Message ordering in GCP - Commands & Configuration

Introduction

When sending messages in a system, sometimes the order matters. Message ordering ensures that messages are received in the same order they were sent. This helps keep data consistent and predictable.

When you want to process user actions in the exact order they happened, like clicks or purchases.

When updating inventory counts where the sequence of changes affects the final number.

When handling chat messages so they appear in the right order to all users.

When processing financial transactions where order affects balances.

When syncing data between systems that depend on the order of updates.

Config File - pubsub-topic.yaml

pubsub-topic.yaml

apiVersion: pubsub.cnrm.cloud.google.com/v1beta1
kind: PubSubTopic
metadata:
  name: ordered-topic
spec:
  messageOrdering: true

This file creates a Google Cloud Pub/Sub topic with message ordering enabled. The messageOrdering: true setting ensures messages published to this topic keep their order.

Commands

This command creates a Pub/Sub topic named 'ordered-topic' with message ordering enabled so messages keep their order.

Terminal

gcloud pubsub topics create ordered-topic --message-ordering

Expected Output

Created topic [projects/your-project/topics/ordered-topic].

→

--message-ordering - Enables message ordering on the topic.

This command creates a subscription to the 'ordered-topic' with message ordering enabled to receive messages in order.

Terminal

gcloud pubsub subscriptions create ordered-subscription --topic=ordered-topic --enable-message-ordering

Expected Output

Created subscription [projects/your-project/subscriptions/ordered-subscription].

→

--enable-message-ordering - Ensures the subscription respects message ordering.

This command publishes the first message with an ordering key to keep it in sequence with related messages.

Terminal

gcloud pubsub topics publish ordered-topic --ordering-key=order-key --message='First message'

Expected Output

messageIds: ["1234567890"]

→

--ordering-key - Specifies the key to order messages by.

This command publishes the second message with the same ordering key to keep it after the first message.

Terminal

gcloud pubsub topics publish ordered-topic --ordering-key=order-key --message='Second message'

Expected Output

messageIds: ["1234567891"]

→

--ordering-key - Keeps messages with the same key in order.

This command pulls two messages from the subscription, showing they arrive in the order sent.

Terminal

gcloud pubsub subscriptions pull ordered-subscription --auto-ack --limit=2

Expected Output

ACKNOWLEDGED 2 messages.

→

--auto-ack - Automatically acknowledges messages after pulling.

→

--limit - Limits the number of messages pulled.

Key Concept

If you remember nothing else from this pattern, remember: message ordering requires enabling it on both the topic and subscription and using the same ordering key when publishing messages.

Common Mistakes

Not enabling message ordering on the subscription after enabling it on the topic.

Messages may arrive out of order because the subscription does not enforce ordering.

Always enable message ordering on both the topic and the subscription.

Publishing messages without specifying the same ordering key.

Messages with different or missing ordering keys are not ordered relative to each other.

Use the same ordering key for related messages to keep their order.

Not acknowledging messages in order or quickly enough.

Pub/Sub holds back messages with the same ordering key until previous messages are acknowledged, causing delays.

Acknowledge messages promptly and in the order received to maintain flow.

Summary

Create a Pub/Sub topic with message ordering enabled using the --message-ordering flag.

Create a subscription with message ordering enabled using the --enable-message-ordering flag.

Publish messages with the same ordering key to keep them in sequence.

Pull messages from the subscription to receive them in the order sent.

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