Message ordering in GCP - Mini Project: Build & Apply

Message Ordering with Google Cloud Pub/Sub

📖 Scenario: You are building a messaging system where the order of messages is important. For example, a bank transaction system where deposits and withdrawals must be processed in the exact order they occur.

🎯 Goal: Set up a Google Cloud Pub/Sub topic and subscription that ensures messages are delivered in the order they were published.

📋 What You'll Learn

Create a Pub/Sub topic with message ordering enabled

Create a subscription to the topic with message ordering enabled

Publish messages with ordering keys

Ensure the subscription receives messages in the correct order

💡 Why This Matters

🌍 Real World

Message ordering is critical in systems like banking, inventory management, and event processing where the sequence of events affects the system state.

💼 Career

Understanding message ordering in Pub/Sub is essential for cloud engineers and developers building reliable, event-driven applications on Google Cloud.

1

Create a Pub/Sub topic with message ordering enabled

Create a Pub/Sub topic named bank-transactions with message ordering enabled by setting messageOrdering to true in the topic configuration.

GCP

# Create the Pub/Sub topic with message ordering enabled
# Your code here

Hint

Use the gcloud pubsub topics create command with the --message-ordering flag.

2

Create a subscription with message ordering enabled

Create a subscription named bank-transactions-sub to the bank-transactions topic with message ordering enabled by setting --enable-message-ordering flag.

GCP

gcloud pubsub topics create bank-transactions --message-ordering
# Create the subscription with message ordering enabled
# Your code here

Hint

Use the gcloud pubsub subscriptions create command with the --enable-message-ordering flag.

3

Publish messages with ordering keys

Publish two messages to the bank-transactions topic with the ordering key account-123. Use the --ordering-key=account-123 flag for both messages.

GCP

gcloud pubsub topics create bank-transactions --message-ordering
gcloud pubsub subscriptions create bank-transactions-sub --topic=bank-transactions --enable-message-ordering
# Publish first message with ordering key
# Your code here

Hint

Use the gcloud pubsub topics publish command with the --ordering-key flag for each message.

4

Configure subscription to acknowledge messages in order

Ensure the subscription bank-transactions-sub is configured to acknowledge messages in order by setting --enable-message-ordering and verify the subscription is ready to process ordered messages.

GCP

gcloud pubsub topics create bank-transactions --message-ordering
gcloud pubsub subscriptions create bank-transactions-sub --topic=bank-transactions --enable-message-ordering
gcloud pubsub topics publish bank-transactions --message="Deposit $100" --ordering-key=account-123
gcloud pubsub topics publish bank-transactions --message="Withdraw $50" --ordering-key=account-123
# Confirm subscription is configured for message ordering
# Your code here

Hint

Use gcloud pubsub subscriptions describe bank-transactions-sub to verify the subscription settings.

Practice

1.

What does message ordering ensure in a messaging system?

easy

A. Messages are duplicated for backup

B. Messages are received in the same order they were sent

C. Messages are encrypted during transmission

D. Messages are sent to multiple receivers simultaneously

Solution

1. Step 1: Understand message ordering concept

   Message ordering means messages keep the order they were sent in.

2. Step 2: Identify the correct behavior

   Only Messages are received in the same order they were sent describes messages arriving in the same order they were sent.

3. Final Answer:

   Messages are received in the same order they were sent -> Option B

4. Quick Check:

   Message ordering = same send and receive order [OK]

Hint: Ordering means messages keep their send order [OK]

Common Mistakes:

• Confusing ordering with encryption
• Thinking ordering means message duplication
• Assuming ordering means broadcasting

2.

Which of the following is the correct way to enable message ordering in Google Cloud Pub/Sub?

publisher.publish(topic, data, ...)

easy

A. Add ordering_key attribute with a string value

B. Set priority attribute to high

C. Use batch_size parameter

D. Enable encryption_key for the topic

Solution

1. Step 1: Recall how to enable ordering in Pub/Sub

   Ordering requires setting an ordering_key on messages to group them.

2. Step 2: Match options to ordering feature

   Only Add ordering_key attribute with a string value correctly uses ordering_key to enable ordering.

3. Final Answer:

   Add ordering_key attribute with a string value -> Option A

4. Quick Check:

   Ordering enabled by ordering_key [OK]

Hint: Use ordering_key to group messages for ordering [OK]

Common Mistakes:

• Confusing ordering_key with priority
• Thinking batch_size controls ordering
• Assuming encryption enables ordering

3.

Given the following code snippet, what will be the order of messages received if ordering_key='group1' is used for all messages?

publisher.publish(topic, b'Msg1', ordering_key='group1')
publisher.publish(topic, b'Msg2', ordering_key='group1')
publisher.publish(topic, b'Msg3', ordering_key='group1')

medium

A. Order is random

B. Msg3, Msg2, Msg1

C. Msg2, Msg1, Msg3

D. Msg1, Msg2, Msg3

Solution

1. Step 1: Understand ordering_key effect

   All messages share the same ordering_key 'group1', so they keep send order.

2. Step 2: Determine message order

   Messages will be received in the order published: Msg1, Msg2, Msg3.

3. Final Answer:

   Msg1, Msg2, Msg3 -> Option D

4. Quick Check:

   Same ordering_key means ordered delivery [OK]

Hint: Same ordering_key means messages keep send order [OK]

Common Mistakes:

• Assuming order reverses
• Thinking order is random without ordering_key
• Mixing ordering_key values

4.

What is the likely cause if messages with the same ordering_key arrive out of order in Google Cloud Pub/Sub?

medium

A. Ordering is not enabled on the subscription

B. Messages have different data payloads

C. The topic is encrypted

D. The publisher is using batching

Solution

1. Step 1: Identify ordering requirements

   Both publisher and subscriber must support ordering for it to work.

2. Step 2: Analyze cause of disorder

   If subscription does not enable ordering, messages may arrive out of order despite ordering_key.

3. Final Answer:

   Ordering is not enabled on the subscription -> Option A

4. Quick Check:

   Subscription must enable ordering [OK]

Hint: Enable ordering on both publisher and subscriber [OK]

Common Mistakes:

• Blaming message content for order
• Assuming encryption affects order
• Thinking batching breaks ordering

5.

You have two message groups with ordering_key='groupA' and ordering_key='groupB'. How does Google Cloud Pub/Sub handle message ordering across these groups?

hard

A. Messages from groupB are always delivered before groupA

B. All messages from both groups are merged and ordered globally

C. Messages within each group are ordered, but groups are independent

D. Ordering is ignored when multiple ordering keys are used

Solution

1. Step 1: Understand ordering_key grouping

   Ordering keys group messages that must be ordered together.

2. Step 2: Analyze cross-group ordering

   Messages in different ordering_key groups are independent and can be delivered in any order relative to each other.

3. Final Answer:

   Messages within each group are ordered, but groups are independent -> Option C

4. Quick Check:

   Ordering keys isolate message order groups [OK]

Hint: Ordering keys create separate ordered groups [OK]

Common Mistakes:

• Assuming global ordering across keys
• Thinking one group always precedes another
• Believing ordering breaks with multiple keys