Concurrency control

This document provides information about using concurrency control with messages published to a topic.

Concurrency control helps you override the default number of background (I/O) threads used by the client library to publish messages. This lets the publisher clients to send messages in parallel. You can also implement your own concurrency control when you're using a low-level library.

Support for concurrency control depends on the programming language of the client library. For language implementations that support parallel threads, such as C++, Go, and Java, the client libraries make a default choice for the number of threads.

Before you begin

Before configuring the publish workflow, ensure you have completed the following tasks:

Required roles

To get the permissions that you need to publish messages to a topic, ask your administrator to grant you the Pub/Sub Publisher (roles/pubsub.publisher) IAM role on topic. For more information about granting roles, see Manage access.

You might also be able to get the required permissions through custom roles or other predefined roles.

You need additional permissions to create or update topics and subscriptions.

Use Concurrency control with messages

Support for concurrency depends on your programming language. Refer to the API Reference documentation for more information.

The following sample illustrates how to control concurrency in a publisher:


Before trying this sample, follow the C++ setup instructions in Quickstart: Using Client Libraries. For more information, see the Pub/Sub C++ API reference documentation.

namespace pubsub = ::google::cloud::pubsub;
using ::google::cloud::future;
using ::google::cloud::GrpcBackgroundThreadPoolSizeOption;
using ::google::cloud::Options;
using ::google::cloud::StatusOr;
[](std::string project_id, std::string topic_id) {
  auto topic = pubsub::Topic(std::move(project_id), std::move(topic_id));
  // Override the default number of background (I/O) threads. By default the
  // library uses `std::thread::hardware_concurrency()` threads.
  auto options = Options{}.set<GrpcBackgroundThreadPoolSizeOption>(8);
  auto publisher = pubsub::Publisher(
      pubsub::MakePublisherConnection(std::move(topic), std::move(options)));

  std::vector<future<void>> ids;
  for (char const* data : {"1", "2", "3", "go!"}) {
            .then([data](future<StatusOr<std::string>> f) {
              auto s = f.get();
              if (!s) return;
              std::cout << "Sent '" << data << "' (" << *s << ")\n";
  // Block until they are actually sent.
  for (auto& id : ids) id.get();


Before trying this sample, follow the Go setup instructions in Quickstart: Using Client Libraries. For more information, see the Pub/Sub Go API reference documentation.

import (


func publishSingleGoroutine(w io.Writer, projectID, topicID, msg string) error {
	// projectID := "my-project-id"
	// topicID := "my-topic"
	// msg := "Hello World"
	ctx := context.Background()
	client, err := pubsub.NewClient(ctx, projectID)
	if err != nil {
		return fmt.Errorf("pubsub.NewClient: %w", err)
	defer client.Close()

	t := client.Topic(topicID)
	t.PublishSettings.NumGoroutines = 1

	result := t.Publish(ctx, &pubsub.Message{Data: []byte(msg)})
	// Block until the result is returned and a server-generated
	// ID is returned for the published message.
	id, err := result.Get(ctx)
	if err != nil {
		return fmt.Errorf("Get: %w", err)
	fmt.Fprintf(w, "Published a message; msg ID: %v\n", id)
	return nil


Before trying this sample, follow the Java setup instructions in Quickstart: Using Client Libraries. For more information, see the Pub/Sub Java API reference documentation.

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;

public class PublishWithConcurrencyControlExample {
  public static void main(String... args) throws Exception {
    // TODO(developer): Replace these variables before running the sample.
    String projectId = "your-project-id";
    String topicId = "your-topic-id";

    publishWithConcurrencyControlExample(projectId, topicId);

  public static void publishWithConcurrencyControlExample(String projectId, String topicId)
      throws IOException, ExecutionException, InterruptedException {
    TopicName topicName = TopicName.of(projectId, topicId);
    Publisher publisher = null;
    List<ApiFuture<String>> messageIdFutures = new ArrayList<>();

    try {
      // Provides an executor service for processing messages. The default
      // `executorProvider` used by the publisher has a default thread count of
      // 5 * the number of processors available to the Java virtual machine.
      ExecutorProvider executorProvider =

      // `setExecutorProvider` configures an executor for the publisher.
      publisher = Publisher.newBuilder(topicName).setExecutorProvider(executorProvider).build();

      // schedule publishing one message at a time : messages get automatically batched
      for (int i = 0; i < 100; i++) {
        String message = "message " + i;
        ByteString data = ByteString.copyFromUtf8(message);
        PubsubMessage pubsubMessage = PubsubMessage.newBuilder().setData(data).build();

        // Once published, returns a server-assigned message id (unique within the topic)
        ApiFuture<String> messageIdFuture = publisher.publish(pubsubMessage);
    } finally {
      // Wait on any pending publish requests.
      List<String> messageIds = ApiFutures.allAsList(messageIdFutures).get();

      System.out.println("Published " + messageIds.size() + " messages with concurrency control.");

      if (publisher != null) {
        // When finished with the publisher, shutdown to free up resources.
        publisher.awaitTermination(1, TimeUnit.MINUTES);


Before trying this sample, follow the Ruby setup instructions in Quickstart: Using Client Libraries. For more information, see the Pub/Sub Ruby API reference documentation.

# topic_id = "your-topic-id"

pubsub =

topic = pubsub.topic topic_id, async: {
  threads: {
    # Use exactly one thread for publishing message and exactly one thread
    # for executing callbacks
    publish:  1,
    callback: 1
topic.publish_async "This is a test message." do |result|
  raise "Failed to publish the message." unless result.succeeded?
  puts "Message published asynchronously."

# Stop the async_publisher to send all queued messages immediately.

What's next