What is the Observer Pattern? Where is it used in real life? (Event handling, Pub/Sub)

Difficulty: 5/10

event handling, pub/sub, decoupling

Observer Pattern

The Observer Pattern establishes a one-to-many relationship where a subject publishes state changes or events and observers react to them. It is common in GUI event handling, domain events and reactive systems. In distributed systems, the same conceptual idea appears in Pub/Sub messaging, although a message broker introduces additional concerns such as delivery semantics, retries and durability.

javascript

1
Subject publishes events to registered observers.
2
Observers can be added without changing the subject's core behavior.
3
Common in UI events and in-process domain events.
4
Distributed Pub/Sub extends the concept across processes or services.
5
Important concerns include observer lifecycle, ordering, failures and memory leaks.

Scenario Questions

0-2 years experience

2-5 years experience

1We have a chat application where multiple UI widgets need to react to incoming messages. Walk me through how you'd structure observers and what trade‑offs you consider.

What is the Observer Pattern? Where is it used in real life? (Event handling, Pub/Sub)

Observer Pattern

Scenario Questions

Follow-up Questions