The mediator topology pattern is used to design systems/processes that will need some level of coordination/orchestration in order to process the event. The ideal example of this scenario could be the order processing example, where there are multiple steps, such as order entry, inventory validation, finance validation, and so on. All these steps require some level of orchestration in order to assess whether they can be performed serially or in parallel. There are four main components within the mediator topology:

• Event queue
• Event mediator
• Event channels
• Event processors

All these components are depicted in the following diagram:

The client sends an event, which is then received by the event queue. The event queue transfers the event to the event mediator. The event mediator receives the event and orchestrates it. This is done by sending additional asynchronous events to the various event channels, which in turn will execute each step of the process. Event processors receive the event from the event channel and apply business logic to process the event. There can be any number of event queues in an EDA. An event queue can be implemented as a message queue, web service component, or in any other form that is suitable for the system under consideration. There are two types of events that are provided by this pattern:

• Initial event: This refers to the original event that is received by the mediator
• Processing event: This refers to the events that are generated by the mediator and are sent to the event processing components

The event mediator is mainly responsible for performing orchestration of the various steps that are present within the initial event. In order to perform each step in the initial event, the event mediator sends a specific processing event to the event channel. This processing event is received and processed by the event processor. Event channels are used to pass processing events associated with each step to the event processors. Event channels can either be in the form of message queues or in the form of message topics. The application logic that is required for processing the events is present in the event processor. Event processors are typically highly decoupled architectural components that are associated with a specific task in the system.