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Writing and responding to Pub/Sub messages

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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 an App Engine app.

Prerequisites

Cloning the sample app

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

Go

git clone https://github.com/GoogleCloudPlatform/golang-samples.git
cd golang-samples/appengine_flexible/pubsub

Java

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

Node.js

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

PHP

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

Python

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

Ruby

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

.NET

git clone https://github.com/GoogleCloudPlatform/dotnet-docs-samples
cd dotnet-docs-samples/appengine/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:

Go

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.

Java

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.

Node.js

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.

PHP

gcloud pubsub topics create YOUR_TOPIC_NAME
gcloud pubsub subscriptions create YOUR_SUBSCRIPTION_NAME \
    --topic YOUR_TOPIC_NAME

Python

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.

Ruby

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.

.NET

gcloud pubsub topics create YOUR_TOPIC
gcloud pubsub subscriptions create YOUR_SUBSCRIPTION `
    --topic YOUR_TOPIC `
    --push-endpoint `
    https://YOUR_PROJECT_ID.REGION_ID.r.appspot.com/pubsub/push?token=YOUR_SECRET_TOKEN `
    --ack-deadline 10

Setting environment variables

Go

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

Java

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>

Node.js

Edit the app.flexible.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

PHP

Edit your index.php file to set the environment variables for your topic and subscription:

$container->set('topic', 'php-example-topic');
$container->set('subscription', 'php-example-subscription');

Python

Edit the app.yaml file 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

Ruby

Edit the app.yaml file 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>

.NET

Edit the appsettings.json file to set your project ID:

{
  "Pubsub": {
    "ProjectId": "your-project-id",
    "VerificationToken": "your-secret-token",
    "TopicId": "your-topic",
    "SubscriptionId": "your-subscription",
    "AuthSubscriptionId": "your-auth-subscription",
    "ServiceAccountEmail": "your-service-account-email"
  },
  
  "Logging": {
    "IncludeScopes": false,
    "LogLevel": {
      "Default": "Debug",
      "System": "Information",
      "Microsoft": "Information"
    }
  }
}

Code review

The sample app uses the Pub/Sub Client Library.

Go

The sample app uses the environment variables you set in the app.yaml file (PUBSUB_TOPIC and PUBSUB_VERIFICATION_TOKEN) for configuration.

The messages received by this instance are stored in a slice:

messages   []string

The pushHandler function receives pushed messages, verifies the token, and adds the message to the messages slice:


func pushHandler(w http.ResponseWriter, r *http.Request) {
	// Verify the token.
	if r.URL.Query().Get("token") != token {
		http.Error(w, "Bad token", http.StatusBadRequest)
		return
	}
	msg := &pushRequest{}
	if err := json.NewDecoder(r.Body).Decode(msg); err != nil {
		http.Error(w, fmt.Sprintf("Could not decode body: %v", err), http.StatusBadRequest)
		return
	}

	messagesMu.Lock()
	defer messagesMu.Unlock()
	// Limit to ten.
	messages = append(messages, string(msg.Message.Data))
	if len(messages) > maxMessages {
		messages = messages[len(messages)-maxMessages:]
	}
}

The publishHandler function publishes new messages to the topic.


func publishHandler(w http.ResponseWriter, r *http.Request) {
	ctx := context.Background()

	msg := &pubsub.Message{
		Data: []byte(r.FormValue("payload")),
	}

	if _, err := topic.Publish(ctx, msg).Get(ctx); err != nil {
		http.Error(w, fmt.Sprintf("Could not publish message: %v", err), 500)
		return
	}

	fmt.Fprint(w, "Message published.")
}

Java

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());
    }
  }

Node.js

The sample app uses the values you set in the app.flexible.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:

// The following environment variables are set by app.flexible.yaml when
// running on App Engine, but will need to be manually set when running locally.
var PUBSUB_VERIFICATION_TOKEN = process.env.PUBSUB_VERIFICATION_TOKEN;
var pubsub = gcloud.pubsub({
    projectId: process.env.GOOGLE_CLOUD_PROJECT
});
var topic = pubsub.topic(process.env.PUBSUB_TOPIC);

