This is an example Java RESTful web service for a coffee shop.
It is designed as an event sourcing system where all data is stored as events.
Also read model is separated from update model by following the Command Query Responsibility Segregation(CQRS) pattern.
Dropwizard is used as the application framework.
PostgreSQL is used as the database.
Apache Kafka is used for messaging.
With event Sourcing, all changes to application state are stored as a sequence of events.
This forms an append-only event log. Events are immutable, they cannot be changed.
An entity’s current state can be created by replaying all the events in order of occurrence.
Events work best when you're modeling how something evolves over time.
Command: An operation that changes the state of the application.
Query: An operation that reads the state of the application.
With CQRS pattern, the data for commands and queries are split in the database.
By splitting the commands and queries, the CQRS allows developers to use different models to read and write data.
The coffee shop has to manage its stocks.
The stocks go up when new products are added through the PUT /stocks endpoint.
The stocks go down when the products are sold through the PUT /checkout endpoint.
All updates in the stocks are saved as events in the events table which corresponds to the update model.
Then these events are projected to the stocks table which corresponds to the read model.
Run mvn package to build project with Maven.
Run docker-compose --profile local up to start application with Docker.
To check that your application is running enter url http://localhost:8080/
You may see application's health at http://localhost:8081/healthcheck