These patterns are used to measure events that are related to specific measurable changes in conditions. These patterns are used in scenarios that demand real-time flow of work to be triggered without any other constraints or considerations. Parameters such as lag time and costs related to business are not taken into consideration while using simple event processing patterns in architecture. Some scenarios for usage of this type of pattern could be a detection of temperature/pressure changes by a sensor. Let us explain simple event processing with the help of the order management system example we used at the beginning of the chapter. The diagram is repeated here for quick reference:
In this example, after the order enters the order management system, the first event is triggering a notification to the warehouse management system for checking inventory levels and to the financial system for payment validation. For simplicity, we will consider only one flow of events: the events related to the warehouse management (related to inventory check). Once the order enters the warehouse management system, the items in the order are checked against the inventory present in the warehouse using the check inventory service. The check inventory service allocates the inventory pertaining to the items present in the order and then checks the remaining inventory for optimal levels of inventory threshold. If the stock in the warehouse falls under the available threshold, the check inventory service generates a low inventory threshold event. This event is received by the simple event processing engine, as depicted in the following diagram. The event processing rules in this example will initiate two sets of events to handle the low inventory threshold situation: the first one would be a process to reorder the inventory, and the second one would be for publishing the event for consumption by the subscribers. In this particular example, the subscribers are inventory buyers and a notification to the inventory controller would also be generated. All these activities are depicted in the following diagram:
