Which pattern allows multiple modules to receive notifications when a specific event occurs?

The Observer or Publish-Subscribe pattern is specifically designed to facilitate communication among multiple modules or components in a system when a certain event occurs. In this pattern, there are two main roles: the "subject" that emits events and "observers" that listen and respond to those events.

When an event happens in the subject, it notifies all registered observers, allowing them to act independently based on the event. This decouples the modules, meaning that the subject does not need to have direct references to the observers, making the system more flexible and modular. Observers can be added or removed without changing the subject, which enhances maintainability and scalability.

In contrast, the other patterns listed do not serve the purpose of enabling multiple modules to receive notifications in the same way. The Strategy pattern is focused on defining a family of algorithms and making them interchangeable, rather than event notification. The Facade pattern provides a simplified interface to a complex subsystem, shielding the client from the complexities rather than facilitating communication among modules. The Singleton pattern ensures that a class has only one instance and provides a global point of access to it; it does not deal with event notifications or communication between multiple observers.

Thus, the Observer or Publish-Subscribe pattern is ideal for