An example of reactive microservice using Lagom, Kafka, Akka Play HTTP server, and PostgreSQL

This sample application presents a simple product catalog and shopping cart service built with Lagom, Kafka, Akka Play HTTP server, and PostgreSQL.

It contains the following two services,

Shopping cart service:

• It saves data to PostgreSQL database using Lagom's persistence API. It provides CRUD APIs on the shopping cart to view, add, update, and delete the products from the cart.

Product catalog service:

• It consumes a stream of events published to Kafka from the shopping cart service, and shows how to consume Kafka event stream work using Lagom.
• When the cart will be checked out, cart service will publish an event which product service will receive and adjust the product catalog.
• The catalog is currently maintained in a memory store so it won't be replicated over the cluster.

Technologies Used

• Lagom
• Lightbend
• Akka
• Play HTTP
• PostgreSQL
• Kafka
• Docker
• Java
• curl

What is a reactive service

• https://www.reactivemanifesto.org/
• https://medium.com/@filia.aleks/microservice-performance-battle-spring-mvc-vs-webflux-80d39fd81bf0
• https://sites.google.com/a/mammatustech.com/mammatusmain/reactive-microservices

Other reactive service frameworks

• Springboot Async
• Eclipse Vert.x

Running the application

Prerequisites

• Download and install Docker desktop for your version of OS.
• Install Java, either Oracle or OpenJDK, and configure the JAVA_PATH.
• Install some IDE like IntelliJ or Eclipse. It will allow you to debug or extend the code.
• Optionally install the Docker plugin to start or stop docker from the IDE.
• Download, install, and configure Maven.

Running Docker

• Run the following docker-compose command. It will make the PostgresQL and Kafka available.
• Postgres will be available on port 5432 and Kafka on port 9092.

cd into the project base directory
docker-compose up -d

Running services

mvn lagom:runAll

Catalog service API

The catalog service offers two REST endpoints:

• Get the inventory of an item:

curl http://localhost:9000/catalog/123

• Add product quantity to a catalog: The following command is adding 4 items to product id 123

curl -H "Content-Type: application/json" -d 4 -X POST http://localhost:9000/calalog/123
curl -H "Content-Type: application/json" -d 6 -X POST http://localhost:9000/calalog/456

Note: Before using the cart APIs do call the add items API so that catalog service can decrement the item quantities upon kafka event when the cart is checked out.

Shopping cart service

The shopping cart service offers four REST endpoints:

• Get the current contents of the shopping cart:

curl http://localhost:9000/cart/1

• Get a report of the shopping cart creation and checkout dates:

curl http://localhost:9000/cart/1/report

• Add an item in the shopping cart:

curl -H "Content-Type: application/json" -d '{"itemId": "123", "quantity": 2}' -X POST http://localhost:9000/cart/1
curl -H "Content-Type: application/json" -d '{"itemId": "456", "quantity": 1}' -X POST http://localhost:9000/cart/1

• Remove an item in the shopping cart:

curl -X DELETE http://localhost:9000/cart/1/item/123

• Adjust an item's quantity in the shopping cart:

curl -H "Content-Type: application/json" -X PATCH -d '{"quantity": 2}' http://localhost:9000/cart/1/item/123

• Check out the shopping cart (i.e., complete the transaction)

curl -X POST http://localhost:9000/cart/1/checkout

When the shopping cart is checked out, an event is published to the Kafka topic called shopping-cart by the shopping cart service. Such events look like this:

{
  "id": "1",
  "items": [
    {"itemId": "123", "quantity": 2},
    {"itemId": "456", "quantity": 1}
  ],
  "checkedOut": true
}

The catalog service will decrement the item quantities which can be verified by calling the following APIs

curl http://localhost:9000/catalog/123
curl http://localhost:9000/catalog/456