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This chapter is from the book

Scalability Doesn't Equal Performance

Another thing you should have gotten out of this chapter is that scalability is not the same as performance. The two have different metrics and measure distinct things.

Performance has to do with the raw speed of the application, perhaps in a vacuum where only one user is using it. When we talk about performance, we mean response time—it's as simple as that. Optimizing performance has to do with improving the performance for that one user. If we measure average response time of 100 concurrent users, our performance challenge is to improve the average response time of the same 100 concurrent users.

Scalability, on the other hand, has to do with the ability to accommodate increasing demand. A primary metric for scalability is throughput, which measures transactions or users per second. There is no such thing as infinite scalability—the ability to handle arbitrary demand. Every application has its limits. In fact, for many deployments it is satisfying to achieve just linear scalability, although the optimizer in all of us wants to achieve much better than that. Not unexpectedly, the most successful examples of scalability are those that simply minimize the rate at which new resources are required.

Measure Scalability by Comparison

Scalability is difficult to ensure because its metrics don't allow you to compare it easily to an average (nonlinearly scalable) baseline and make some conclusions. One thing you can do, however, is measure how the scalability of your application evolves. First, define what kind of throughput is reasonable: Create (or buy) an automated stress-testing system that identifies whether your current system achieves that goal for a reasonable number of users. Then, as the application evolves, periodically retest and determine if it's improving relative to past scalability—this is without a doubt something that even your end users will notice.

Another strategy is to measure throughput as the number of users increases and identify important trends. For example, measure the throughput of your applications with 100 concurrent transactions, then with 1,000, and then with 10,000 transactions. Look at how your throughput changes and see how it compares with linear scalability. This comparison will likely give you a better sense for whether your application architecture is inherently scalable.

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