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

// List of all messages received by this instance
var messages = [];

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

app.post('/pubsub/push', jsonBodyParser, (req, res) => {
  if (req.query.token !== PUBSUB_VERIFICATION_TOKEN) {
    res.status(400).send();
    return;
  }

  // The message is a unicode string encoded in base64.
  const message = Buffer.from(req.body.message.data, 'base64').toString(
    'utf-8'
  );

  messages.push(message);

  res.status(200).send();
});

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

app.get('/', (req, res) => {
  res.render('index', {messages, tokens, claims});
});

app.post('/', formBodyParser, async (req, res, next) => {
  if (!req.body.payload) {
    res.status(400).send('Missing payload');
    return;
  }

  const data = Buffer.from(req.body.payload);
  try {
    const messageId = await topic.publish(data);
    res.status(200).send(`Message ${messageId} sent.`);
  } catch (error) {
    next(error);
  }
});

PHP

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:

runtime: php
env: flex

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

$messages = [];

The pull method retrieves messages from the topic you created and adds them to the messages list:

// get PULL pubsub messages
$pubsub = new PubSubClient([
    'projectId' => $projectId,
]);
$subscription = $pubsub->subscription($subscriptionName);
$pullMessages = [];
foreach ($subscription->pull(['returnImmediately' => true]) as $pullMessage) {
    $pullMessages[] = $pullMessage;
    $messages[] = $pullMessage->data();
}
// acknowledge PULL messages
if ($pullMessages) {
    $subscription->acknowledgeBatch($pullMessages);
}

The publish method publishes new messages to the topic:

if ($message = (string) $request->getBody()) {
    // Publish the pubsub message to the topic
    $pubsub = new PubSubClient([
        'projectId' => $projectId,
    ]);
    $topic = $pubsub->topic($topicName);
    $topic->publish(['data' => $message]);
    return $response->withStatus(204);
}

Python

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

Ruby

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
  @claims = claims
  @messages = messages

  slim :index
end

post "/publish" do
  topic.publish params[:payload]

  redirect "/", 303
end

.NET

[HttpGet]
[HttpPost]
public IActionResult Index(MessageForm messageForm)
{
    var model = new MessageList();
    if (!_options.HasGoodProjectId())
    {
        model.MissingProjectId = true;
        return View(model);
    }
    if (!string.IsNullOrEmpty(messageForm.Message))
    {
        // Publish the message.
        var pubsubMessage = new PubsubMessage()
        {
            Data = ByteString.CopyFromUtf8(messageForm.Message)
        };
        pubsubMessage.Attributes["token"] = _options.VerificationToken;
        _publisher.PublishAsync(pubsubMessage);
        model.PublishedMessage = messageForm.Message;
    }
    // Render the current list of messages.
    model.Messages = s_receivedMessages.ToArray();
    model.AuthMessages = s_authenticatedMessages.ToArray();
    return View(model);
}

Running the sample locally

When running locally, you can use the Google Cloud CLI 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.

Go

Set environment variables before starting your application:

export GOOGLE_CLOUD_PROJECT=[your-project-id]
export PUBSUB_VERIFICATION_TOKEN=[your-token]
export PUBSUB_TOPIC=[your-topic]
go run pubsub.go

Java

mvn clean package

Set environment variables before starting your application:

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

Node.js

Set environment variables before starting your application:

export GOOGLE_CLOUD_PROJECT=[your-project-id]
export PUBSUB_VERIFICATION_TOKEN=[your-verification-token]
export PUBSUB_TOPIC=[your-topic]
npm install
npm start

PHP

Install dependencies using Composer:

composer install

Then set environment variables before starting your application:

export GOOGLE_CLOUD_PROJECT=[your-project-id]
export PUBSUB_VERIFICATION_TOKEN=[your-verification-token]
export PUBSUB_TOPIC=[your-topic]
php -S localhost:8080

Python

Install dependencies, preferably in a virtual environment.

