Publishing messages in GCP - Time & Space Complexity

Time Complexity: Publishing messages

O(n)

Understanding Time Complexity

When sending messages to a cloud messaging service, it is important to understand how the time taken grows as we send more messages.

We want to know how the number of messages affects the total time and operations involved.

Scenario Under Consideration

Analyze the time complexity of the following operation sequence.

// Publish multiple messages to a Pub/Sub topic
for (int i = 0; i < n; i++) {
  publisher.publish(PubsubMessage.newBuilder()
    .setData(ByteString.copyFromUtf8("Message " + i))
    .build());
}
    

This code sends n messages one by one to a Pub/Sub topic.

Identify Repeating Operations

Identify the API calls, resource provisioning, data transfers that repeat.

• Primary operation: The publish API call to send a single message.
• How many times: This call happens once for each message, so n times.

How Execution Grows With Input

Each message requires one API call, so as the number of messages grows, the total calls grow at the same rate.

Input Size (n)	Approx. API Calls/Operations
10	10 publish calls
100	100 publish calls
1000	1000 publish calls

Pattern observation: The number of API calls grows directly in proportion to the number of messages.

Final Time Complexity

Time Complexity: O(n)

This means the time to publish messages grows linearly with the number of messages sent.

Common Mistake

[X] Wrong: "Publishing multiple messages at once takes the same time as publishing one message."

[OK] Correct: Each message requires its own API call, so more messages mean more calls and more time.

Interview Connect

Understanding how message publishing scales helps you design efficient cloud systems and answer questions about performance growth confidently.

Self-Check

"What if we batch multiple messages into a single publish call? How would the time complexity change?"

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