Python Observer vs Pub/Sub — ELI5

Imagine two ways to share exciting news at school.

Way 1 — The Observer pattern: You have a best friend list. When something happens, you personally walk up to each friend and tell them. You know exactly who you’re telling, and they know it’s coming from you. If your friend moves to a different school, you have to update your list.

Way 2 — Pub/Sub: There’s a bulletin board in the hallway. When something happens, you pin a note to the board. You don’t know who reads it. Kids who care about that kind of news check the board regularly. If a new student shows up and cares about your topic, they just start reading — you never have to know they exist.

Both patterns do the same basic job: “something happened, and other parts of the program need to know.” The difference is how directly connected the sender and receivers are.

With Observer, the sender (called the “subject”) keeps a list of receivers (called “observers”) and notifies them directly. It’s like having someone’s phone number — you call them specifically.

With Pub/Sub, there’s a middleman (called a “broker” or “message bus”) that handles delivery. Publishers post messages to topics. Subscribers listen to topics. Publishers and subscribers never talk to each other directly — the broker is the bulletin board.

When does each make sense? If your program is small and everything runs in one place, Observer is simpler — just keep a list and notify it. If your program is spread across multiple services or you want maximum flexibility, Pub/Sub gives you that separation.

One thing to remember: Observer is “I know who I’m telling.” Pub/Sub is “I’m announcing it, and whoever cares will hear it.” Both share information, but Pub/Sub puts a middleman between sender and receiver.