Writing and Responding to Pub/Sub Messages

Google Cloud Pub/Sub provides reliable, many-to-many, asynchronous messaging between applications. Publisher applications can send messages to a topic, and other applications can subscribe to that topic to receive the messages.

This document describes how to use the Google Cloud Client Library to send and receive Google Cloud Pub/Sub messages in an app running in the flexible environment.

Prerequisites

  • Follow the instructions in "Hello, World!" for Python on App Engine to set up your environment and project, and to understand how App Engine Python apps are structured.
  • Write down and save your project ID, because you will need it to run the sample application described in this document.

    Clone the sample app

    Copy the sample apps to your local machine, and navigate to the pubsub directory:

    git clone https://github.com/GoogleCloudPlatform/python-docs-samples
    cd python-docs-samples/appengine/flexible/pubsub
    

    Create a topic and subscription

    Create a topic and subscription, which includes specifying the endpoint to which the Pub/Sub server should send requests:

    gcloud beta pubsub topics create YOUR_TOPIC_NAME
    gcloud beta pubsub subscriptions create YOUR_SUBSCRIPTION_NAME \
        --topic YOUR_TOPIC_NAME \
        --push-endpoint \
        https://YOUR_PROJECT_ID.appspot.com/pubsub/push?token=YOUR_TOKEN \
        --ack-deadline 10
    

    Replace YOUR_TOKEN with a secret random token. The push endpoint uses this to verify requests.

    Edit app.yaml

    Edit app.yaml to set the environment variables for your project ID, topic, and verification token:

    env_variables:
        PUBSUB_TOPIC: your-topic
        # This token is used to verify that requests originate from your
        # application. It can be any sufficiently random string.
        PUBSUB_VERIFICATION_TOKEN: 1234abc

    Code review

    The sample app uses the Google Cloud Client Libraries.

    The sample app uses the values you set in the app.yaml file to configure environment variables. The push request handler uses these values to confirm that the request came from Pub/Sub and originated from a trusted source:

    app.config['PUBSUB_VERIFICATION_TOKEN'] = \
        os.environ['PUBSUB_VERIFICATION_TOKEN']
    app.config['PUBSUB_TOPIC'] = os.environ['PUBSUB_TOPIC']
    

    The sample app maintains a global list to store messages received by this instance:

    MESSAGES = []
    

    The pubsub_push() method receives pushed messages and adds them to the MESSAGES global list:

    @app.route('/pubsub/push', methods=['POST'])
    def pubsub_push():
        if (request.args.get('token', '') !=
                current_app.config['PUBSUB_VERIFICATION_TOKEN']):
            return 'Invalid request', 400
    
        envelope = json.loads(request.data.decode('utf-8'))
        payload = base64.b64decode(envelope['message']['data'])
    
        MESSAGES.append(payload)
    
        # Returning any 2xx status indicates successful receipt of the message.
        return 'OK', 200

    The index() method interacts with the App Engine web app to publish new messages and display received messages:

    @app.route('/', methods=['GET', 'POST'])
    def index():
        if request.method == 'GET':
            return render_template('index.html', messages=MESSAGES)
    
        data = request.form.get('payload', 'Example payload').encode('utf-8')
    
        publisher = pubsub_v1.PublisherClient()
        topic_path = publisher.topic_path(
            current_app.config['PROJECT'],
            current_app.config['PUBSUB_TOPIC'])
    
        publisher.publish(topic_path, data=data)
    
        return 'OK', 200

    Run the sample locally

    When running locally, you can use the Google Cloud SDK to provide authentication to use Google Cloud APIs. Assuming you set up your environment as described in Prerequisites, you have already run the gcloud init command, which provides this authentication.

    Install dependencies, preferably with a virtualenv:

    virtualenv env
    source env/bin/activate
    pip install -r requirements.txt
    

    Then set environment variables before starting your application:

    export GCLOUD_PROJECT=[your-project-id]
    export PUBSUB_VERIFICATION_TOKEN=[your-verification-token]
    export PUBSUB_TOPIC=[your-topic]
    python main.py
    

    Simulate push notifications

    The application can send messages locally, but it is not able to receive push messages locally. You can, however, simulate a push message by making an HTTP request to the local push notification endpoint. The sample includes the file sample_message.json.

    You can use curl or httpie to send an HTTP POST request:

    curl -i -H "Content-Type: application/json" --data @sample_message.json "localhost:8080/pubsub/push?token=[your-token]"
    

    Or

    http POST ":8080/pubsub/push?token=[your-token]" < sample_message.json
    

    Response:

    HTTP/1.0 200 OK
    Content-Length: 2
    Content-Type: text/html; charset=utf-8
    Date: Mon, 10 Aug 2015 17:52:03 GMT
    Server: Werkzeug/0.10.4 Python/2.7.10
    
    OK
    

    After the request completes, you can refresh localhost:8080 and see the message in the list of received messages.

    Run on App Engine

    To deploy the demo app to App Engine by using the gcloud command-line tool, you run the following command from the directory where your app.yaml is located:

    gcloud app deploy
    

    You can now access the application at https://[YOUR_PROJECT_ID].appspot.com. You can use the form to submit messages, but there's no guarantee of which instance of your application will receive the notification. You can send multiple messages and refresh the page to see the received message.

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