Pub-Sub

easy · observer, topics

Brief-reported interview attribution (unverified): Amazon, Microsoft, Uber

Design a publish-subscribe system. Subscribers register a callback for a topic; publish(topic, message) delivers the message to every current subscriber of that topic, in subscription order, synchronously. Delivery count is returned.

class PubSub:
    def __init__(self):
    def subscribe(self, topic, subscriber_id, callback):
        # -> True if newly subscribed, False if this subscriber is already on the topic
    def unsubscribe(self, topic, subscriber_id):
        # -> True if the subscription existed, False otherwise
    def publish(self, topic, message):
        # -> number of subscribers the message was delivered to
    def topics(self):
        # -> sorted list of topics that currently have at least one subscriber

Rules. A subscriber may be on several topics. Publishing to a topic with no subscribers delivers nothing and returns 0. Publishing must not break if a callback unsubscribes during delivery (deliver to a snapshot of the subscriber list). topics() lists only topics with subscribers — publishing alone does not create a topic.

Tests use explicit inputs and injected clocks/callbacks. No network, real threads or application-server execution is involved.

Public demo

# Public demo: run after implementing the class API above.
p = PubSub()
inbox = []
print(p.subscribe('news', 'alice', lambda message: inbox.append(('alice', message))))
print(p.subscribe('news', 'alice', lambda message: None))

Starter code

class PubSub:

    def __init__(self):
        """Initialize the state described in the problem."""
        pass

    def subscribe(self, topic, subscriber_id, callback):
        """-> True if newly subscribed, False if this subscriber is already on the topic"""
        pass

    def unsubscribe(self, topic, subscriber_id):
        """-> True if the subscription existed, False otherwise"""
        pass

    def publish(self, topic, message):
        """-> number of subscribers the message was delivered to"""
        pass

    def topics(self):
        """-> sorted list of topics that currently have at least one subscriber"""
        pass

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