Use Pub/Sub to send and receive real-time messages
Use Pub/Sub to send and receive real-time messages
Introduction
Pub/Sub is a fully-managed real-time messaging service that allows you to
send and receive messages between independent applications. This tutorial gives a brief
introduction to the command-line interface for Pub/Sub, using the gcloud
command-line tool.
Project setup
Google Cloud organizes resources into projects. This allows you to collect all of the related resources for a single application in one place.
Begin by creating a new project or selecting an existing project for this tutorial.
For details, see Creating a project.
Open Cloud Shell
In this tutorial, you do much of your work in Cloud Shell, which is a built-in command-line tool for the Cloud Console.
Open Cloud Shell by clicking the
Create your first topic
A topic is a named resource to which you send messages. Create your first topic with the following command:
gcloud pubsub topics create my-topic
Add a subscription
To receive messages, you need to create subscriptions. A subscription needs to have a corresponding topic. Create your first subscription with the following command:
gcloud pubsub subscriptions \
create my-sub --topic my-topic \
--ack-deadline=60
This command creates a subscription named my-sub attached to the topic
my-topic. All of the messages published to my-topic will be delivered to this
subscription.
The ack-deadline option sets an acknowledgement deadline of 60 seconds
for this subscription. This deadline is explained in more detail later.
List topics and subscriptions
Before sending your first message, check whether the topic and the subscription have been successfully created.
List your topic and subscription using the following commands:
gcloud pubsub topics list
gcloud pubsub subscriptions list
Publish messages to the topic
Send two messages with the following commands:
gcloud pubsub topics publish my-topic --message hello
gcloud pubsub topics publish my-topic --message goodbye
Each of these commands sends a message. The first message is hello and the
second is goodbye. When you successfully publish a message, you should see the
messageId returned from the server. This is a unique ID automatically assigned
by the server to each message.
Pull messages from the subscription
Now, pull the messages with the following command:
gcloud pubsub subscriptions \
pull --auto-ack --limit=2 my-sub
This should return the two messages that you have just published. The messages have
the data, hello and goodbye, as well as MESSAGE_ID. The MESSAGE_ID is a
unique ID of the message that the server assigned.
Pub/Sub doesn't guarantee the order of the messages. It is also possible that only one message was returned; in that case, run the same command again until you see the other message.
Acknowledging messages
After you pull a message and process it, you must notify Pub/Sub that you successfully received the message. This action is called acknowledgement.
If you do not acknowledge the message before the acknowledgement deadline has passed, Pub/Sub will re-send the message.
The --auto-ack flag passed with the pull command automatically acknowledges
a message when it is pulled.
Manual acknowledgement
Send a new message
Send a new message with the following command:
gcloud pubsub \
topics publish my-topic --message thanks
Pull messages again
Pull the messages with this command:
gcloud pubsub subscriptions \
pull my-sub
This should display the thanks message, as well as MESSAGE_ID and ACK_ID.
The ACK_ID is an ID that you can use for acknowledging the message. Copy
the ACK_ID value, which you will paste into an acknowledgement in the next
step.
Acknowledge the message
Acknowledge the message with the following command, replacing [ACK_ID] with
the ID that you copied in the previous step:
gcloud pubsub subscriptions ack \
my-sub --ack-ids [ACK_ID]
See the topic and the subscription in the GCP Console
This concludes the gcloud command-line tutorial, but let's look at the web
interface for the Cloud Console before finishing the tutorial.
Open the Navigation menu in the upper-left corner of the console, and select Pub/Sub.
You can use this web interface to create and manage topics and subscriptions.
Clean up
Now that you have completed the tutorial, you should delete the resources that you created for the tutorial.
Select the checkbox next to the topic that you created, and click the Delete button to permanently delete the topic.
Conclusion
Congratulations!
You have just walked through the basic concepts of Pub/Sub. The next step is to create your awesome applications!
For more information, see the Pub/Sub documentation and explore our code samples.
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