What are Publish-Subscribe (Pub-Sub) patterns in Spring Boot? Spring Boot में Publish-Subscribe (Pub-Sub) patterns क्या हैं?

Pub-Sub is a messaging pattern where publishers send messages to topics without knowing subscribers, and subscribers receive all messages from subscribed topics. Enables loose coupling and one-to-many communication in distributed systems.

// Spring Cloud Stream Pub-Sub Example

// Publisher
@RestController
@RequestMapping('/api/orders')
public class OrderPublisher {
    @Autowired
    private StreamBridge streamBridge;
    
    @PostMapping
    public void createOrder(@RequestBody Order order) {
        // Publish to 'order-events' destination
        streamBridge.send('order-events', order);
    }
}

// Subscriber 1: Inventory Service
@Service
public class InventoryService {
    @Bean
    public Consumer<Order> inventoryProcessor() {
        return order -> {
            System.out.println('Updating inventory for order: ' + 
                order.getId());
            // Update inventory
        };
    }
}

// Subscriber 2: Notification Service
@Service
public class NotificationService {
    @Bean
    public Consumer<Order> notificationProcessor() {
        return order -> {
            System.out.println('Sending notification for order: ' + 
                order.getId());
            // Send email/SMS
        };
    }
}

// Subscriber 3: Analytics Service
@Service
public class AnalyticsService {
    @Bean
    public Consumer<Order> analyticsProcessor() {
        return order -> {
            System.out.println('Recording order metric: ' + 
                order.getId());
            // Log metrics
        };
    }
}

// Configuration
spring:
  cloud:
    function:
      definition: inventoryProcessor;notificationProcessor;analyticsProcessor
    stream:
      bindings:
        inventoryProcessor-in-0:
          destination: order-events
          group: inventory-group
        notificationProcessor-in-0:
          destination: order-events
          group: notification-group
        analyticsProcessor-in-0:
          destination: order-events
          group: analytics-group

// Pub-Sub guarantees:
// 1. Fan-out - single message to multiple subscribers
// 2. Loose coupling - publishers don't know subscribers
// 3. Scalability - add subscribers without changing publisher
// 4. Independent processing - each subscriber own pace
// 5. Message durability - queue persistence