Using a rule engine provides a framework that allows a way to externalize business logic in a common place. This will in turn empower business users and subject matter experts of the business to easily change and manage the rules. Coding such rules directly into the application makes application maintenance difficult and expensive because the rules change so often. This article goes into detail on how to architect and build a service that uses Drools to provide business decisions. This service can be part of the overall enterprise SOA infrastructure. As such, it can either be a standalone service that is consumed in a one-to-many model by all contracted consumers, or part of a composite service that provides a complex business functionality. To illustrate this point, the article shows how a service using the Drools rule engine can hide the complexity of automating mortgage underwriting decisions that a mortgage company needs to make on a daily basis.