Subscribe with concurrency control

Configures number of threads for receiving messages and processing message callbacks.

Explore further

For detailed documentation that includes this code sample, see the following:

• Process more messages with concurrency control

Code sample

C++

Before trying this sample, follow the C++ setup instructions in the Pub/Sub quickstart using client libraries. For more information, see the Pub/Sub C++ API reference documentation.

To authenticate to Pub/Sub, set up Application Default Credentials. For more information, see Set up authentication for a local development environment.

namespace pubsub = ::google::cloud::pubsub;
using ::google::cloud::future;
using ::google::cloud::GrpcBackgroundThreadPoolSizeOption;
using ::google::cloud::Options;
using ::google::cloud::StatusOr;
auto sample = [](std::string project_id, std::string subscription_id) {
  // Create a subscriber with 16 threads handling I/O work, by default the
  // library creates `std::thread::hardware_concurrency()` threads.
  auto subscriber = pubsub::Subscriber(pubsub::MakeSubscriberConnection(
      pubsub::Subscription(std::move(project_id), std::move(subscription_id)),
      Options{}
          .set<pubsub::MaxConcurrencyOption>(8)
          .set<GrpcBackgroundThreadPoolSizeOption>(16)));

  // Create a subscription where up to 8 messages are handled concurrently. By
  // default the library uses `std::thread::hardware_concurrency()` as the
  // maximum number of concurrent callbacks.
  auto session = subscriber.Subscribe(
      [](pubsub::Message const& m, pubsub::AckHandler h) {
        // This handler executes in the I/O threads, applications could use,
        // std::async(), a thread-pool, or any other mechanism to transfer the
        // execution to other threads.
        std::cout << "Received message " << m << "\n";
        std::move(h).ack();
        PleaseIgnoreThisSimplifiesTestingTheSamples();
      });
  return std::make_pair(subscriber, std::move(session));
};

C#

Before trying this sample, follow the C# setup instructions in the Pub/Sub quickstart using client libraries. For more information, see the Pub/Sub C# API reference documentation.

To authenticate to Pub/Sub, set up Application Default Credentials. For more information, see Set up authentication for a local development environment.

using Google.Cloud.PubSub.V1;
using System;
using System.Threading;
using System.Threading.Tasks;

public class SubscriberConcurrencyControlSample
{
    public async Task<int> SubscribeWithConcurrencyControlAsync(string projectId, string subscriptionId)
    {
        SubscriptionName subscriptionName = SubscriptionName.FromProjectSubscription(projectId, subscriptionId);

        SubscriberClient subscriber = await new SubscriberClientBuilder
        {
            SubscriptionName = subscriptionName,
            // Normally the number of clients depends on the number of processors.
            // Here we explicitly request 2 concurrent clients instead.
            ClientCount = 2
        }.BuildAsync();

        int count = 0;
        Task startTask = subscriber.StartAsync((PubsubMessage message, CancellationToken cancellationToken) =>
        {
            string text = message.Data.ToStringUtf8();
            Console.WriteLine($"Received message: {text}");
            Interlocked.Increment(ref count);
            return Task.FromResult(SubscriberClient.Reply.Ack);
        });
        // Run for 10 seconds.
        await Task.Delay(10_000);
        await subscriber.StopAsync(CancellationToken.None);
        // Lets make sure that the start task finished successfully after the call to stop.
        await startTask;
        return count;
    }
}

Go

Before trying this sample, follow the Go setup instructions in the Pub/Sub quickstart using client libraries. For more information, see the Pub/Sub Go API reference documentation.

To authenticate to Pub/Sub, set up Application Default Credentials. For more information, see Set up authentication for a local development environment.

import (
	"context"
	"fmt"
	"io"
	"sync/atomic"
	"time"

	"cloud.google.com/go/pubsub"
)

func pullMsgsConcurrencyControl(w io.Writer, projectID, subID string) error {
	// projectID := "my-project-id"
	// subID := "my-sub"
	ctx := context.Background()
	client, err := pubsub.NewClient(ctx, projectID)
	if err != nil {
		return fmt.Errorf("pubsub.NewClient: %w", err)
	}
	defer client.Close()

	sub := client.Subscription(subID)
	// Must set ReceiveSettings.Synchronous to false (or leave as default) to enable
	// concurrency pulling of messages. Otherwise, NumGoroutines will be set to 1.
	sub.ReceiveSettings.Synchronous = false
	// NumGoroutines determines the number of goroutines sub.Receive will spawn to pull
	// messages.
	sub.ReceiveSettings.NumGoroutines = 16
	// MaxOutstandingMessages limits the number of concurrent handlers of messages.
	// In this case, up to 8 unacked messages can be handled concurrently.
	// Note, even in synchronous mode, messages pulled in a batch can still be handled
	// concurrently.
	sub.ReceiveSettings.MaxOutstandingMessages = 8

	// Receive messages for 10 seconds, which simplifies testing.
	// Comment this out in production, since `Receive` should
	// be used as a long running operation.
	ctx, cancel := context.WithTimeout(ctx, 10*time.Second)
	defer cancel()

	var received int32

	// Receive blocks until the context is cancelled or an error occurs.
	err = sub.Receive(ctx, func(_ context.Context, msg *pubsub.Message) {
		atomic.AddInt32(&received, 1)
		msg.Ack()
	})
	if err != nil {
		return fmt.Errorf("sub.Receive returned error: %w", err)
	}
	fmt.Fprintf(w, "Received %d messages\n", received)

	return nil
}

Java

Before trying this sample, follow the Java setup instructions in the Pub/Sub quickstart using client libraries. For more information, see the Pub/Sub Java API reference documentation.

