off issue, we decided to promote this to a full fledged performance check. So we wrote Zoompf check: #316: Duplicate Cookies to detect this issue.

Want to see what performance problems your website has? Duplicate Cookies is just two of the 300+ web performance issues Zoompf detects when scanning your web applications. Get your instant free web performance assessment at Zoompf.com today!

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