This article covers the difference between Pub/Sub push and pull subscriptions, what each one is good at, the requirements for setting up a push subscription, and how the Associate Cloud Engineer exam tests this distinction.
It does not cover the deeper Pub/Sub topics like ordered delivery, message retention configuration, dead-letter queues, or the differences between Pub/Sub and Pub/Sub Lite. Those exist but the ACE exam coverage is narrower.
In Pub/Sub, a publisher sends messages to a topic. A subscriber receives those messages. The subscription is the link between the topic and the subscriber. The subscription type determines how messages get from Pub/Sub to the subscriber. There are two options. Pull and push.
With a pull subscription, the subscriber actively asks Pub/Sub for messages. The subscriber sends a request, gets back a batch of messages, processes them, and then acknowledges them so Pub/Sub knows they were handled successfully.
The control sits with the subscriber. If the subscriber is slow or temporarily unavailable, no problem. Messages stay in Pub/Sub until the subscriber comes back and pulls them. The subscriber pulls at whatever rate it can handle, which means it never gets overwhelmed.
Pull is the right choice for high-volume batch processing where throughput matters more than latency. A worker that processes a million messages per hour will use a pull subscription so it can drain messages in batches at its own pace.
With a push subscription, Pub/Sub actively sends messages to the subscriber as soon as they arrive. The subscriber does not ask. Pub/Sub just delivers.
The control sits with Pub/Sub. As soon as a message lands in a topic, Pub/Sub forwards it to the subscriber. If the subscriber is fast enough, latency is very low - the message reaches the subscriber within milliseconds of being published.
The catch is that the subscriber has to be ready to receive whatever Pub/Sub sends. Specifically, the subscriber must be a webhook endpoint that accepts POST over HTTPS. Pub/Sub does the HTTP call. The subscriber's job is to accept it, process the message, and respond with an HTTP 2xx to acknowledge.
Push is the right choice for low-latency, real-time streaming use cases where you want messages delivered as soon as they are published. A function that triggers on every event in real time will use a push subscription so it never has to poll.
This is a detail the Associate Cloud Engineer exam tests directly. To use a push subscription, the subscriber has to be reachable as an HTTPS POST endpoint. That is a real constraint. If your subscriber cannot accept HTTPS POST requests, you cannot use push.
In practice, Cloud Run services, Cloud Functions HTTP triggers, and App Engine endpoints all qualify. They all accept HTTPS POST and they all integrate cleanly with push subscriptions. A worker process running on Compute Engine that does not expose an HTTPS endpoint cannot be a push subscriber, but it can be a pull subscriber.
Pull is better when. You have high message volume and want to process in batches. The subscriber's processing rate varies and you want backpressure to be natural. The subscriber does not have an HTTPS endpoint. You want maximum throughput, and a small amount of polling latency is acceptable.
Push is better when. You need the lowest possible latency between publish and process. The subscriber is already an HTTPS service. The message rate is moderate. You do not want to manage polling logic.
The exam patterns are consistent.
If a scenario describes a high-volume batch processing system that consumes messages from Pub/Sub, the answer is a pull subscription.
If a scenario describes a real-time, low-latency delivery requirement where messages should hit the subscriber immediately, the answer is a push subscription.
If a scenario describes a Cloud Run service or webhook endpoint as the subscriber, that is a strong push signal because Cloud Run is naturally an HTTPS endpoint.
If you see in the question batch delivery, large message volume, or backend worker processing, think pull. If you see real-time, low-latency, webhook, or HTTPS endpoint, think push.
The exam may also test the requirement directly: which subscription type requires the subscriber to accept HTTPS POST. That is push.
Pull subscriptions let the subscriber pull messages at its own pace. Best for batch and high-volume use cases. Push subscriptions have Pub/Sub send messages directly to a webhook endpoint over HTTPS POST. Best for low-latency, real-time use cases.
For the Associate Cloud Engineer exam, recognize the latency-versus-throughput trade-off and the HTTPS POST requirement for push. Those two ideas cover most of what gets tested.
My Associate Cloud Engineer course covers push and pull subscriptions in the messaging section alongside the broader Pub/Sub topics that the ACE exam tests.
