Handle transient spikes with flow control

Data pipelines sometimes get spikes in published traffic. The traffic spikes can overwhelm subscribers unless you're prepared for it. A simple solution to avoiding traffic spikes is to dynamically increase Pub/Sub subscriber resources to process more messages. However, this solution might drive up costs or not work instantaneously. For example, you might require many VMs.

Flow control on the subscriber side lets the subscriber regulate the rate at which messages are ingested. Flow control thus handles traffic spikes without driving up costs or until the subscriber is scaled up.

Flow control is an available feature in the Pub/Sub high-level client library. You can also implement your own flow control programming when you're using a low-level client library.

The need for flow control indicates that messages are being published at a higher rate than they are being consumed. If this scenario is a persistent state, rather than a transient spike in message volume, consider increasing the number of subscriber client instances.

Flow control configuration

Flow control lets you configure the maximum number of bytes allocated for outstanding requests, and the maximum number of outstanding messages permitted. Set these limits according to the throughput capacity of your client machines.

The default values for the flow control variables and the names of the variables might differ across client libraries. For example, in the Java client library, the following variables configure flow control:

  • setMaxOutstandingElementCount(). Defines the maximum number of messages for which Pub/Sub has not received acknowledgments or negative acknowledgments.

  • setMaxOutstandingRequestBytes(). Defines the maximum size of messages for which Pub/Sub has not received acknowledgments or negative acknowledgments.

If the limit for setMaxOutstandingElementCount() or setMaxOutstandingRequestBytes() is crossed, the subscriber client does not pull more messages. This behavior continues until the messages that are already pulled get acknowledged or negatively acknowledged. We can thus align throughput with the cost associated with running more subscribers.

Code samples for flow control

To control the rate at which the subscriber client receives messages, use the flow control features of the subscriber. These flow control features are illustrated in the following samples:


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::Options;
using ::google::cloud::StatusOr;
auto sample = [](std::string project_id, std::string subscription_id) {
  // Change the flow control watermarks, by default the client library uses
  // 0 and 1,000 for the message count watermarks, and 0 and 10MiB for the
  // size watermarks. Recall that the library stops requesting messages if
  // any of the high watermarks are reached, and the library resumes
  // requesting messages when *both* low watermarks are reached.
  auto constexpr kMiB = 1024 * 1024L;
  auto subscriber = pubsub::Subscriber(pubsub::MakeSubscriberConnection(
      pubsub::Subscription(std::move(project_id), std::move(subscription_id)),
          .set<pubsub::MaxOutstandingBytesOption>(8 * kMiB)));

  auto session = subscriber.Subscribe(
      [](pubsub::Message const& m, pubsub::AckHandler h) {
        std::cout << "Received message " << m << "\n";
  return std::make_pair(subscriber, std::move(session));


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.

using Google.Api.Gax;
using Google.Cloud.PubSub.V1;
using System;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

public class PullMessagesWithFlowControlAsyncSample
    public async Task<int> PullMessagesWithFlowControlAsync(string projectId, string subscriptionId, bool acknowledge)
        SubscriptionName subscriptionName = SubscriptionName.FromProjectSubscription(projectId, subscriptionId);
        int messageCount = 0;
        SubscriberClient subscriber = await new SubscriberClientBuilder
            SubscriptionName = subscriptionName,
            Settings = new SubscriberClient.Settings
                AckExtensionWindow = TimeSpan.FromSeconds(4),
                AckDeadline = TimeSpan.FromSeconds(10),
                FlowControlSettings = new FlowControlSettings(maxOutstandingElementCount: 100, maxOutstandingByteCount: 10240)
        // SubscriberClient runs your message handle function on multiple
        // threads to maximize throughput.
        Task startTask = subscriber.StartAsync((PubsubMessage message, CancellationToken cancel) =>
            string text = System.Text.Encoding.UTF8.GetString(message.Data.ToArray());
            Console.WriteLine($"Message {message.MessageId}: {text}");
            Interlocked.Increment(ref messageCount);
            return Task.FromResult(acknowledge ? SubscriberClient.Reply.Ack : SubscriberClient.Reply.Nack);
        // Run for 5 seconds.
        await Task.Delay(5000);
        await subscriber.StopAsync(CancellationToken.None);
        // Lets make sure that the start task finished successfully after the call to stop.
        await startTask;
        return messageCount;


