Writing and Responding to Pub/Sub Messages

Region ID

The REGION_ID is an abbreviated code that Google assigns based on the region you select when you create your app. The code does not correspond to a country or province, even though some region IDs may appear similar to commonly used country and province codes. For apps created after February 2020, REGION_ID.r is included in App Engine URLs. For existing apps created before this date, the region ID is optional in the URL.

Learn more about region IDs.

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 Cloud Client Library to send and receive Pub/Sub messages in a Java app.

Prerequisites

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

    Cloning the sample app

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

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

    Creating a topic and subscription

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

    gcloud pubsub topics create YOUR_TOPIC_NAME
    gcloud pubsub subscriptions create YOUR_SUBSCRIPTION_NAME \
        --topic YOUR_TOPIC_NAME \
        --push-endpoint \
        https://YOUR_PROJECT_ID.REGION_ID.r.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.

    Setting environment variables

    Edit the app.yaml file to set the environment variables for your 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-verification-token>

    Code review

    The sample app uses the Cloud Client Library.

    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:

    String pubsubVerificationToken = System.getenv("PUBSUB_VERIFICATION_TOKEN");
    

    The sample app maintains a Cloud Datastore database instance to store messages. The PubSubPush servlet receives pushed messages and adds them to the messageRepository database instance:

    @WebServlet(value = "/pubsub/push")
    public class PubSubPush extends HttpServlet {
    
      @Override
      public void doPost(HttpServletRequest req, HttpServletResponse resp)
          throws IOException, ServletException {
        String pubsubVerificationToken = System.getenv("PUBSUB_VERIFICATION_TOKEN");
        // Do not process message if request token does not match pubsubVerificationToken
        if (req.getParameter("token").compareTo(pubsubVerificationToken) != 0) {
          resp.setStatus(HttpServletResponse.SC_BAD_REQUEST);
          return;
        }
        // parse message object from "message" field in the request body json
        // decode message data from base64
        Message message = getMessage(req);
        try {
          messageRepository.save(message);
          // 200, 201, 204, 102 status codes are interpreted as success by the Pub/Sub system
          resp.setStatus(HttpServletResponse.SC_OK);
        } catch (Exception e) {
          resp.setStatus(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
        }
      }

    The PubSubPublish servlet interacts with the App Engine web app to publish new messages and display received messages:

    @WebServlet(name = "Publish with PubSub", value = "/pubsub/publish")
    public class PubSubPublish extends HttpServlet {
    
      @Override
      public void doPost(HttpServletRequest req, HttpServletResponse resp)
          throws IOException, ServletException {
        Publisher publisher = this.publisher;
        try {
          String topicId = System.getenv("PUBSUB_TOPIC");
          // create a publisher on the topic
          if (publisher == null) {
            publisher = Publisher.newBuilder(
                ProjectTopicName.of(ServiceOptions.getDefaultProjectId(), topicId))
                .build();
          }
          // construct a pubsub message from the payload
          final String payload = req.getParameter("payload");
          PubsubMessage pubsubMessage =
              PubsubMessage.newBuilder().setData(ByteString.copyFromUtf8(payload)).build();
    
          publisher.publish(pubsubMessage);
          // redirect to home page
          resp.sendRedirect("/");
        } catch (Exception e) {
          resp.sendError(HttpStatus.SC_INTERNAL_SERVER_ERROR, e.getMessage());
        }
      }

    Running the sample locally

    When running locally, you can use the 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.

    mvn clean package
    

    Then set environment variables before starting your application:

    export PUBSUB_VERIFICATION_TOKEN=[your-verification-token]
    export PUBSUB_TOPIC=[your-topic]
    mvn jetty:run
    

    Simulating 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 a httpie client to send an HTTP POST request:

    curl -H "Content-Type: application/json" -i --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/1.1 200 OK
    Date: Wed, 26 Apr 2017 00:03:28 GMT
    Content-Length: 0
    Server: Jetty(9.3.8.v20160314)
    

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

    Running 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 file is located:

    mvn package appengine:deploy -Dapp.deploy.projectId=PROJECT_ID

    Replace PROJECT_ID with the ID of your Cloud project. If your pom.xml file already specifies your project ID, you don't need to include the -Dapp.deploy.projectId property in the command you run.

    You can now access the application at https://PROJECT_ID.REGION_ID.r.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.