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.