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.

Prerequisites

  • Follow the instructions in "Hello, World!" for Ruby on App Engine to set up your environment and project, and to understand how App Engine Ruby 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/ruby-docs-samples
    cd ruby-docs-samples/appengine/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_APP_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-name>
        # This token is used to verify that requests originate from your
        # application. It can be any sufficiently random string.
        PUBSUB_VERIFICATION_TOKEN: <your-token>

    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:

    topic  = pubsub.topic ENV["PUBSUB_TOPIC"]
    PUBSUB_VERIFICATION_TOKEN = ENV["PUBSUB_VERIFICATION_TOKEN"]

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

    # List of all messages received by this instance
    messages = []

    This method receives pushed messages and adds them to the messages global list:

    post "/pubsub/push" do
      halt 400 if params[:token] != PUBSUB_VERIFICATION_TOKEN
    
      message = JSON.parse request.body.read
      payload = Base64.decode64 message["message"]["data"]
    
      messages.push payload
    end

    This method interacts with the App Engine web app to publish new messages and display received messages:

    get "/" do
      @messages = messages
    
      slim :index
    end
    
    post "/publish" do
      topic.publish params[:payload]
    
      redirect "/"
    end

    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:

    bundle install
    

    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]
    bundle exec ruby app.rb -p 8080
    

    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 --data @sample_message.json ":8080/pubsub/push?token=[your-token]"
    

    Or

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

    Response:

    HTTP/1.1 200 OK
    Content-Type: text/html;charset=utf-8
    Content-Length: 13
    X-Xss-Protection: 1; mode=block
    X-Content-Type-Options: nosniff
    X-Frame-Options: SAMEORIGIN
    Server: WEBrick/1.3.1 (Ruby/2.3.0/2015-12-25)
    Date: Wed, 20 Apr 2016 20:56:23 GMT
    Connection: Keep-Alive
    
    Hello, World!
    

    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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