Designing an Event Forwarding Framework in Java involves building a system that acts as an intermediary between event producers (source systems) and event consumers (target systems)(LLD).

This design promotes decoupling between components and enables scalability, reliability, and flexibility in integrating diverse systems.

✅ 1. Problem Statement

Design a framework where:

• Events are produced by one or more systems.

• Events need to be forwarded and consumed by one or more downstream systems.

• The framework should be extensible, support asynchronous forwarding, and ensure reliable delivery.

✅ 2. Key Requirements

Functional:

• Support registration of multiple event sources and consumers.

• Route/forward events to appropriate consumers.

• Support multiple event types.

• Support filtering, transformation, or enrichment of events before forwarding.

Non-Functional:

• Thread-safe, low-latency event delivery.

• Extensible (plug in new sources/consumers).

• Fault-tolerant and resilient.

• Optionally persistent (in-memory or disk-backed).
  

✅ 3. High-Level Architecture

           +----------------+
           |  Event Source  |  ← one or many
           +--------+-------+
                    |
                    v
          +--------------------+
          |  Event Dispatcher  |  ← central broker/router
          +--------+-----------+
                   | 
        +----------+----------+
        |          |          |
        v          v          v
  +---------+  +---------+  +---------+
  | Consumer|  | Consumer|  | Consumer| ← multiple
  +---------+  +---------+  +---------+

✅ 4. Core Components

1. Event

• Base class or interface for all event types.

public interface Event {
    String getType();
    long getTimestamp();
}

2. EventSource

• Interface to represent a producer of events.

public interface EventSource {
    void generateEvent(EventDispatcher dispatcher);
}

3. EventConsumer

• Interface for consumers that subscribe to and handle events.

  public interface EventConsumer {
      boolean supports(Event event);
      void consume(Event event);
  }
  

4. EventDispatcher

• Central component that receives events from sources and forwards them to appropriate consumers.

• Can support routing/filtering/transformation logic.

public class EventDispatcher {
    private final List<EventConsumer> consumers = new CopyOnWriteArrayList<>();

    public void registerConsumer(EventConsumer consumer) {
        consumers.add(consumer);
    }

    public void dispatch(Event event) {
        for (EventConsumer consumer : consumers) {
            if (consumer.supports(event)) {
                // Dispatch asynchronously (optional)
                CompletableFuture.runAsync(() -> consumer.consume(event));
            }
        }
    }
}

5. EventRouter (optional)

• Handles advanced routing (e.g., filter by type or metadata).

• Can be plugged into the dispatcher.

---

6. EventTransformer (optional)

• Allows transformation/enrichment of events before delivery.

---

7. EventQueue (optional for durability)

• Use in-memory queues like LinkedBlockingQueue or message queues (Kafka, RabbitMQ).
  

🎯 Pattern Mapping to Components

1. Observer Pattern

• Participants:

  • EventDispatcher = Subject

  • EventConsumer = Observer

• Purpose:

  • Notify all interested consumers when a new event is published.

  // Dispatcher calls all registered consumers' consume method
  for (EventConsumer consumer : consumers) {
      if (consumer.supports(event)) {
          consumer.consume(event);
      }
  }
  

  2. Strategy Pattern

  • Participants:

    • Interface: RoutingStrategy, TransformationStrategy, etc.

    • Implementations: e.g., TypeBasedRouter, PriorityRouter, AuditTransformer.

  • Purpose:

    • Easily switch logic for routing/filtering/transforming without modifying core flow.

    interface RoutingStrategy {
        List<EventConsumer> route(Event event, List<EventConsumer> all);
    }
    

    3. Factory Pattern

    • Participants:

      • EventFactory class to convert raw data (e.g., JSON, string) into event objects.

    • Purpose:

      • Abstract creation logic of event objects, useful when new event types are added dynamically.
        

     public class EventFactory {
        public static Event createEvent(String rawPayload) {
            // parse and return appropriate Event implementation
        }
    }
    

    4. Singleton Pattern

    • Participants:

      • EventDispatcher or EventQueue

    • Purpose:

      • Avoid multiple instances that could lead to conflicting state or double dispatch.

    public class EventDispatcher {
        private static final EventDispatcher instance = new EventDispatcher();
        private EventDispatcher() {}
        public static EventDispatcher getInstance() {
            return instance;
        }
    }
    

    5. Adapter Pattern

    • Participants:

      • Adapters for LegacySystemProducer or KafkaSourceAdapter

    • Purpose:

      • Wrap incompatible source systems to conform to EventSource interface.

    class KafkaEventSourceAdapter implements EventSource {
        public void generateEvent(EventDispatcher dispatcher) {
            // Read from Kafka and forward to dispatcher
        }
    }
    

    6. Decorator Pattern (Optional)

    • Participants:

      • EventTransformer interface with layered decorators (e.g., enrichment, masking)

    • Purpose:

      • Dynamically attach additional responsibilities to events.

    ---

    7. Chain of Responsibility (Optional)

    • Participants:

      • A chain of EventHandler classes processing the event.

    • Purpose:

      • Create a processing pipeline for events: validate → enrich → route → notify.

    interface EventHandler {
        void handle(Event event, EventHandler next);
    }
    

    8. Command Pattern (Optional, Advanced)

    • Participants:

      • Encapsulate each event delivery attempt as a command object.

    • Purpose:

      • Add retry, audit logging, or delayed execution for event delivery.
        

    class DeliverEventCommand {
        private final EventConsumer consumer;
        private final Event event;
    
        public void execute() {
            consumer.consume(event);
        }
    }