SysAdmin's Journey

Tip for "Split Components Across Domains" Performance Goal From Yahoo!

Just thought I’d pass this little tidbit out there - we fixed it by pure luck on the first try. Yahoo unselfishly provides a document titled Best Practices for Speeding Up Your Website. While some of the rules offered there aren’t applicable for all sites, it’s a great document and if you run a website, you should read it. At $work, part of our last code drop was to push out a feature that enabled “Split Components Across Domains”. From the article Performance Research, Part 4: Maximizing Parallel Downloads in the Carpool Lane:

Our rule of thumb is to increase the number of parallel downloads by using at least two, but no more than four hostnames. Once again, this underscores the number one rule for improving response times: reduce the number of components in the page.

I’m here to tell you, if you have AOL users surfing your site, do not use four hostnames. When we pushed this feature up to production, we had one hostname that served up the HTML, and we had four hostnames that served up imagery (all these hostnames pointed back to the same IP, but doing this allows a performance boost in the browser). For this example, let’s say that is the HTML hostname;,,, and were the imagery servers. This most certainly improved performance on the client side, but we started receiving some reports from a few users that they were no longer able to see any imagery on the site since we dropped the new code. We immediately knew what was causing the issue, but had no idea why, or how far spread out it was. Well, after poking around some of the “big boys” websites such as Amazon, we noticed that while all of them separated their components as suggested by Yahoo!, all of them used only one hostname for the imagery. We quickly configured our webapp to only use for the HTML, and for the imagery. Once we did that, our AOL users were again able to see imagery. Now, I have no idea how widespread the issue was. I know it was limited to users of the AOL browser, and I suspect it’s probably a bug in a specific version of their browser. However, if your site needs to provide compatibility to the most users possible, you may want to use just one separate hostname for splitting components. I hope this helps someone else!