appendix_ds1_table.asciidoc

Appendix A: Summary Diagram and Table

Here’s what our architecture looks like by the end of the book:

The components of our architecture and what they all do recaps each pattern and what it does.

Table 1. The components of our architecture and what they all do

Layer	Component	Description
Domain Defines the business logic.	Entity	A domain object whose attributes may change but that has a recognizable identity over time.
	Value object	An immutable domain object whose attributes entirely define it. It is fungible with other identical objects.
	Aggregate	Cluster of associated objects that we treat as a unit for the purpose of data changes. Defines and enforces a consistency boundary.
	Event	Represents something that happened.
	Command	Represents a job the system should perform.
Service Layer Defines the jobs the system should perform and orchestrates different components.	Handler	Receives a command or an event and performs what needs to happen.
	Unit of work	Abstraction around data integrity. Each unit of work represents an atomic update. Makes repositories available. Tracks new events on retrieved aggregates.
	Message bus (internal)	Handles commands and events by routing them to the appropriate handler.
Adapters (Secondary) Concrete implementations of an interface that goes from our system to the outside world (I/O).	Repository	Abstraction around persistent storage. Each aggregate has its own repository.
	Event publisher	Pushes events onto the external message bus.
Entrypoints (Primary adapters) Translate external inputs into calls into the service layer.	Web	Receives web requests and translates them into commands, passing them to the internal message bus.
	Event consumer	Reads events from the external message bus and translates them into commands, passing them to the internal message bus.
N/A	External message bus (message broker)	A piece of infrastructure that different services use to intercommunicate, via events.