An Eventuate project

This project is part of Eventuate, which is a microservices collaboration platform.

Eventuate Tram Sagas Customers and Orders

This application demonstrates how to maintain data consistency in an Java/JDBC/JPA-based microservice architecture using sagas.

The application consists of two services:

• Order Service - creates orders

• Customer Service - manages customers

Both services are implemented using Spring Boot, JPA and the Eventuate Tram Saga framework

The Order Service uses a saga to enforce the customer’s credit limit when creating orders.

About sagas

Sagas are a mechanism for maintaining data consistency in a microservice 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. Please see example to learn about choreography-based sagas. This example uses orchestration-based sagas, where a saga (orchestration) object invokes the participants.

A saga orchestrator is a persistent object that does one of two things:

1. On startup, it sends a command message to a participant

2. When it receives a reply, it updates its state and sends a command message to the next participant.

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

• the Saga pattern

• my microXchg 2018 presentation

• read about sagas in my Microservice patterns book

The Create Order Saga

The following diagrams shows how the saga for creating an Order works:

It consists of the follow steps:

1. The Order Service creates an Order in a pending state

2. The Order Service creates a CreateOrderSaga to coordinate the creation of the order.

3. The CreateOrderSaga sends a reserveCredit command to the CustomerService

4. The Customer Service receives the command and attempts to reserve credit for that Order. It replies with a message indicating the outcome.

5. The CreateOrderSaga receives the reply

6. It send either an ApproveOrder or a RejectOrder command to the OrderService

7. The Order Service receives the command and changes state of the order to either approved or rejected.

Architecture

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

The application consists of two services:

• Customer Service - implements the REST endpoints for managing customers. The service persists the Customer JPA entity in a MySQL/Postgres database. Using the Eventuate Tram Saga framework, it processes command messages, updates its the Customer entity, and sends back a reply message.

• Order Service - implements a REST endpoint for managing orders. The service persists the Order JPA entity and the CreateOrderSaga in MySQL/Postgres database. Using the Eventuate Tram Saga framework, it sends command messages and processes replies.

The Eventuate Tram CDC service tracks inserts into the MESSAGE table using the MySQL binlog and publishes messages to Apache Kafka.

See below for a tour of the code.

Building and running

Note: you do not need to install Gradle since it will be downloaded automatically. You just need to have Java 8 installed.

First, build the application

./gradlew assemble

Next, launch the services using Docker Compose:

./gradlew mysqlComposeBuild
./gradlew mysqlComposeUp

Note:

If the containers aren’t accessible via localhost - e.g. you are using Docker Toolbox, you will have to use ${DOCKER_HOST_IP} instead of localhost. See this guide to setting DOCKER_HOST_IP for more information.

You can also run the Postgres version using ./gradlew postgresComposeBuild and ./gradlew postgresComposeUp

Using the application

Once the application has started, you can use the application via the Swagger UI:

• Order Service - http://localhost:8081/swagger-ui.html

• Customer Service - http://localhost:8082/swagger-ui.html

• API Gateway - http://localhost:8083/swagger-ui.html

You can also use curl to interact with the services. First, let’s create a customer:

$ curl -X POST --header "Content-Type: application/json" -d '{
  "creditLimit": {
    "amount": 5
  },
  "name": "Jane Doe"
}' http://localhost:8082/customers

HTTP/1.1 200
Content-Type: application/json;charset=UTF-8

{
  "customerId": 1
}

Next, create an order:

$ curl -X POST --header "Content-Type: application/json" -d '{
  "customerId": 1,
  "orderTotal": {
    "amount": 4
  }
}' http://localhost:8081/orders

HTTP/1.1 200
Content-Type: application/json;charset=UTF-8

{
  "orderId": 1
}

Finally, check the status of the Order:

$ curl -X GET http://localhost:8081/orders/1

HTTP/1.1 200
Content-Type: application/json;charset=UTF-8

{
  "orderId": 1,
  "orderState": "APPROVED"
}

Get a tour of the code

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

In Visual Studio Code

1. Install the Code Tour extension from the Visual Studio Code Marketplace

2. Use the CodeTour: Start Tour command from the command palette to start the tour

In a Github Codespace

1. Create the codespace

2. If necessary, install the Code Tour extension from the Visual Studio Code Marketplace.

3. Use the CodeTour: Start Tour command from the command palette to start the tour.

Got questions?

Don’t hesitate to create an issue or see

• Mailing list

• Slack. Get invite

• Contact us.