Requirements

Imagine you're building a simplified message queueing system. This system allows producers to publish messages to specific topics, and consumers to subscribe to these topics to receive messages. A key aspect of this system is the ability to handle multiple consumers subscribing to the same topic and ensuring each message is delivered to all subscribers. This system needs to be robust, scalable, and thread-safe. Let's dive deep into the core of a message queueing system, focusing on the object-oriented design aspects rather than the complexities of a distributed infrastructure. # Requirements ## Functional Requirements 1. **Topic Creation:** The system must allow the creation of new topics. 2. **Publishing Messages:** Producers should be able to publish messages to a specific topic. The message should be a simple string payload. 3. **Subscription to Topics:** Consumers should be able to subscribe to a topic. 4. **Message Delivery:** When a message is published to a topic, it should be delivered to all subscribed consumers. 5. **Unsubscription from Topics:** Consumers should be able to unsubscribe from a topic. 6. **Message Persistence (Optional):** The system should optionally persist messages to disk, to survive restarts. This persistence mechanism should be pluggable (different storage backends should be supported). 7. **Message Ordering (Optional):** The system should optionally guarantee message delivery in the order they were published to a topic. ## Non-Functional Requirements 1. **Thread Safety:** The system must be thread-safe, allowing multiple producers and consumers to interact with the queue concurrently without data corruption or race conditions. Proper synchronization mechanisms should be used. 2. **Extensibility:** The design should be extensible, allowing new features (e.g., different message formats, delivery guarantees, storage options) to be added easily without significant code changes. Use design patterns to achieve this. 3. **Modularity:** The system should be modular, with well-defined responsibilities for each component. This will improve maintainability and testability. 4. **Testability:** The system should be designed to be easily testable. Dependencies should be easily mockable, and unit tests should be possible for each component. 5. **Resource Management:** Efficiently manage resources (e.g., memory, threads) to prevent leaks and ensure optimal performance. Consider using a thread pool for message delivery if the number of consumers is very high. 6. **Persistence mechanism:** Persistence mechanism should be pluggable so that different storage options can be supported. ## Core Entities 1. **Topic:** Represents a channel for message delivery. It maintains a list of subscribers (consumers). 2. **Message:** Represents the data being published. For simplicity, it will be a String. A more complex system might have a `Message` class with metadata and payload. 3. **Producer:** Publishes messages to a specific topic. 4. **Consumer:** Subscribes to a topic and receives messages. It has a callback or method that gets executed when a message is received. 5. **MessageQueue:** The central component that manages topics, producers, and consumers. It handles message routing and delivery. 6. **Subscriber:** Represents a subscriber to the topic and encapsulates the action to be performed on receiving a message. 7. **Storage (interface):** Abstraction for message persistence, allowing for different storage implementations (e.g., file system, database).

Developing a message queueing system.

Requirements

Think like an Architect

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