Publishing messages in GCP - Mini Project: Build & Apply

Publishing messages

📖 Scenario: You are working on a cloud project where you need to send messages to a messaging service for other parts of your application to process asynchronously.Google Cloud Pub/Sub is a service that lets you publish messages to a topic. Other services can subscribe to this topic to receive messages.

🎯 Goal: Create a simple Google Cloud Pub/Sub publisher that sends messages to a topic.

📋 What You'll Learn

Create a variable called project_id with the exact value "my-gcp-project".

Create a variable called topic_id with the exact value "my-topic".

Create a Pub/Sub client object called publisher.

Publish a message with the exact text "Hello, Pub/Sub!" to the topic.

Add a line to confirm the message was published by storing the message ID in a variable called message_id.

💡 Why This Matters

🌍 Real World

Publishing messages to a Pub/Sub topic is common in cloud applications to decouple components and enable asynchronous processing.

💼 Career

Understanding how to publish messages to Pub/Sub is essential for cloud engineers and developers working with event-driven architectures on Google Cloud.

1

Set up project and topic variables

Create a variable called project_id and set it to "my-gcp-project". Then create a variable called topic_id and set it to "my-topic".

GCP

# Your code here

Hint

Use simple assignment to create the two variables with the exact names and values.

2

Create the Pub/Sub publisher client

Import the pubsub_v1 module from google.cloud. Then create a Pub/Sub publisher client object called publisher.

GCP

project_id = "my-gcp-project"
topic_id = "my-topic"
# Your code here

Hint

Use from google.cloud import pubsub_v1 and then create publisher = pubsub_v1.PublisherClient().

3

Create the full topic path and publish a message

Use publisher.topic_path(project_id, topic_id) to create a variable called topic_path. Then publish the message "Hello, Pub/Sub!" to topic_path using publisher.publish(). Store the returned future in a variable called future.

GCP

from google.cloud import pubsub_v1

project_id = "my-gcp-project"
topic_id = "my-topic"

publisher = pubsub_v1.PublisherClient()
# Your code here

Hint

Use publisher.topic_path() to get the full topic path string. Then publish the message as bytes with publisher.publish().

4

Get the message ID after publishing

Call future.result() and store the returned message ID string in a variable called message_id.

GCP

from google.cloud import pubsub_v1

project_id = "my-gcp-project"
topic_id = "my-topic"

publisher = pubsub_v1.PublisherClient()

topic_path = publisher.topic_path(project_id, topic_id)

future = publisher.publish(topic_path, b"Hello, Pub/Sub!")
# Your code here

Hint

Use future.result() to wait for the publish to complete and get the message ID.

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