So far, we've seen how to use rules to make decisions based on a set of data (called facts). This information is pretty much any group of Java objects describing the state of the domain on which we're making the decisions, but it has always represented the state of the world at one particular point in time. In this chapter, we're going to see a set of concepts, configurations, and rule syntax components that will allow us to make decisions based on time relationships between facts. These concepts are often called complex event processing (CEP).

Drools provides support for this under the name of Drools Fusion or Drools CEP, a conceptual module fully integrated into the Drools core features. This is just a conceptual separation as all the CEP features are fully supported by the same modules that provide the rule engine functionality. To fully understand this, the chapter will cover the following topics: