This repository is a microservice reference example that is intended to teach the basics of event sourcing in Spring Boot applications.

For this reference, I chose to create a simple example domain with a high degree of relationships between data stored on separate microservices. In the architecture diagram below, you'll see an abstract component diagram that describes an event-driven microservice architecture containing two domain services and one aggregate processor.

One of the main problems I see today when describing components of a microservice architecture is a general ambiguity in the roles of separate services. For this reason, this example will describe a set of conventions for the roles of separate services.

Domain services are microservices that own the system of record for a portion of the application's domain.

Domain services:

• Manage the storage of domain data that it owns.
• Produce the API contract for the domain data that it owns.
• Produce events when the state of any domain data changes.
• Maintain relationship integrity to domain data owned by other services.

Aggregate services are microservices that replicate eventually consistent views of domain data owned by separate domain services.

Aggregate services:

• Subscribe to domain events emitted by separate domain services.
• Maintain an ordered immutable event log for events it receives.
• Create connected query projections of distributed domain data.
• Provide performant read-access to complex views of domain data.

The example domain is a social network of users who can establish friend relationships with one another. I chose this domain because it has high join complexity when the domain data is split across separate services.

The diagram above is a domain graph that shows User nodes and Friend relationships. In a microservice architecture we may decide to decompose this domain graph into two separate domain services, a user-service and a friend-service. For this reason, we'll have foreign-key relationships stored in the friend-service that reference the unique identity of a User stored on the user-service.

This project is licensed under Apache License 2.0.