The Observer pattern has subjects directly maintain and notify a list of observers within the same process, while the Publish/Subscribe model decouples publishers and subscribers entirely through an intermediary broker or message channel.
Key Points: • Observer involves direct references: the subject knows its observers and calls their update methods directly. • Pub/Sub introduces a broker or event bus between publishers and subscribers, so neither knows about the other. • Observer is typically synchronous and in-process; Pub/Sub is often asynchronous and can span multiple processes or services. • Pub/Sub scales better for distributed systems since publishers and subscribers can be added or removed independently of each other. • Observer is simpler to implement for small, in-app event handling; Pub/Sub suits event-driven microservice architectures.
Example: A Java Swing button click notifying its registered ActionListeners is a classic Observer scenario, whereas a microservice publishing an OrderPlaced event to a Kafka topic that multiple unrelated services subscribe to is a Pub/Sub scenario.
Interview Tip: A concise interview answer is:
"Observer has the subject directly hold and notify its observers in-process, while Pub/Sub decouples publishers and subscribers through a broker or channel so they never reference each other directly — that makes Pub/Sub much better suited to distributed, event-driven systems, while Observer is simpler for in-app notifications."