Pub/Sub: Qwik Start - Python - GSP094

Overview

The Pub/Sub service allows applications to exchange messages reliably, quickly, and asynchronously. To accomplish this, a data producer publishes messages to a Cloud Pub/Sub topic. A subscriber client then creates a subscription to that topic and consumes messages from the subscription. Cloud Pub/Sub persists messages that could not be delivered reliably for up to seven days.

In this lab, you will learn how to get started publishing messages with Pub/Sub using the Python client library.

What you'll do

In this lab, you'll do the following:

• Learn the basics of Pub/Sub

• Create, delete, and list Pub/Sub topics and subscriptions

• Publish messages to a topic.

• Use a pull subscriber to output individual topic messages.

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Task 1. Create a virtual environment

Python virtual environments are used to isolate package installation from the system.

1. Install the virtualenv environment:

sudo apt-get install -y virtualenv

2. Build the virtual environment:

python3 -m venv venv

3. Activate the virtual environment.

source venv/bin/activate

Task 2. Install the client library

1. Run the following to install the client library:

pip install --upgrade google-cloud-pubsub

2. Get the sample code by cloning a GitHub repository:

git clone https://github.com/googleapis/python-pubsub.git

3. Navigate to the directory:

cd python-pubsub/samples/snippets

Task 3. Pub/Sub - the Basics

Pub/Sub is an asynchronous global messaging service. There are three terms in Pub/Sub that appear often: topics, publishing, and subscribing.

A topic is a shared string that allows applications to connect with one another through a common thread.

Publishers push (or publish) a message to a Pub/Sub topic. Subscribers will then make a subscription to that thread, where they will either pull messages from the topic or configure webhooks for push subscriptions. Every subscriber must acknowledge each message within a configurable window of time.

In sum, a publisher creates and sends messages to a topic and a subscriber creates a subscription to a topic to receive messages from it.

Pub/Sub in Google CLoud

Pub/Sub comes preinstalled in Cloud Shell, so there are no installations or configurations required to get started with this service. In this lab you use Python to create the topic, subscriber, and then view the message. You use a gcloud command to publish the message to the topic.

Task 4. Create a topic

To publish data to Pub/Sub you create a topic and then configure a publisher to the topic.

1. In Cloud Shell, your Project ID should automatically be stored in the environment variable GOOGLE_CLOUD_PROJECT:

echo $GOOGLE_CLOUD_PROJECT

2. Ensure the output is the same as the Project ID in your CONNECTION DETAILS.

publisher.py is a script that demonstrates how to perform basic operations on topics with the Cloud Pub/Sub API. View the content of publisher script:

cat publisher.py

Note: Alternatively, you can use the shell editors that are installed on Cloud Shell, such as nano or vim or use the Cloud Shell code editor to view python-pubsub/samples/snippets/publisher.py.

3. For information about the publisher script:

python publisher.py -h

Example output:

usage: publisher.py [-h]
                    project
                    {list,create,delete,publish,publish-with-custom-attributes,publish-with-futures,publish-with-error-handler,publish-with-batch-settings}
                    ...

This application demonstrates how to perform basic operations on topics
with the Cloud Pub/Sub API.

For more information, see the README.md under /pubsub and the documentation
at https://cloud.google.com/pubsub/docs.

positional arguments:
  project               Your Google Cloud project ID
  {list,create,delete,publish,publish-with-custom-attributes,publish-with-futures,publish-with-error-handler,publish-with-batch-settings}
    list                Lists all Pub/Sub topics in the given project.
    create              Create a new Pub/Sub topic.
    delete              Deletes an existing Pub/Sub topic.
    publish             Publishes multiple messages to a Pub/Sub topic.
    publish-with-custom-attributes
                        Publishes multiple messages with custom attributes to
                        a Pub/Sub topic.
    publish-with-futures
                        Publishes multiple messages to a Pub/Sub topic and
                        prints their message IDs.
    publish-with-error-handler
                        Publishes multiple messages to a Pub/Sub topic with an
                        error handler.
    publish-with-batch-settings
                        Publishes multiple messages to a Pub/Sub topic with
                        batch settings.

optional arguments:
  -h, --help            show this help message and exit

4. Run the publisher script to create Pub/Sub Topic:

python publisher.py $GOOGLE_CLOUD_PROJECT create MyTopic

Example output:

Topic created: name: "projects/qwiklabs-gcp-fe27729bc161fb22/topics/MyTopic"

Test Completed Task

Click Check my progress to verify your performed task. If you have successfully created a Cloud Pub/Sub topic, you will see an assessment score.

Create a topic.