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 pullMsgsFlowControlSettings(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)
	// MaxOutstandingMessages is the maximum number of unprocessed messages the
	// subscriber client will pull from the server before pausing. This also configures
	// the maximum number of concurrent handlers for received messages.
	// For more information, see https://cloud.google.com/pubsub/docs/pull#streamingpull_dealing_with_large_backlogs_of_small_messages.
	sub.ReceiveSettings.MaxOutstandingMessages = 100
	// MaxOutstandingBytes is the maximum size of unprocessed messages,
	// that the subscriber client will pull from the server before pausing.
	sub.ReceiveSettings.MaxOutstandingBytes = 1e8
	err = sub.Receive(ctx, func(ctx context.Context, msg *pubsub.Message) {
		fmt.Fprintf(w, "Got message: %q\n", string(msg.Data))
	if err != nil {
		return fmt.Errorf("sub.Receive: %w", err)
	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 com.google.api.gax.batching.FlowControlSettings;
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 SubscribeWithFlowControlSettingsExample {
  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";

    subscribeWithFlowControlSettingsExample(projectId, subscriptionId);

  public static void subscribeWithFlowControlSettingsExample(
      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());

    Subscriber subscriber = null;

    // The subscriber will pause the message stream and stop receiving more messsages from the
    // server if any one of the conditions is met.
    FlowControlSettings flowControlSettings =
            // 1,000 outstanding messages. Must be >0. It controls the maximum number of messages
            // the subscriber receives before pausing the message stream.
            // 100 MiB. Must be >0. It controls the maximum size of messages the subscriber
            // receives before pausing the message stream.
            .setMaxOutstandingRequestBytes(100L * 1024L * 1024L)

    try {
      subscriber =
          Subscriber.newBuilder(subscriptionName, receiver)

      // Start the subscriber.
      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.


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

 * TODO(developer): Uncomment these variables before running the sample.
// const subscriptionNameOrId = 'YOUR_SUBSCRIPTION_NAME_OR_ID';
// const maxInProgress = 5;
// const timeout = 10;

// Imports the Google Cloud client library
const {PubSub} = require('@google-cloud/pubsub');

// Creates a client; cache this for further use
const pubSubClient = new PubSub();

async function subscribeWithFlowControlSettings(
) {
  const subscriberOptions = {
    flowControl: {
      maxMessages: maxInProgress,

  // References an existing subscription.
  // Note that flow control settings are not persistent across subscribers.
  const subscription = pubSubClient.subscription(

    `Subscriber to subscription ${subscription.name} is ready to receive messages at a controlled volume of ${maxInProgress} messages.`

  const messageHandler = message => {
    console.log(`Received message: ${message.id}`);
    console.log(`\tData: ${message.data}`);
    console.log(`\tAttributes: ${message.attributes}`);

    // "Ack" (acknowledge receipt of) the message

  subscription.on('message', messageHandler);

  // Wait a while for the subscription to run. (Part of the sample only.)
  setTimeout(() => {
  }, timeout * 1000);


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

from concurrent.futures import TimeoutError
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

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}.")

# Limit the subscriber to only have ten outstanding messages at a time.
flow_control = pubsub_v1.types.FlowControl(max_messages=10)

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

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


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.

# subscription_id = "your-subscription-id"

pubsub = Google::Cloud::Pubsub.new

subscription = pubsub.subscription subscription_id
subscriber   = subscription.listen inventory: 10 do |received_message|
  puts "Received message: #{received_message.data}"

# Let the main thread sleep for 60 seconds so the thread for listening
# messages does not quit
sleep 60

What's next

Read about the other delivery options you can configure for a subscription: