Observer Pattern vs. Pub/Sub Models

Observer and pub-sub both distribute reactions to events, but they operate at different scales and with different assumptions.

    flowchart LR
	    A["Observer"] --> B["Subject knows listener registration"]
	    B --> C["Usually in-process and direct"]
	    D["Pub-sub"] --> E["Publisher and subscriber decoupled by topic or broker"]
	    E --> F["Often distributed or infrastructure-backed"]

Observer

Observer is usually:

• in-process
• direct
• synchronous unless designed otherwise
• built around explicit listener registration

Pub-Sub

Pub-sub is usually:

• brokered or topic-based
• more loosely coupled in addressability
• often asynchronous
• more infrastructure-aware

Java Shortcut

If one object inside the same process needs to notify listeners, start by thinking Observer.

If events need routing through topics, infrastructure, or service boundaries, think pub-sub or event-driven messaging instead.