Check my progress

5. This command returns a list of all Pub/Sub topics in a given project:

python publisher.py $GOOGLE_CLOUD_PROJECT list

Example output:

name: "projects/qwiklabs-gcp-fe27729bc161fb22/topics/MyTopic"

You can also view the topic you just made in the Cloud Console.

6. Navigate to Navigation menu > Pub/Sub > Topics.

You should see MyTopic.

Task 5. Create a subscription

1. Create a Pub/Sub subscription for topic with subscriber.py script:

python subscriber.py $GOOGLE_CLOUD_PROJECT create MyTopic MySub

Test Completed Task

Click Check my progress to verify your performed task. If you have successfully created a Cloud Pub/Sub subscription, you will see an assessment score.

Create a subscription.

Check my progress

2. This command returns a list of subscribers in given project:

python subscriber.py $GOOGLE_CLOUD_PROJECT list-in-project

You'll see only one subscription because you've made only one subscription.

Example output:

projects/qwiklabs-gcp-7877af129f04d8b3/subscriptions/MySub

3. Check out the subscription you just made in the console. In the left pane, click Subscriptions. You should see the subscription name and other details.

4. For information about the subscriber script:

python subscriber.py -h

Output:

usage: subscriber.py [-h]
                     project
                     {list_in_topic,list_in_project,create,create-push,delete,update,receive,receive-custom-attributes,receive-flow-control,receive-synchronously,listen_for_errors}
                     ...

This application demonstrates how to perform basic operations on
subscriptions with the Cloud Pub/Sub API.

For more information, see the README.md under /pubsub and the documentation
at https://cloud.google.com/pubsub/docs.

positional arguments:
  project               Your Google Cloud project ID
  {list_in_topic,list_in_project,create,create-push,delete,update,receive,receive-custom-attributes,receive-flow-control,receive-synchronously,listen_for_errors}
    list_in_topic       Lists all subscriptions for a given topic.
    list_in_project     Lists all subscriptions in the current project.
    create              Create a new pull subscription on the given topic.
    create-push         Create a new push subscription on the given topic.
    delete              Deletes an existing Pub/Sub topic.
    update              Updates an existing Pub/Sub subscription's push
                        endpoint URL. Note that certain properties of a
                        subscription, such as its topic, are not modifiable.
    receive             Receives messages from a pull subscription.
    receive-custom-attributes
                        Receives messages from a pull subscription.
    receive-flow-control
                        Receives messages from a pull subscription with flow
                        control.
    receive-synchronously
                        Pulling messages synchronously.
    listen_for_errors   Receives messages and catches errors from a pull
                        subscription.

optional arguments:
  -h, --help            show this help message and exit

Task 6. Publish messages

Now that you've set up MyTopic (the topic) and a subscription to MyTopic (MySub), use gcloud commands to publish a message to MyTopic.

1. Publish the message "Hello" to MyTopic:

gcloud pubsub topics publish MyTopic --message "Hello"

2. Publish a few more messages to MyTopic—run the following commands (replacing <YOUR NAME> with your name and <FOOD> with a food you like to eat):

gcloud pubsub topics publish MyTopic --message "Publisher's name is <YOUR NAME>"

gcloud pubsub topics publish MyTopic --message "Publisher likes to eat <FOOD>"

gcloud pubsub topics publish MyTopic --message "Publisher thinks Pub/Sub is awesome"

Task 7. View messages

Now that you've published messages to MyTopic, pull and view the messages using MySub.

1. Use MySub to pull the message from MyTopic:

python subscriber.py $GOOGLE_CLOUD_PROJECT receive MySub

Example output:

Listening for messages on projects/qwiklabs-gcp-7877af129f04d8b3/subscriptions/MySub
Received message: Message {
  data: 'Publisher thinks Pub/Sub is awesome'
  attributes: {}
}
Received message: Message {
  data: 'Hello'
  attributes: {}
}
Received message: Message {
  data: "Publisher's name is Harry"
  attributes: {}
}
Received message: Message {
  data: 'Publisher likes to eat cheese'
  attributes: {}
}

2. Click Ctrl+c to stop listening.

Task 8. Test your understanding

Below are multiple-choice questions to reinforce your understanding of this lab's concepts. Answer them to the best of your abilities.

Google Cloud Pub/Sub service allows applications to exchange messages reliably, quickly, and asynchronously.

• True

• False

A _____ is a shared string that allows applications to connect with one another.

• topic

• message

• subscription

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Solution of Lab

curl -LO raw.githubusercontent.com/quiccklabs/Labs_solutions/master/PubSub%20Qwik%20Start%20Python/quicklabgsp094.sh
sudo chmod +x quicklabgsp094.sh
./quicklabgsp094.sh