Fundamentals of Software Architecture

The mediator component has knowledge and control over the workflow, something the broker topology does not have. Because the mediator controls the workflow, it can maintain event state and manage error handling, recoverability, and restart capabilities. For example, suppose in the prior example the payment was not applied due to the credit card being expired. In this case the mediator receives this error condition, and knowing the order cannot be fulfilled (step 3) until payment is applied, stops the workflow and records the state of the request in its own persistent datastore. Once payment is eventually applied, the workflow can be restarted from where it left off