Notification Service

The negotiator features a comprehensive notification service for notifying users about various events tied to negotiations. The system is built using a strategy pattern with event-driven architecture to ensure extensibility and maintainability.

Architecture Overview

The notification system consists of several key components:

Core Components

1. NotificationListener - A generic event listener that receives Spring Application Events and dispatches them to appropriate handlers
2. NotificationStrategy - An interface that defines how specific events should be handled
3. NotificationService - The service responsible for creating and managing in-app notifications

Notifications vs Emails

The system handles two types of communications:

• Notifications - In-app notifications stored in the database that users can view within the negotiator interface
• Emails - Email notifications sent to users' email addresses for important events

Both are triggered by the same events but serve different purposes. Notifications provide an audit trail and in-app alerts, while emails ensure users are notified even when not actively using the platform.

Event Flow

1. Business events (e.g., negotiation state changes, new posts) are published as Spring Application Events
2. The NotificationListener receives these events asynchronously via @TransactionalEventListener
3. The listener dispatches events to registered NotificationStrategy implementations
4. Each strategy handles its specific event type and creates appropriate notifications and/or emails

Email Implementation

Email delivery is implemented using SpringMail.

Configuration

To configure Spring Mail, set the following environment variables or update the properties file for the appropriate profile:

SPRING_MAIL_HOST="smtp.example"
SPRING_MAIL_PORT=1025
SPRING_MAIL_USERNAME="user"
SPRING_MAIL_PASSWORD="pass"

Email Templates

Email templates use Thymeleaf as the template engine for dynamic content. Templates can be managed through the negotiator's admin interface.

Developer Guide: Extending the Notification System

Adding New Event Types and Handlers

To add support for new events:

1. Create an Event Class

public class MyCustomEvent extends ApplicationEvent {
    private final String negotiationId;
    
    public MyCustomEvent(Object source, String negotiationId) {
        super(source);
        this.negotiationId = negotiationId;
    }
    // Getters...
}

2. Implement a Notification Strategy

@Component
public class MyCustomEventHandler implements NotificationStrategy<MyCustomEvent> {
    private final NotificationService notificationService;

    @Override
    public Class<MyCustomEvent> getSupportedEventType() {
        return MyCustomEvent.class;
    }

    @Override
    @Transactional
    public void notify(MyCustomEvent event) {
        // Create notifications and/or trigger emails
        NotificationCreateDTO notification = new NotificationCreateDTO(
            userIds, "Title", "Body", event.getNegotiationId()
        );
        notificationService.createNotifications(notification);
    }
}

3. Publish the Event

@Service
public class MyBusinessService {
    private final ApplicationEventPublisher eventPublisher;

    public void performBusinessAction(String negotiationId) {
        // Business logic...
        eventPublisher.publishEvent(new MyCustomEvent(this, negotiationId));
    }
}

The NotificationListener will automatically discover and register new handlers, dispatching events to appropriate strategies.