Event-driven architectures (EDA) are based on commands and events that are created each time an application changes state. According to Martin Fowler, there are four patterns that are used to build software systems using this approach. 

In this chapter, we are going to learn about these four patterns and look at how messaging can be tied together to take full advantage of a programming model based on messages. Even when it's not a requirement, messaging can be used to add more capabilities into applications that are built using an event-driven architectural style. 

In this chapter, we will look at the following topics:

• Underlying concepts and key aspects associated with event-driven architectures:
  • Commands
  • Events
• Common patterns used within event-driven architectures:
  • Event notification
  • Event-carried state transfer
  • Event sourcing
  • CQRS