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Cloud Platforms > Google Cloud Platform (GCP) > Cloud Pub/Sub

Cloud Pub/Sub

Author: Venkata Sudhakar

Google Cloud Pub/Sub is a fully managed, real-time messaging service that allows you to send and receive messages between independent applications. It is a scalable, durable event ingestion and delivery system that serves as the backbone for streaming analytics and event-driven systems.

Key Concepts:

1. Topic - A named resource to which messages are sent by publishers.

2. Subscription - A named resource representing the stream of messages from a topic.

3. Publisher - An application that creates and sends messages to a topic.

4. Subscriber - An application that receives messages from a subscription.

5. Message - The combination of data and optional attributes that a publisher sends.

The below example shows how to publish and subscribe to messages using the Java client library.

import com.google.cloud.pubsub.v1.Publisher;
import com.google.cloud.pubsub.v1.Subscriber;
import com.google.cloud.pubsub.v1.AckReplyConsumer;
import com.google.cloud.pubsub.v1.MessageReceiver;
import com.google.protobuf.ByteString;
import com.google.pubsub.v1.ProjectSubscriptionName;
import com.google.pubsub.v1.ProjectTopicName;
import com.google.pubsub.v1.PubsubMessage;
import java.util.concurrent.TimeUnit;

public class CloudPubSubExample {
    private static final String PROJECT_ID = "your-project-id";
    private static final String TOPIC_ID = "my-topic";
    private static final String SUBSCRIPTION_ID = "my-subscription";

public static void main(String[] args) throws Exception {
        // Publish a message
        ProjectTopicName topicName = ProjectTopicName.of(PROJECT_ID, TOPIC_ID);
        Publisher publisher = Publisher.newBuilder(topicName).build();

String messageText = "Hello from Cloud Pub/Sub!";
        PubsubMessage pubsubMessage = PubsubMessage.newBuilder()
            .setData(ByteString.copyFromUtf8(messageText))
            .putAttributes("source", "java-example")
            .build();

String messageId = publisher.publish(pubsubMessage).get();
        System.out.println("Published message ID: " + messageId);
        publisher.shutdown();

// Subscribe and receive messages
        ProjectSubscriptionName subscriptionName =
            ProjectSubscriptionName.of(PROJECT_ID, SUBSCRIPTION_ID);

MessageReceiver receiver = (PubsubMessage message, AckReplyConsumer consumer) -> {
            System.out.println("Received: " + message.getData().toStringUtf8());
            System.out.println("Attributes: " + message.getAttributesMap());
            consumer.ack();
        };

Subscriber subscriber = Subscriber.newBuilder(subscriptionName, receiver).build();
        subscriber.startAsync().awaitRunning();
        TimeUnit.SECONDS.sleep(10);
        subscriber.stopAsync();
    }
}

It gives the following output,

Published message ID: 7661234567890123
Received: Hello from Cloud Pub/Sub!
Attributes: {source=java-example}

Delivery Modes:

Push delivery - Pub/Sub sends messages to a specified HTTPS endpoint (webhook). Best for low-latency real-time processing.

Pull delivery - Subscribers explicitly call the API to pull messages. Best for large volumes where subscribers control the processing rate.

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