[BACKEND]

To do:

When new order is created, you need to send an email to users with ADMIN role
create a NotificationClient interface and a NotificationClientImpl class with the implementation of notification client with a single method: sendEmailNotification.
In the sendEmailNotification you should call notification-app endpoint which sends emails
use Kafka and RestTemplate to communicate between microservices (kafka must be paid for heroku
create 2 implementations of NotificationClient. One is KafkaNotificationClient and another is NotificationClientImpl which will be default implementation

I suggest you to use RestTemplate because if you deploy application to Heroku - kafka is not going to be in the free plan. But for the local purposes you can use kafka to communicate between microservices.