Publishing messages in GCP - Commands & Configuration

Introduction

Sometimes you want to send information from one part of your system to another without them being directly connected. Publishing messages lets you send these pieces of information to a shared space where others can pick them up when ready.

When you want to send notifications from a web app to multiple services without waiting for each to respond.

When you need to log events from different parts of your system in a central place.

When you want to trigger background tasks after a user action without slowing down the user experience.

When you want to decouple parts of your system so they can work independently.

When you want to broadcast updates to many subscribers at once.

Commands

This command creates a new topic named 'example-topic' where messages can be published.

Terminal

gcloud pubsub topics create example-topic

Expected Output

Created topic [projects/your-project-id/topics/example-topic].

This command sends the message 'Hello, world!' to the 'example-topic' topic.

Terminal

gcloud pubsub topics publish example-topic --message="Hello, world!"

Expected Output

messageIds:

- "1234567890123456"

→

--message - Specifies the message content to publish.

This command creates a subscription named 'example-subscription' to receive messages from 'example-topic'.

Terminal

gcloud pubsub subscriptions create example-subscription --topic=example-topic

Expected Output

Created subscription [projects/your-project-id/subscriptions/example-subscription].

→

--topic - Specifies the topic to subscribe to.

This command pulls one message from 'example-subscription' and acknowledges it automatically.

Terminal

gcloud pubsub subscriptions pull example-subscription --auto-ack --limit=1

Expected Output

receivedMessages:

- ackId: "ACK_ID"

  message:

    data: "SGVsbG8sIHdvcmxkIQ=="

    messageId: "1234567890123456"

    publishTime: "2024-06-01T12:00:00.000Z"

→

--auto-ack - Automatically acknowledges the message after pulling.

→

--limit - Limits the number of messages to pull.

Key Concept

If you remember nothing else from this pattern, remember: publishing messages lets different parts of your system talk to each other without waiting or being directly connected.

Common Mistakes

Trying to publish a message to a topic that does not exist.

The command fails because the topic must exist before you can send messages to it.

Always create the topic first using 'gcloud pubsub topics create' before publishing messages.

Not creating a subscription before trying to pull messages.

Without a subscription, there is no way to receive messages from the topic.

Create a subscription with 'gcloud pubsub subscriptions create' linked to the topic before pulling messages.

Forgetting to acknowledge messages after pulling them.

Messages not acknowledged will be redelivered, causing duplicates.

Use the '--auto-ack' flag or manually acknowledge messages after processing.

Summary

Create a topic to hold messages using 'gcloud pubsub topics create'.

Publish messages to the topic with 'gcloud pubsub topics publish'.

Create a subscription to receive messages from the topic.

Pull and acknowledge messages from the subscription to process them.