Architecting for Scale

Chapter 2. Five Focuses to Improve Application Availability

Building a scalable application that has high availability is not easy and does not come automatically. Problems can crop up in unexpected ways, that can cause your beautifully functioning application to stop working for all or some of your customers.

These availability problems often arise from the areas you least expect, and some of the most serious availability problems can originate from extremely benign sources.

A Simple Icon Failure

A classic example of the pitfalls of ignoring dependency failure occurred in a real-life application I worked on. The application provided a service to customers, and on the top of every page was a customizable icon representing the currently logged-in user. The icon was generated by a third-party system.

One day, the third-party system that generated the icon failed. Our application, which assumed that system would always work, didn’t know what to do. As a result, our application failed as well. Our entire application failed simply because the icon-generation system—a very minor “feature”—failed.

How could we have avoided this problem? If we had simply anticipated that the third-party system might fail, we would have walked through this failure scenario during design and discovered that our application would fail subsequently. We could then have added logic to detect the failure and remove the icon if the failure occurred, or simply catch the error when it occurred and not ...