Mac OS / Linux

  1. Create an isolated Python environment:
    python3 -m venv env
    source env/bin/activate
  2. If you're not in the directory that contains the sample code, navigate to the directory that contains the hello_world sample code. Then install dependencies:
    cd YOUR_SAMPLE_CODE_DIR
    pip install -r requirements.txt

Windows

Use PowerShell to run your Python packages.

  1. Locate your installation of PowerShell.
  2. Right-click on the shortcut to PowerShell and start it as an administrator.
  3. Create an isolated Python environment.
    python -m venv env
    .\env\Scripts\activate
  4. Navigate to your project directory and install dependencies. If you're not in the directory that contains the sample code, navigate to the directory that contains the hello_world sample code. Then, install dependencies:
    cd YOUR_SAMPLE_CODE_DIR
    pip install -r requirements.txt

Then set environment variables before starting your application:

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

Ruby

Install dependencies:

bundle install

Then set environment variables before starting your application:

export GOOGLE_CLOUD_PROJECT=[your-project-id]
export PUBSUB_VERIFICATION_TOKEN=[your-verification-token]
export PUBSUB_TOPIC=[your-topic]
bundle exec ruby app.rb -p 8080

.NET

To run the sample app locally:

Visual Studio

  1. Open dotnet-docs-samples\appengine\flexible\AppEngineFlex.sln with Visual Studio.

  2. In Solution Explorer, right-click Pubsub, and choose Debug > Start new instance.

Command line

  1. Run the following commands from the dotnet-docs-samples\appengine\flexible\Pubsub directory:

    dotnet restore
    dotnet run
    
  2. In your web browser, enter the following address:

    http://localhost:5000/
    

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.

Go

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: Tue, 13 Nov 2018 16:04:18 GMT
Content-Length: 0

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

Java

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.

Node.js

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
Connection: keep-alive
Date: Mon, 31 Aug 2015 22:19:50 GMT
Transfer-Encoding: chunked
X-Powered-By: Express

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

PHP

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

curl -i --data @sample_message.json "localhost:4567/pubsub/push?token=[your-token]"

Or

http POST ":4567/pubsub/push?token=[your-token]" < sample_message.json

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

Python

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

curl -i --data @sample_message.json "localhost:8080/push-handlers/receive_messages?token=[your-token]"

Or

http POST ":8080/push-handlers/receive_messages?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.

Ruby

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

curl -i --data @sample_message.json "localhost:4567/pubsub/push?token=[your-token]"

Or

http POST ":4567/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.

.NET

To send an HTTP POST request:

Get-Content -Raw .\sample_message.json | Invoke-WebRequest -Uri
http://localhost:5000/Push?token=your-secret-token -Method POST -ContentType
'text/json' -OutFile out.txt

After the request completes, you can refresh localhost:5000 and see the message in the list of received messages.

Running on App Engine

To deploy the demo app to App Engine using the gcloud command-line tool:

Go

Run the following command from the directory where your app.yaml is located:

gcloud app deploy

Java

To deploy your app using Maven, run the following:

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.

Node.js

Run the following command from the directory where your app.flexible.yaml is located:

gcloud app deploy app.flexible.yaml

PHP

Run the following command from the directory where your app.yaml is located:

gcloud app deploy

Python

Run the following command from the directory where your app.yaml is located:

gcloud app deploy

Ruby

Run the following command from the directory where your app.yaml is located:

gcloud app deploy app.yaml

.NET

Visual Studio

To deploy the Hello World app:

  1. Open dotnet-docs-samples\appengine\flexible\AppEngineFlex.sln with Visual Studio.
  2. In Solution Explorer, right-click Pubsub, and choose Publish to Google Cloud...

  3. Click App Engine Flex.

  4. Click Publish.

Command line

Run the following commands from the dotnet-docs-samples\appengine\flexible\Pubsub directory:

  dotnet restore
  dotnet publish
  cd bin\Debug\netcoreapp2.1\publish
  gcloud app deploy

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.