spring-boot-event-driven-patterns

# Spring Boot Event-Driven Patterns

## Overview

Implement Event-Driven Architecture (EDA) patterns in Spring Boot 3.x using domain events, ApplicationEventPublisher, `@TransactionalEventListener`, and distributed messaging with Kafka and Spring Cloud Stream.

## When to Use

- Implementing event-driven microservices with Kafka messaging
- Publishing domain events from aggregate roots in DDD architectures
- Setting up transactional event listeners that fire after database commits
- Adding async messaging with producers and consumers via Spring Kafka
- Ensuring reliable event delivery using the transactional outbox pattern
- Replacing synchronous calls with event-based communication between services

## Quick Reference

| Concept | Description |
|---------|-------------|
| **Domain Events** | Immutable events extending `DomainEvent` base class with eventId, occurredAt, correlationId |
| **Event Publishing** | `ApplicationEventPublisher.publishEvent()` for local, `KafkaTemplate` for distributed |
| **Event Listening** | `@TransactionalEventListener(phase = AFTER_COMMIT)` for reliable handling |
| **Kafka** | `@KafkaListener(topics = "...")` for distributed event consumption |
| **Spring Cloud Stream** | Functional programming model with `Consumer` beans |
| **Outbox Pattern** | Atomic event storage with business data, scheduled publisher |

## Examples

### Monolithic to Event-Driven Refactoring

**Before (Anti-Pattern):**
```java
@Transactional
public Order processOrder(OrderRequest request) {
    Order order = orderRepository.save(request);
    inventoryService.reserve(order.getItems()); // Blocking
    paymentService.charge(order.getPayment()); // Blocking
    emailService.sendConfirmation(order); // Blocking
    return order;
}
```

**After (Event-Driven):**
```java
@Transactional
public Order processOrder(OrderRequest request) {
    Order order = Order.create(request);
    orderRepository.save(order);

    // Publish event after transaction commits
    eventPublisher.publishEvent(new OrderCreatedEvent(order.getId(), order.getItems()));

    return order;
}

@Component
public class OrderEventHandler {
    @TransactionalEventListener(phase = TransactionPhase.AFTER_COMMIT)
    public void handleOrderCreated(OrderCreatedEvent event) {
        // Execute asynchronously after the order is saved
        inventoryService.reserve(event.getItems());
        paymentService.charge(event.getPayment());
    }
}
```

See [examples.md](references/examples.md) for complete working examples.

## Instructions

### 1. Design Domain Events

Create immutable event classes extending a base `DomainEvent` class:

```java
public abstract class DomainEvent {
    private final UUID eventId;
    private final LocalDateTime occurredAt;
    private final UUID correlationId;
}

public class ProductCreatedEvent extends DomainEvent {
    private final ProductId productId;
    private final String name;
    private final BigDecimal price;
}
```

See [domain-events-design.md](references/domain-events-design.md) for patterns.

### 2. Publish Events from Aggregates

Add domain events to aggregate roots, publish via `ApplicationEventPublisher`:

```java
@Service
@Transactional
public class ProductService {
    public Product createProduct(CreateProductRequest request) {
        Product product = Product.create(request.getName(), request.getPrice(), request.getStock());
        repository.save(product);

        product.getDomainEvents().forEach(eventPublisher::publishEvent);
        product.clearDomainEvents();

        return product;
    }
}
```

See [aggregate-root-patterns.md](references/aggregate-root-patterns.md) for DDD patterns.

### 3. Handle Events Transactionally

Use `@TransactionalEventListener` for reliable event handling:

```java
@Component
public class ProductEventHandler {
    @TransactionalEventListener(phase = TransactionPhase.AFTER_COMMIT)
    public void onProductCreated(ProductCreatedEvent event) {
        notificationService.sendProductCreatedNotification(event.getName());
    }
}
```

**Validate:** Confirm the event handler fires only after the transaction commits by checking that the database state is committed before the handler executes.

See [event-handling.md](references/event-handling.md) for handling patterns.

### 4. Configure Kafka Infrastructure

Configure KafkaTemplate for publishing, `@KafkaListener` for consuming:

```yaml
spring:
  kafka:
    bootstrap-servers: localhost:9092
    producer:
      value-serializer: org.springframework.kafka.support.serializer.JsonSerializer
```

**Validate:** Send a test event via `KafkaTemplate` and confirm it appears in the consumer logs before proceeding to production patterns.

See [dependency-setup.md](references/dependency-setup.md) and [configuration.md](references/configuration.md).

### 5. Implement Outbox Pattern

Create `OutboxEvent` entity for atomic event storage:

```java
@Entity
public class OutboxEvent {
    private UUID id;
    private String aggregateId;
    private String eventType;
    private String payload;
    private LocalDateTime publishedAt;
}
```

**Validate:** Confirm the scheduled processor picks up pending events by checking the `publishedAt` timestamp is set after the scheduled run.

Scheduled processor publishes pending events. See [outbox-pattern.md](references/outbox-pattern.md).

### 6. Handle Failure Scenarios

Implement retry logic, dead-letter queues, idempotent handlers:

```java
@RetryableTopic(attempts = "3")
@KafkaListener(topics = "product-events")
public void handleProductEvent(ProductCreatedEventDto event) {
    orderService.onProductCreated(event);
}
```

**Validate:** Confirm messages reach the dead-letter topic after exhausting retries before moving to observability.

### 7. Add Observability

Enable Spring Cloud Sleuth for distributed tracing, monitor metrics.

## Best Practices

- **Use past tense naming**: `ProductCreated` (not `CreateProduct`)
- **Keep events immutable**: All fields should be final
-