Feedback Control for Computer Systems

Preface

This is a book about feedback control—not a topic that programmers (among others) tend to know much about. This is a pity, because feedback control was originally devised to solve a problem that should be all too familiar to software engineers, especially those who are working on enterprise systems. Feedback control is a way to make sure that large, complicated systems run reliably, even when subject to external disturbances, and to make efficient use of constrained resources.

If you are looking for a system that can spin up some additional servers when traffic in your data center spikes and take them down again when the rush is over, then you have come to the right place.

What Is Feedback?

Feedback works by constantly comparing the actual behavior of a system to its desired behavior. If the actual behavior differs from the desired one, a corrective action is applied to counteract the deviation and drive the system back to its target. This process is repeated constantly, as long as the system is running.

One appealing feature of feedback control is that it requires relatively little knowledge about the controlled system. As long as one knows which direction to “nudge” the system when it has gotten off course, one can build a feedback loop. For this reason, feedback is an attractive technique for controlling large, complex, and opaque systems.

Moreover, feedback systems are self-correcting even in the presence of external disturbances. Because the system’s behavior is monitored ...