mediumSystem Design

What is the publish-subscribe pattern?

Q: What is the publish-subscribe pattern?

## Publish-subscribe In **pub/sub**, publishers send messages to a **topic**, and the broker **fans out** a copy to **every** subscriber of that topic. Publishers and subscribers never reference each other — they're fully **decoupled**, and you can add subscribers without touching the publisher. Publisher Topic orders <rect

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pub-sub

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In-memory broker (subscribe / publish)

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from collections import defaultdict

class Broker:
    def __init__(self):
        self.subs = defaultdict(list)      # topic -> [callbacks]
    def subscribe(self, topic, fn):
        self.subs[topic].append(fn)
    def publish(self, topic, msg):
        for fn in self.subs[topic]:        # fan-out: every subscriber gets it
            fn(msg)

broker = Broker()
broker.subscribe("orders", lambda m: print("email service got", m))
broker.subscribe("orders", lambda m: print("analytics got", m))
broker.publish("orders", {"id": 1, "total": 4999})

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