Writing and Responding to Pub/Sub Messages

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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 Cloud Pub/Sub messages in an app running in the flexible environment.

Prerequisites

• Follow the instructions in "Hello, World!" for PHP on App Engine to set up your environment and project, and to understand how App Engine PHP 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/php-docs-samples.git
cd php-docs-samples/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

Setting environment variables

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

pubsub/app/index.php

View on GitHub

$app['topic'] = 'php-example-topic';
$app['subscription'] = 'php-example-subscription';

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:

pubsub/app/app.yaml.flexible

View on GitHub

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:

pubsub/app/app.php

View on GitHub

// 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:

pubsub/app/app.php

View on GitHub

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

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.

Install dependencies using Composer:

composer install

Then set environment variables before starting your application:

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 httpie to send an HTTP POST request:

Or

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

Response:

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:

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.