To authenticate to Pub/Sub, set up Application Default Credentials. For more information, see Set up authentication for a local development environment.

import com.google.api.gax.core.ExecutorProvider;
import com.google.api.gax.core.InstantiatingExecutorProvider;
import com.google.cloud.pubsub.v1.AckReplyConsumer;
import com.google.cloud.pubsub.v1.MessageReceiver;
import com.google.cloud.pubsub.v1.Subscriber;
import com.google.pubsub.v1.ProjectSubscriptionName;
import com.google.pubsub.v1.PubsubMessage;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

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

    subscribeWithConcurrencyControlExample(projectId, subscriptionId);
  }

  public static void subscribeWithConcurrencyControlExample(
      String projectId, String subscriptionId) {
    ProjectSubscriptionName subscriptionName =
        ProjectSubscriptionName.of(projectId, subscriptionId);

    // Instantiate an asynchronous message receiver.
    MessageReceiver receiver =
        (PubsubMessage message, AckReplyConsumer consumer) -> {
          // Handle incoming message, then ack the received message.
          System.out.println("Id: " + message.getMessageId());
          System.out.println("Data: " + message.getData().toStringUtf8());
          consumer.ack();
        };

    Subscriber subscriber = null;
    try {
      // Provides an executor service for processing messages. The default `executorProvider` used
      // by the subscriber has a default thread count of 5.
      ExecutorProvider executorProvider =
          InstantiatingExecutorProvider.newBuilder().setExecutorThreadCount(4).build();

      // `setParallelPullCount` determines how many StreamingPull streams the subscriber will open
      // to receive message. It defaults to 1. `setExecutorProvider` configures an executor for the
      // subscriber to process messages. Here, the subscriber is configured to open 2 streams for
      // receiving messages, each stream creates a new executor with 4 threads to help process the
      // message callbacks. In total 2x4=8 threads are used for message processing.
      subscriber =
          Subscriber.newBuilder(subscriptionName, receiver)
              .setParallelPullCount(2)
              .setExecutorProvider(executorProvider)
              .build();

      // Start the subscriber.
      subscriber.startAsync().awaitRunning();
      System.out.printf("Listening for messages on %s:\n", subscriptionName.toString());
      // Allow the subscriber to run for 30s unless an unrecoverable error occurs.
      subscriber.awaitTerminated(30, TimeUnit.SECONDS);
    } catch (TimeoutException timeoutException) {
      // Shut down the subscriber after 30s. Stop receiving messages.
      subscriber.stopAsync();
    }
  }
}

Python

Before trying this sample, follow the Python setup instructions in the Pub/Sub quickstart using client libraries. For more information, see the Pub/Sub Python API reference documentation.

To authenticate to Pub/Sub, set up Application Default Credentials. For more information, see Set up authentication for a local development environment.

from concurrent import futures
from google.cloud import pubsub_v1

# TODO(developer)
# project_id = "your-project-id"
# subscription_id = "your-subscription-id"
# Number of seconds the subscriber should listen for messages
# timeout = 5.0

# An optional executor to use. If not specified, a default one with maximum 10
# threads will be created.
executor = futures.ThreadPoolExecutor(max_workers=5)
# A thread pool-based scheduler. It must not be shared across SubscriberClients.
scheduler = pubsub_v1.subscriber.scheduler.ThreadScheduler(executor)

subscriber = pubsub_v1.SubscriberClient()
subscription_path = subscriber.subscription_path(project_id, subscription_id)

def callback(message: pubsub_v1.subscriber.message.Message) -> None:
    print(f"Received {message.data!r}.")
    message.ack()

streaming_pull_future = subscriber.subscribe(
    subscription_path, callback=callback, scheduler=scheduler
)
print(f"Listening for messages on {subscription_path}..\n")

# Wrap subscriber in a 'with' block to automatically call close() when done.
with subscriber:
    try:
        # When `timeout` is not set, result() will block indefinitely,
        # unless an exception is encountered first.
        streaming_pull_future.result(timeout=timeout)
    except TimeoutError:
        streaming_pull_future.cancel()  # Trigger the shutdown.
        streaming_pull_future.result()  # Block until the shutdown is complete.

Ruby

Before trying this sample, follow the Ruby setup instructions in the Pub/Sub quickstart using client libraries. For more information, see the Pub/Sub Ruby API reference documentation.

To authenticate to Pub/Sub, set up Application Default Credentials. For more information, see Set up authentication for a local development environment.

# subscription_id = "your-subscription-id"

pubsub = Google::Cloud::Pubsub.new

subscription = pubsub.subscription subscription_id
# Use 2 threads for streaming, 4 threads for executing callbacks and 2 threads
# for sending acknowledgements and/or delays
subscriber   = subscription.listen streams: 2, threads: {
  callback: 4,
  push:     2
} do |received_message|
  puts "Received message: #{received_message.data}"
  received_message.acknowledge!
end

subscriber.start
# Let the main thread sleep for 60 seconds so the thread for listening
# messages does not quit
sleep 60
subscriber.stop.wait!

What's next

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