Publishing messages in GCP - Step-by-Step Execution

Process Flow - Publishing messages

Create Topic

↓

Prepare Message

↓

Publish Message to Topic

↓

Message Stored in Topic

↓

Subscribers Receive Message

This flow shows how a message is created, published to a topic, stored, and then delivered to subscribers.

Execution Sample

GCP

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'My message data')
print(future.result())

This code publishes a message to a Google Cloud Pub/Sub topic and waits for confirmation.

Process Table

Step	Action	Input	Result	State Change
1	Create Topic Path	project='my-project', topic='my-topic'	topic_path='projects/my-project/topics/my-topic'	Topic path ready for publishing
2	Prepare Message	Message data='My message data'	Message bytes ready	Message encoded as bytes
3	Publish Message	topic_path, message bytes	Future object returned	Message sent to Pub/Sub service
4	Wait for Result	Future object	Message ID received	Message stored in topic, ready for delivery
5	Message Delivery	Subscribers listening	Subscribers receive message	Message delivered to subscribers

💡 Message published and confirmed with message ID; ready for subscribers.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
publisher	None	PublisherClient instance	PublisherClient instance	PublisherClient instance	PublisherClient instance	PublisherClient instance
topic_path	None	'projects/my-project/topics/my-topic'	'projects/my-project/topics/my-topic'	'projects/my-project/topics/my-topic'	'projects/my-project/topics/my-topic'	'projects/my-project/topics/my-topic'
message_data	None	None	b'My message data'	b'My message data'	b'My message data'	b'My message data'
future	None	None	None	Future object	Future object	Message ID string

Key Moments - 3 Insights

Why do we convert the message to bytes before publishing?

Pub/Sub requires messages as bytes to ensure consistent data format; see execution_table step 2 where message is encoded.

What does the Future object represent after publishing?

The Future object represents the asynchronous result of publishing; it confirms message acceptance when resolved, as shown in step 4.

Does publishing guarantee immediate delivery to subscribers?

Publishing confirms message storage in the topic (step 4), but delivery to subscribers happens asynchronously afterward (step 5).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'topic_path' after step 1?

A 'projects/my-project/topics/my-topic'

B 'my-topic'

C None

D 'my-project'

💡 Hint

Check the 'topic_path' variable in variable_tracker after step 1.

At which step does the message get encoded into bytes?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Action' and 'Result' columns in execution_table for message preparation.

If the Future object fails to resolve, what happens to the message?

A Message is confirmed published

B Message is lost or not stored

C Message is delivered immediately

D Message is duplicated

💡 Hint

Refer to step 4 in execution_table about waiting for the Future result.

Concept Snapshot

Publishing messages in GCP Pub/Sub:
- Create a topic path with project and topic names
- Prepare message data as bytes
- Publish message asynchronously
- Wait for confirmation via Future
- Message stored and delivered to subscribers asynchronously

Full Transcript

Publishing messages in Google Cloud Pub/Sub involves creating a topic path, preparing the message as bytes, publishing it asynchronously, and waiting for confirmation. Once confirmed, the message is stored in the topic and delivered to subscribers. The Future object returned by the publish method confirms successful storage. Messages must be bytes to ensure consistent format. Delivery to subscribers happens after publishing confirmation.

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