The Control Bus describes a number of approaches to monitor the health of the message processing system. Each component in the system can publish periodic "heartbeat" messages to the control bus in order to keep the monitoring mechanism informed that the component is still active. The heartbeat messages can contain vital stats of the component as well, such as the number of messages processed, the average time required to process a message or the percentage of CPU utilization on the machine.

What happens, though, if a component is actively processing messages, but garbles outgoing messages due to an internal fault?

![TestMessage](./TestMessage.gif)

Therefore, use Test Message to assure the health of message processing components (see Figure).

The Test Message pattern relies on the following components: