In the request-response example, we have built a WCF service that acts as a hub in the hub-and-spoke architecture, which connects different integration participants. The workflow we presented is quite simplistic; the only action it performs is aggregating the results of requests that are sent to providers.
In a more sophisticated solution, more actions can be required. The most typical ones are message validation, message transformation, and message routing (they are explained in the following list). Message hubs that perform these actions are called message brokers.