Pub/Sub questions on the Professional Cloud Architect exam tend to lean on a small set of vocabulary words. If you do not have crisp definitions for publisher, subscriber, topic, and subscription, the wording of the answer choices will trip you up. I want to walk through these four terms the way I think about them when I am designing a messaging system on Google Cloud.
A publisher is an entity that creates and sends messages to Pub/Sub. It can be an application, a microservice, an IoT device, or any process that has data to emit. The defining property of a publisher is that it does not need to know who will receive or process the messages. It just sends them.
A subscriber is an entity on the other end. It receives messages from Pub/Sub, acknowledges that it got them, and processes them. The subscriber also has no knowledge of the publisher. It does not care which application produced the message or how many other consumers exist.
The "Pub" in Pub/Sub literally stands for publisher and the "Sub" stands for subscriber. That sounds obvious, but on the exam I have seen answer choices written so that you need to disambiguate which side of the system is being described. Get the vocabulary down and these questions get easier.
The reason this decoupling matters is that publishers and subscribers can scale independently. You can add five more publishers without touching any subscriber code. You can spin up additional subscribers without notifying any publisher. That is the whole point of using Pub/Sub instead of having services call each other directly.
A topic is a named resource that publishers send their messages to. Think of it as a channel or a category. Publishers do not address messages to a specific subscriber. They publish to a topic, and the topic is responsible for fanning the message out.
A subscription is also a named resource, but it represents the stream of messages flowing from a topic to a subscriber. Creating a subscription is how a consumer says "I want every message that gets published to this topic."
This is the part that catches people on the Professional Cloud Architect exam. The topic and the subscription are both named resources, but they sit on opposite sides of the system. The topic is what publishers write to. The subscription is what subscribers read from. They are linked, but they are not the same object.
Picture three publishers all sending messages to a single topic. Now picture four subscriptions attached to that same topic, each one feeding a different downstream subscriber. Every subscription gets its own copy of every message. It does not matter how many subscriptions you create. Each one receives every message published to the topic.
That fan-out behavior is the property the exam tends to test. If a question asks how to deliver the same stream of events to four independent processing pipelines, the answer is one topic with four subscriptions. You do not need four topics. You do not need to publish four times. You publish once, and each subscription independently receives the message.
The same idea works in reverse. Multiple publishers can write to one topic without any coordination between them. The topic serves as the meeting point, and neither side needs to know how many entities are on the other side.
Pub/Sub questions on the Professional Cloud Architect exam are usually about choosing the right messaging pattern, not about API mechanics. If you understand that publishers and subscribers are decoupled through topics and subscriptions, and that each subscription gets its own copy of each message, you can reason through almost any scenario question that involves event-driven architecture, log ingestion, or microservice communication on Google Cloud.
My Professional Cloud Architect course covers Pub/Sub publishers, subscribers, topics, and subscriptions alongside the rest of the messaging and pipelines material.
