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.

Practice

1. What is the main purpose of publishing messages in Google Cloud Pub/Sub?

easy

A. To send data to a topic so other services can receive it

B. To store data permanently in a database

C. To create virtual machines automatically

D. To monitor network traffic in real-time

Solution

1. Step 1: Understand the role of publishing in Pub/Sub

   Publishing messages means sending data to a topic so subscribers can get it.

2. Step 2: Compare with other options

   Storing data, creating VMs, or monitoring traffic are unrelated to publishing messages.

3. Final Answer:

   To send data to a topic so other services can receive it -> Option A

4. Quick Check:

   Publishing = sending data to topic [OK]

Hint: Publishing means sending messages to topics for others [OK]

Common Mistakes:

• Confusing publishing with storing data
• Thinking publishing creates resources like VMs
• Mixing publishing with monitoring tasks

2. Which of the following is the correct way to publish a message using the Google Cloud Pub/Sub Python client?

easy

A. publisher.send_message(topic_path, data="Hello World")

B. publisher.publish(topic_path, data=b"Hello World")

C. publisher.publish(topic_path, message="Hello World")

D. publisher.send(topic_path, "Hello World")

Solution

1. Step 1: Recall the correct method and parameters

   The correct method is publish(), and the message data must be bytes, so data=b"Hello World".

2. Step 2: Check other options for errors

   send() and send_message() are not valid methods; also, data must be bytes, not string.

3. Final Answer:

   publisher.publish(topic_path, data=b"Hello World") -> Option B

4. Quick Check:

   Use publish() with bytes data [OK]

Hint: Use publish() method with bytes data to send messages [OK]

Common Mistakes:

• Passing string instead of bytes as data
• Using incorrect method names like send()
• Using wrong parameter names like message=

3. Given the following Python code snippet, what will be printed?

from google.cloud import pubsub_v1
publisher = pubsub_v1.PublisherClient()
topic_path = publisher.topic_path('my-project', 'my-topic')
future = publisher.publish(topic_path, b'Message 1')
print(future.result())

medium

A. The content of the message 'Message 1'

B. The topic path string

C. An error because publish() returns None

D. The message ID of the published message

Solution

1. Step 1: Understand what publish() returns

   publish() returns a future that resolves to the message ID after publishing.

2. Step 2: Understand future.result() behavior

   Calling future.result() waits for publishing to complete and returns the message ID string.

3. Final Answer:

   The message ID of the published message -> Option D

4. Quick Check:

   future.result() = message ID [OK]

Hint: future.result() returns message ID after publish completes [OK]

Common Mistakes:

• Expecting future.result() to return message content
• Thinking publish() returns None
• Confusing topic path with message ID

4. You wrote this code to publish a message but get a TypeError: 'str' object cannot be interpreted as bytes. What is the mistake?

publisher.publish(topic_path, 'Hello World')

medium

A. The message data must be bytes, not a string

B. The topic_path variable is not defined

C. publish() method does not exist

D. You must call future.result() after publish()

Solution

1. Step 1: Identify the error cause

   The error says a string was given where bytes are expected.

2. Step 2: Fix the data type for message

   Message data must be bytes, so prefix string with b like b'Hello World'.

3. Final Answer:

   The message data must be bytes, not a string -> Option A

4. Quick Check:

   Message data = bytes, not string [OK]

Hint: Always prefix message with b to make bytes [OK]

Common Mistakes:

• Passing string instead of bytes as message data
• Ignoring error message details
• Assuming publish() method is missing

5. You want to publish multiple messages to a topic and ensure all are delivered before your program exits. Which approach is best?

hard

A. Publish messages using a loop but do not wait for any future results

B. Publish all messages and ignore the futures, then exit immediately

C. Publish each message and call future.result() immediately to wait for delivery

D. Publish messages and call future.cancel() to stop waiting

Solution

1. Step 1: Understand message delivery confirmation

   Calling future.result() waits for the message to be confirmed as delivered.

2. Step 2: Ensure program waits for all messages

   Waiting on each future ensures no message is lost before program exits.

3. Step 3: Evaluate other options

   Ignoring futures or cancelling them risks losing messages; not waiting is unsafe.

4. Final Answer:

   Publish each message and call future.result() immediately to wait for delivery -> Option C

5. Quick Check:

   Wait on future.result() to confirm delivery [OK]

Hint: Wait on future.result() for each message to confirm delivery [OK]

Common Mistakes:

• Ignoring futures and exiting too soon
• Cancelling futures instead of waiting
• Assuming publish() is synchronous