Patterns of event-driven architectures

When people talk about event-driven architectures, they often refer to one of the following patterns: 

• Event notification
• Event-carried state transfer
• Event sourcing
• CQRS

At times, more than one of these are used together within the same system, depending on what the business requirements are. Let's review each of these patterns in order to identify the scenarios in which they can be used. 

Event notification

The event notification pattern works by emitting events to subscribers once a command is executed. This can be compared to the observer pattern in which you observe a subject that has a list of many listeners or subscribers that are automatically notified...