Controllability and Observability

These are central concepts from control theory. We will define them as:

• Controllability: the ability to change the behavior of a system by changing its parameters.

• Observability: the ability to observe the state and behavior of a system well enough to control it.

In order to control a system, both controllability and observability are required.

The implications for large software systems are:

• Aspects of software that cannot easily be observed will never be debugged.

• All large software systems must have observability built in.

• Observability is a requirement, not a luxury. The time spent building in observability will be well repaid.

• In-process traces can be turned on by setting bits in a bit vector.