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Google Cloud Pub/Sub is a fully managed, real-time messaging service that allows you to send and receive messages between independent applications. It decouples services that produce events from services that process events, enabling reliable, scalable, and asynchronous communication across your architecture.
Pub/Sub stands for publish-subscribe. It is a messaging pattern where message producers (publishers) send messages to a named resource called a topic, and message consumers (subscribers) receive messages from subscriptions attached to that topic. The publisher does not know which subscribers exist, and subscribers do not know which publisher sent the message.
| Characteristic | Description |
|---|---|
| Fully managed | No servers to provision, patch, or scale — Google handles everything |
| Global | A single topic can span multiple regions with automatic replication |
| At-least-once delivery | Every message is delivered to every subscription at least once |
| Real-time | Messages are typically delivered within milliseconds |
| Serverless | Pay only for what you use — no idle costs beyond storage |
| Durable | Messages are persisted to multiple zones for durability |
In modern cloud-native architectures, services must communicate without tight coupling. Synchronous REST calls create dependencies that reduce resilience and scalability. Pub/Sub solves this by introducing an intermediary message broker.
Decoupling — Publishers and subscribers operate independently. You can add new subscribers without modifying the publisher. If a subscriber goes offline, messages accumulate and are delivered when it comes back.
Scalability — Pub/Sub automatically scales to handle millions of messages per second. You do not need to provision capacity in advance or manage throughput units.
Reliability — Messages are stored redundantly across multiple availability zones. Pub/Sub guarantees at-least-once delivery, and with ordering keys, it can guarantee ordered delivery within a key.
Fan-out — A single published message can be delivered to many subscribers simultaneously. This is ideal for event-driven architectures where multiple services need to react to the same event.
Load levelling — When traffic spikes, Pub/Sub absorbs the burst. Subscribers process messages at their own pace, preventing downstream services from being overwhelmed.
A topic is a named resource to which publishers send messages. Think of it as a broadcast channel. A topic can have zero or more subscriptions. When a message is published to a topic, Pub/Sub automatically delivers a copy of that message to every subscription.
A subscription is a named resource that represents the stream of messages from a single topic to a specific subscriber. Each subscription receives a copy of every message published to its topic. A subscription can be pull-based (the subscriber polls for messages) or push-based (Pub/Sub sends messages to an endpoint).
A message is the unit of data sent through Pub/Sub. It consists of:
| Component | Description |
|---|---|
| Data | The message body — up to 10 MB of binary or text data |
| Attributes | Key-value pairs of metadata (up to 100 attributes) |
| Message ID | A unique identifier assigned by Pub/Sub |
| Publish time | Timestamp when the message was accepted by Pub/Sub |
| Ordering key | Optional key for ordered delivery within a group |
When a subscriber receives a message, it must acknowledge (ack) it to tell Pub/Sub that processing was successful. Until a message is acknowledged, Pub/Sub considers it undelivered and will redeliver it after the acknowledgement deadline expires.
If the subscriber does not acknowledge within the deadline, Pub/Sub redelivers the message. This ensures no message is lost, even if a subscriber crashes mid-processing.
# Create a topic
gcloud pubsub topics create my-topic
# List all topics in the project
gcloud pubsub topics list
# Create a pull subscription
gcloud pubsub subscriptions create my-sub --topic=my-topic
# Create a push subscription
gcloud pubsub subscriptions create my-push-sub \
--topic=my-topic \
--push-endpoint=https://example.com/push-handler
# Publish a simple text message
gcloud pubsub topics publish my-topic --message="Hello, Pub/Sub!"
# Publish with attributes
gcloud pubsub topics publish my-topic \
--message="Order created" \
--attribute="orderId=12345,priority=high"
# Pull messages (auto-acknowledge)
gcloud pubsub subscriptions pull my-sub --auto-ack --limit=10
Pub/Sub integrates deeply with other Google Cloud services:
| Integration | Description |
|---|---|
| Cloud Functions | Trigger a function on every message |
| Cloud Run | Push messages to a containerised HTTP service |
| Dataflow | Stream messages into Apache Beam pipelines |
| BigQuery | Write messages directly to BigQuery tables via subscriptions |
| Cloud Storage | Export messages to GCS buckets for archival |
| Cloud Logging | Route log entries through Pub/Sub |
| Eventarc | Unified eventing that uses Pub/Sub as the transport layer |
Pub/Sub pricing is based on the volume of data published and delivered:
| Component | Cost Basis |
|---|---|
| Message ingestion | Per TiB of data published |
| Message delivery | Per TiB of data delivered to subscriptions |
| Seek and snapshot | Per TiB of retained acknowledged message backlog |
| Storage | Per GiB-month of retained messages beyond 7 days |
The first 10 GiB per month is free, making Pub/Sub cost-effective for development and small workloads.
Google Cloud Pub/Sub is a fully managed, globally distributed messaging service that decouples publishers from subscribers. It provides at-least-once delivery, automatic scaling, and seamless integration with the broader GCP ecosystem. Understanding the core concepts — topics, subscriptions, messages, and acknowledgements — is the foundation for building reliable event-driven architectures on Google Cloud. In the following lessons, we will dive deeper into topics, subscriptions, delivery modes, and advanced features.