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mediumSystem Design

What is the publish-subscribe pattern?

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01

Understand the problem

Explain pub/sub.

pub-sub
02

Attempt it yourself

Sketch your approach before reading the solution — that's what interviews test.

Nudge consolestandby

Stuck? Beam a request up — the console returns a conceptual nudge that guides your logic without spoiling the implementation.

03

Study the solution

Step 1: Outline use cases and constraints

Gather requirements and scope the problem. Ask questions to clarify use cases and constraints. Discuss assumptions.

Use cases

We'll scope the problem to handle only the following use cases

  • User needs to understand pub-sub to make architecture decisions

Solution ready — 2 min read

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04

Read the code

In-memory broker (subscribe / publish)
Run Playground
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})
05

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