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CH12 Integrating via Messaging

Table of Contents

In this chapter, the authors build a distributed application, that consists of a few services. They call it an "E‐Commerce Application". The services communicate with each other via message buses. The authors use two message platforms: NServiceBus and Mass Transit. They use ASP.NET as a web framework. I’ll use Spring Boot with RabbitMQ and Kafka instead.

Replacements

Table 1. replacements
Original Replacement Comment

ASP.NET

Spring Thymeleaf

JSP also can work

NServiceBus

RabbitMQ

initially ApplicationEventPublisher

Mass Transit

Kafka

As the original code is written on C#, authors use Visual Studio projects. I’ll use Maven. And every Visual Studio subproject will be a separate maven module. Each maven module that wants to produce or consume a message must add a corresponding module with messages classes as a dependency.

The Maven modules naming convention is the following {bounded context}-{messages} like sales-message and {bounded context}-{business-component} like sales-ordes. Also, package structure follows domainName.businessComponent pattern. See the project structure.

Spring Boot

Spring boot has a built-in web server, which makes it easy to develop a web module. However, other components are console applications in the original. They only listen to events or publish events. As they don’t need a web server, Spring Boot is overkill, maybe in the future I’ll rewrite them just with Spring.

Spring Thymeleaf

UI is used only to "place an order" - trigger the process. It’s not necessary, I can trigger web app via curl. But I try to follow the book structure. The web module provides a simple HTML page. I used Spring Thymeleaf But I don’t need any rendering, so just HTML should work.

ApplicationEventPublisher

Spring Framework supports Application Events. Initially, while I was developing everything in a single module I used ApplicationEventPublisher as a message bus.

RabbitMQ

Java classes must implement Serializable interface to be published in RabbitMQ.

I run RabbitMQ separately from docker. I use the default settings so far. The default credentials are guest/guest. RabbitMQ management UI is available at http://localhost:15672/.

docker run -d -p 5672:5672 -p 15672:15672 --name my-rabbit rabbitmq:3-management

Also, it seems it’s not possible to have multi events in one queue. That’s why I configured a few queues. Otherwise, I’m getting an exception.

Cannot convert from [sales.messages.commands.PlaceOrder] to [sales.messages.events.OrderCreated]

Also, I didn’t set up "Second-Level Retries", mentioned in the book.

As RabbitMQ does not support multiple consumers for the same message. That’s why I used fanout feature for OrderCreated event.

Kafka

I use kafka from confluent-local. See the attached docker compose. I use default settings. I use JSON deserializer. Jackson can’t deserialize without setters by default. That’s why OrderCreated in promotions-lucky-winner has getters and setters.

Result Run

Let’s run all apps, "place an order" and see logs.

ui
Figure 1. UI
shipping
Shipping BC storing: Order: {1} User: {42} Shipping Type: {POSTNL} AddressID123
Shipping BC arranged shipping for Order: {1} {42}, Address ID: {AddressID123}
billing
Received order created event: OrderId: 1
# exception logs, simulating PaymentProvider failure
# Caused by: java.lang.RuntimeException: Service unavailable. Down for maintenance.

Received order created event: OrderId: 1
# exception logs, simulating PaymentProvider failure
# Caused by: java.lang.RuntimeException: Service unavailable. Down for maintenance.

Received order created event: OrderId: 1
Received payment accepted notification for Order: {1}. Published PaymentAccepted event
sales
Created order #{1} : Products:{1,2,3} with shipping: {POSTNL} made by user: {42}
promotions bridge
Bridge received order: {1}. About to push it onto Kafka's queue
promotions
Kafka handling order placed event: Order: {1} for User: {42}

Application’s Diagram

@startuml C4_Elements
!include https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Container.puml
!include https://raw.githubusercontent.com/plantuml-stdlib/C4-PlantUML/master/C4_Component.puml
' ContainerDb(db, "Database", "Microsoft SQL", "Holds product, order and invoice information")
'System(systemAlias, "Label", "Optional Description")

AddRelTag("async", $textColor=$ARROW_FONT_COLOR, $lineColor=$ARROW_COLOR, $lineStyle=DashedLine())

Person(user, "user")

System_Boundary(e_commence, "E‐Commerce") {
    Container(web_app, "Web App", "Spring Boot")
    Container(sales, "Sales", "Spring Boot")
    Container(billing, "Billing", "Spring Boot")
    Container(shipping, "Shipping", "Spring Boot")
    Container(promotions_bridge, "Promotions-Bridge", "Spring Boot")

    ContainerQueue(sales_orders_PlaceOrder, "PlaceOrder", "RabbitMQ")
    ContainerQueue(sales_orders_OrderCreated, "OrderCreated", "RabbitMQ")
    ComponentQueue(billing_local, "RecordPaymentAttempt", "RabbitMQ")
    ContainerQueue(billing_payments_PaymentAccepted, "PaymentAccepted", "RabbitMQ")
    ContainerQueue(shipping_ShippingArranged, "ShippingArranged", "RabbitMQ")
}

System_Boundary(promotions_system, "Promotions") {
    Container(promotions, "Promotions", "Spring Boot")
    ContainerQueue(kafka, "Promotions/PlaceOrder", "Kafka")
}

Rel(user, web_app, "POST /orders/place", "HTTP")
Rel(web_app, sales_orders_PlaceOrder, "Publishes", "Command", $tags="async")

Rel(sales, sales_orders_PlaceOrder, "Consumes", "Command", $tags="async")
Rel(sales, sales_orders_OrderCreated, "Publishes", "Event", $tags="async")

Rel(billing, sales_orders_OrderCreated, "Consumes", "Event", $tags="async")
Rel(billing, billing_local, "Publishes", "Command", $tags="async")
Rel(billing, billing_local, "Consumes", "Command", $tags="async")
Rel(billing, billing_payments_PaymentAccepted, "Publishes", "Event", $tags="async")

Rel(shipping, sales_orders_OrderCreated, "Consumes", "Event", $tags="async")
Rel(shipping, billing_payments_PaymentAccepted, "Consumes", "Event", $tags="async")
Rel(shipping, shipping_ShippingArranged, "Publishes", "Event", $tags="async")

Rel(promotions_bridge, sales_orders_OrderCreated, "Consumes", "Event", $tags="async")
Rel(promotions_bridge, kafka, "Publishes", "Event", $tags="async")
Rel(promotions, kafka, "Consumes", "Event", $tags="async")

SHOW_LEGEND()
footer drawn with PlantUML v. %version() and C4-PlantUML v. C4Version()
@enduml

To Don’t List

List of improvements, which nice to do, but I won’t spend time on it. Pull requests are welcome!

  • Find a better way to distinguish events and commands in C4 diagram.

  • Add RabbitMQ to docker compose.

  • Use JSON serializer for RabbitMQ and remove Serializable.

  • Split code into packages: application, domain, infrastructure, like it’s done in book examples

  • Use the latest Spring Boot.

  • When I updated to Spring Boot 3, logging stopped working. That’s why I added exclusions and slf4j to promotions-lucky-winner. Which is strange and should be investigated.

  • Refactor code, fix formatting.

  • Applications are just event listeners and don’t need a web server. Rewrite them to a console app.