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The Art of Capacity Planning by John Allspaw

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Chapter 2. Setting Goals for Capacity

YOU WOULDN'T BEGIN MIXING CONCRETE BEFORE YOU KNEW WHAT YOU WERE BUILDING. SIMILARLY, you shouldn't begin planning for capacity before you determine your site's requirements. Capacity planning involves a lot of assumptions related to why you need the capacity. Some of those assumptions are obvious, others are not.

For example, if you don't know that you should be serving your pages in less than three seconds, you're going to have a tough time determining how many servers you'll need to satisfy that requirement. More important, it will be even tougher to determine how many servers you'll need to add as your traffic grows.

Common sense, right? Yes, but it's amazing how many organizations don't take the time to assemble a rudimentary list of operational requirements. Waiting until users complain about slow responses or time-outs isn't a good strategy.

Establishing the acceptable speed or reliability of each part of your site can be a considerable undertaking, but it will pay off when you're planning for growth and need to know what standard you should maintain. This chapter shows you how to understand the different types of requirements your management and customers will force you to deal with, and how architectural design helps you with this planning.

Different Kinds of Requirements and Measurements

Now that we're talking about requirements—which might be set by others, external to your group—we can look at the different types you'll need to deal with. ...

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