This project is part ofEventuate, which is a microservices collaboration platform.

Sagas are a mechanism for maintaining data consistency in amicroservice architecture.A saga is a sequence of transactions, each of which is local to a service.

There are two main ways to coordinate sagas: orchestration and choreography.This example uses choreography-based sagas, which use domain events for coordination.Each step of a saga updates the local database and publishes a domain event.The domain event is processed by an event handler, which performs the next local transaction.

To learn more about why you need sagas if you are using microservices:

• Look at theOrchestration-based saga example

• Read theSaga pattern

• Look atMicroCPH 2019 presentation

• Read about sagas in myMicroservices patterns book

TheCQRS pattern implements queries that retrieves data from multiple services.It maintains a queryable replica of the data by subscribing to domain events published by the services that own the data.

In this example, theOrder History Service maintains a CQRS view in MongoDB by subscribing to domain events published by theOrder Service andCustomer Service.The CQRS view stores each customer as a MongoDB document that contains information the customer and their orders.

To learn more about why you need CQRS if you are using microservices:

• Read theCQRS pattern

• Look atGOTO Chicago 2019 presentation

• Read about CQRS in myMicroservices patterns book

The services uses theEventuate Tram framework to communicate asynchronously using events.The flow for publishing a domain event using Eventuate Tram is as follows:

1. Eventuate Tram inserts events into theMESSAGE table as part of the ACID transaction that updates the JPA entity.

2. The Eventuate Tram CDC service tracks inserts into theMESSAGE table using the MySQL binlog (or Postgres WAL) and publishes messages to Apache Kafka.

3. A service subscribes to the events, updates its database, and possibly publishes more events.

The following diagram shows the architecture of the Customers and Orders application.

The application consists of three services:Customer Service,Order Service, andOrder History Service

Start the application and the required infrastructure services by running either

or

This command starts the containers on unique ports.It prints out the home page URL.

There are a couple of ways to interact with the application: using the Swagger UIs or usingcurl.Once the application has started, visit the home page URL to see the URLs for the Swagger UIs and the API Gateway.

I’ve configured aCode Tour that will walk through the code in either Visual Studio Code or Github Codespaces.

1. Create a codespace.
2. If necessary, install the Code Tour extension from the Visual Studio Code Marketplace.
3. Use theCodeTour: Start Tour command from the command palette to start the tour.

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