DDD Microservices Example Project in Java: eCommerce

The purpose of this project is to provide a sample implementation of an e-commerce product following Domain-Driven Design (DDD) and Service-Oriented Architecture (SOA) principles.

Programming language is Java with heavy use of Spring Boot, Docker and Kubernetes.

Purpose of the Project

This repository focuses mostly on cross-cutting, infrastructure and deployment concerns.

For the domain and application concepts see the original repository.

Monolith vs Microservices

Both monolithic and microservices deployments are implemented.

To run the monolithic application:

./gradlew :application:bootRun

To set up and run microservices, see the Docker and Kubernetes sections.

Read more about monoliths vs microservices at https://blog.ttulka.com/good-and-bad-monolith

Message Broker

As the message broker a simple Redis instance could be used with Spring profile redis:

docker run --rm --name redis-broker -p 6379:6379 -d redis:6 redis-server

./gradlew :application:bootRun --args='--spring.profiles.active=redis'

Alternatively, RabbitMq could be used as the message broker with Spring profile rabbitmq:

docker run --rm --name rabbitmq-broker -p 5672:5672 -d rabbitmq:3

./gradlew :application:bootRun --args='--spring.profiles.active=rabbitmq'

When neither redis not rabbitmq profiles are active, the system will fall-back to use of Spring application events as the default messaging mechanism.

Messaging Integration

To make the code independent of a concrete messaging implementation and easy to use, Spring application events are used for the internal communication.

In practice, this means that messages are published via EventPublisher abstraction and consumed via Spring's @EventListener.

To make this work, the external messages are re-sent as Spring application events under the hood.

Database

The whole system uses one externalized database with particular tables owned exclusively by services.

In a real-world system this separation would be further implemented by separate schemas/namespaces.

As the database PostgreSQL instance could be used:

docker run --rm --name postgres -p 5432:5432 -e POSTGRES_PASSWORD=secret -d postgres:13

Start the application with Spring profile postgres:

./gradlew :application:bootRun --args='--spring.profiles.active=postgres'

When the postgres profile is not active, the system will fall-back to use H2 as the default database.

Gradle Build

The project is a Gradle multi-project, all sub-projects can be built in a single command:

./gradlew clean build

Docker Containers

Build an image per microservice via Gradle Spring Boot plugin:

./gradlew bootBuildImage

To run the containers:

docker container run --rm -p 8080:8001 ttulka/ecommerce-catalog-service
docker container run --rm -p 8080:8002 ttulka/ecommerce-order-service
docker container run --rm -p 8080:8003 ttulka/ecommerce-cart-service
docker container run --rm -p 8080:8004 ttulka/ecommerce-payment-service
docker container run --rm -p 8080:8005 ttulka/ecommerce-delivery-service
docker container run --rm -p 8080:8006 ttulka/ecommerce-dispatching-service
docker container run --rm -p 8080:8007 ttulka/ecommerce-warehouse-service
docker container run --rm -p 8080:8000 ttulka/ecommerce-portal-service

Active profiles can be set as follows:

docker container run --rm -e "SPRING_PROFILES_ACTIVE=redis,postgres" -p 8080:8001 ttulka/ecommerce-catalog-service

Docker-Compose

Build NGINX reverse proxy image:

docker build -t ttulka/ecommerce-reverseproxy reverseproxy

Start the entire microservices stack:

docker-compose up

Access the Postgres database and init some data:

docker exec -it <containerID> psql -U postgres postgres

INSERT INTO categories VALUES
    ('C1', 'books', 'Books'),
    ('C2', 'games', 'Games');

INSERT INTO products VALUES
    ('P1', 'Domain-Driven Design', 'by Eric Evans', 45.00),
    ('P2', 'Object Thinking', 'by David West', 35.00),
    ('P3', 'Chess', 'Classic game.', 3.20);

INSERT INTO products_in_categories VALUES
    ('P1', 'C1'),
    ('P2', 'C1'),
    ('P3', 'C2');

INSERT INTO products_in_stock VALUES
    ('P1', 5),
    ('P2', 0),
    ('P3', 1);

The NGINX reverse proxy serves as a simple API gateway:

curl localhost:8080/catalog/products
curl localhost:8080/warehouse/stock/5

Kubernetes

To use local images for development with Minikube, run the following command to use local Docker images registry:

minikube start
eval $(minikube docker-env)

Afterwards, build the docker images again for the Minikube's Docker daemon:

./gradlew bootBuildImage

Create deployments:

kubectl apply -f 1-infrastructure.k8s.yml
kubectl apply -f 2-backend-services.k8s.yml
kubectl apply -f 3-frontend-portal.k8s.yml
kubectl apply -f 4-api-gateway.k8s.yml

Set up ports forwarding to access the cluster from your local network:

kubectl port-forward service/reverseproxy 8080:8080

Alternatively, you can create an Ingress:

minikube addons enable ingress

kubectl apply -f 5-ingress.k8s.yml

# get the ingress address 
kubectl get ingress ecommerce-ingress

# add the address into hosts
sudo cp /etc/hosts hosts.bak
sudo echo -e '\n<ingress-address> ecommerce.local' >> /etc/hosts

# access the application in browser: http://ecommerce.local