The other side of the moon

I currently work with the performance team at Yahoo!. This is the team that did the research behind our performance best practices and built YSlow. Most of our past members write and speak about performance, and while I've done a few talks, I've never actually written a public post about web performance. I'm going to try and change that today.

Note, however, that this blog is about many technical topics that interest me and web performance is just a part of that.

I'm never sure how to start a new series, especially one that's been spoken about by others, but since these blog posts also serve as a script for the talks that I do, I thought I'd start with the last performance talk that I did.

Improving a website's performance starts with measuring its current performance. We need a baseline measurement that will help us determine if the changes we make cause an improvement or a regression in performance. Before we start with measurement, however, we need to know what to measure, and for that we need to look at all the factors that contribute to the time it takes for a website to get to the user.

User perceived web app time is spent in looking up stuff, building stuff, downloading stuff, rendering stuff and interacting with stuff.

It's this perceived time that we need to reduce, and consequently measure. All of the above fall into two basic categories:

1. Infrastructure
2. Content structure

Each of these in turn is made up of components that we as developers can control, and those that we cannot. We'd like to be able to measure everything and fix whatever we have control over. I've split the components that I see into this table so we know what can be looked at and who should do the looking.

	Infrastructure	Content
Can control	Web server & App server Database server Web service calls CDNs DNS	HTTP headers HTML Images, Flash CSS Javascript Fonts
Cannot control	ISP's DNS servers ISP's network User's bandwidth User's browser & plugins Other apps using the user's network The internet	Advertisements Third party content included as badges/feeds Third party sites that link to your page

If you have more items to add to this table, leave a comment and I'll add it in.

This is where we can jump to Yahoo!'s performance rules. At the time of this post, there are 34 of them divided into 7 categories. I'll go into more details and refer to these rules in later posts. That's all for this introductory post though.