GCP PubSub Send Example

If you are working on Shopify and you need to monitor webhook events (e.g. an order has been created), chances are you'll be directed to use Google Pub/Sub messaging service. In this blog, I'll show you how to setup PubSub on the Google Cloud Platform (GCP), then send a message and check the message on GCP PubSub cloud console.

Tools and Materials

These are the tools I used to make this example:

• IntelliJ IDEA 2023.3.4 (Community Edition)
• openjdk-17.0.10
• Spring Boot v3.4.5
• Windows 11
• Google account
• Google Cloud SDK Shell
• Git Bash

The example code is here, github.com/jpllosa/gcp-pubsub. You may grab a copy of it.

GCP PubSub Setup

On GCP, create a new project. Like so.

Assign your new project with billing. As of this writing, Google provides free credits to new customers and free usage limits. For Google PubSub, it is 10 GB of messages per month. Please verify for yourselves if this is still the case. Here's what creating a billing account looks like and it being connected to the project ID.

Now that the administrative stuff has all been set up, it's time to create a PubSub topic and subscription.

Google Cloud SDK Shell Setup

Install and initialize the Google Cloud SDK. For installing the Google Cloud SDK Shell, it is better to get it from the horses' mouth. Please follow the Google Cloud documentation on how to install and initialiaze it for your project. Once initialized, enable GCP PubSub. Here are a few checkpoints so you'll know you are heading in the right direction. First and foremost, you must log in with application default credentials using the following command, gcloud auth application-default login. When you are logged, you should have something like below.

Next is to set the shell to your project, gcloud config set project <PROJECT-NAME>. Here's the command to see the available projects, gcloud config projects list. When you have done those, you should have something like below.

Lastly, enable GCP PubSub, gcloud services enable pubsub.googleapis.com. Output would look like below.

For Windows, we can verify that our Google credentials are saved by looking under C:\Users\username\AppData\Roaming\gcloud\application_default_credentials.json. This file should exist.

Spring Boot GCP PubSub Send Code

I used Spring Initialzr to create the skeleton project. The following are the dependencies we need:

• Spring Web - Build web, including RESTful, applications using Spring MVC. Uses Apache Tomcat as the default embedded container.
• Spring Integration - Adds support for Enterprise Integration Patterns. Enables lightweight messaging and supports integration with external systems via declarative adapters.
• Google Cloud Messaging - Adds the Google Cloud Support entry and all the required dependencies so that the Google Cloud Pub/Sub integration work out of the box.

Our pom.xml should at least have the following dependencies.

  
<!-- snipped -->

<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-integration</artifactId>
</dependency>
<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
	<groupId>com.google.cloud</groupId>
	<artifactId>spring-cloud-gcp-starter-pubsub</artifactId>
</dependency>
<dependency>
	<groupId>org.springframework.integration</groupId>
	<artifactId>spring-integration-http</artifactId>
</dependency>

<!-- snipped -->
  

Our GCP PubSub message sender code is short and sweet. When an HTTP POST is received, it then submits the message to GCP PubSub. Here's the code. Explanation follows after.

  
package com.blogspot.jpllosa;

// imports snipped...

@SpringBootApplication
@RestController
public class GcpPubsubApplication {

	public static void main(String[] args) {
		SpringApplication.run(GcpPubsubApplication.class, args);
	}

	@MessagingGateway(defaultRequestChannel = "gcpPubsubOutputChannel")
	public interface GcpPubsubOutboundGateway {
		void sendToGcpPubsub(String text);
	}

	@Bean
	@ServiceActivator(inputChannel = "gcpPubsubOutputChannel")
	public MessageHandler messageSender(PubSubTemplate pubsubTemplate) {
		return new PubSubMessageHandler(pubsubTemplate, "gcp-pubsub-example");
	}

	@Autowired
	private GcpPubsubOutboundGateway messagingGateway;

	@PostMapping("/postMessage")
	public RedirectView postMessage(@RequestParam("message") String message) {
		this.messagingGateway.sendToGcpPubsub(message);
		return new RedirectView("/");
	}

}
  

Since this is a blog about GCP PubSub, I'll focus on explaining the code related to it. I won't be talking about the other annotations (e.g. @SpringBootApplication, @PostMapping, etc.). For example, @PostMapping is related to web and nothing to do with GCP PubSub. We just need that endpoint to trigger a send message to GCP PubSub. We can use other client libraries to communicate with GCP PubSub, not just Spring.

We use Spring Integration messaging gateway to write to a channel, gcpPubsubOutputChannel. This channel is still in Spring land. When a message is in this channel, it is then consumed by the MessageHandler (outbound channel adapter) which in turn publishes the message to GCP PubSub. The outbound channel adapter converts the Spring message into a GCP PubSub message and is published to the topic, gcp-pubsub-example. The @ServiceActivator annotation causes this MessageHandler to be applied to any new messages in inputChannel. Lastly, we got a REST endpoint listening for messages which then forward the message to GCP PubSub. Short and sweet.

Spring Boot GCP PubSub Send in Action

Alrighty, run the Spring Boot app. We can do it command line (e.g. mvn spring-boot:run) or thru IntelliJ. I would suggest running it first on Google Cloud SDK Shell. If it works there then I don't see any reason why it won't work on IntelliJ. When the app is ready, hit the /postMessage endpoint.

  
$ curl --data "message=Eagle has landed!" localhost:8080/postMessage
  

Now, head over to GCP PubSub messages section and we should see the messages appear. If not, try reloading the web page or do a PULL. It should look like below.

Spring Boot GCP PubSub Send Wrap Up

Did you enjoy playing around the GCP PubSub service? We now have an idea on how to send messages via the GCP PubSub messaging service. Spring has made it easy for us by abstracting whatever it is happening under the hood. Thank you for reading.