What is Google Cloud Pub/Sub?

Pub/Sub is a fully managed messaging service that lets applications publish messages to topics and have them delivered asynchronously to one or more subscriptions.

Does Pub/Sub guarantee message ordering?

By default no, but you can enable ordering keys on a subscription so messages sharing the same key are delivered in the order they were published.

What's the difference between push and pull subscriptions?

Push subscriptions have Pub/Sub call a webhook endpoint with each message, while pull subscriptions require the subscriber to actively poll and acknowledge messages.

How long does Pub/Sub retain messages?

By default messages are retained for 7 days after publication or until acknowledged, whichever comes first, and retention can be extended for replay scenarios.

Is Pub/Sub exactly-once or at-least-once?

Pub/Sub guarantees at-least-once delivery by default, with an exactly-once delivery mode available for subscriptions that need stricter processing guarantees.

How does Pub/Sub compare to Kafka?

Pub/Sub is a fully managed, serverless service with less operational overhead, while Kafka offers finer-grained control, on-premises deployment, and long-term log storage typically run and scaled by the user.

Can Pub/Sub trigger Cloud Functions?

Yes, Cloud Functions and Cloud Run both support Pub/Sub triggers, making it a common way to build serverless, event-driven pipelines on Google Cloud.

Cloud

Google Cloud Pub/Sub

By Google Cloud

IntermediateService10K learners

Google Cloud Pub/Sub is a fully managed, globally distributed messaging service that lets applications publish and subscribe to asynchronous event streams at scale.

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Definition

Google Cloud Pub/Sub is a fully managed, globally distributed messaging service that lets applications publish and subscribe to asynchronous event streams at scale.

Overview

Pub/Sub implements the classic publish-subscribe pattern as a serverless, globally available service: publishers send messages to a named topic without knowing who, if anyone, will consume them, and one or more subscriptions independently receive a copy of every message published to that topic. This decoupling lets producers and consumers scale, deploy, and fail independently, which is central to building resilient, loosely coupled distributed systems. Each subscription can use either push delivery, where Pub/Sub calls a webhook endpoint, or pull delivery, where a subscriber polls for messages, acknowledging each one once processed. Unacknowledged messages are redelivered, and Pub/Sub guarantees at-least-once delivery; ordering keys and exactly-once delivery modes are available for workloads that need stricter guarantees. Message retention allows subscribers to replay a backlog for a configurable window, which is useful for recovering from downstream outages. Pub/Sub is a foundational building block across Google Cloud, feeding data pipelines built with Dataflow, triggering Cloud Functions and Cloud Run services, and streaming events into BigQuery for analytics. Its design goals overlap with Apache Kafka and Azure Event Grid, but as a fully managed service it removes the operational burden of running and scaling brokers, at the cost of some of the fine-grained control a self-managed Kafka cluster offers.

Key Features

Fully managed, serverless publish-subscribe messaging with automatic scaling
At-least-once delivery guarantees with optional exactly-once processing
Both push (webhook) and pull (polling) subscription delivery models
Message ordering keys for use cases that require sequential processing
Configurable message retention for replaying a backlog after outages
Global availability with low-latency delivery across regions
Native integration with Dataflow, Cloud Functions, Cloud Run, and BigQuery
Dead-letter topics for isolating messages that repeatedly fail processing

Use Cases

Decoupling microservices through asynchronous event-driven communication

Streaming ingestion pipelines feeding analytics platforms like BigQuery

Triggering serverless functions in response to application events

Distributing real-time notifications to many independent subscribers

Buffering bursty workloads to smooth traffic spikes for downstream systems

Building event sourcing and CQRS architectures

Replicating data changes between services and databases

Alternatives

Azure Event Grid · Microsoft Azure Apache Kafka · Apache Software FoundationAmazon SNS/SQS · AWS

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