NET client SubscriberServiceApiClient.Pull method to return immediately if no messages on topic

[raphambi]

Using the NET client API, how is it possible to achieve the same functionality with  SubscriberServiceApiClient Pull (or PullAsync) method as with the now deprecated returnImmediately parameter such that the PULL method returns to the caller immediately , with no exception raised, if there are no messages present on the topic?  I have tried to set the call setting parameter with a Timeout expiration:

CallSettings.FromExpiration(Expiration.FromTimeout(TimeSpan.FromSeconds(1)))

but this seems to result in an exception of DeadlineExceeded.

The default behaviour seems to timeout around 12 seconds when there are no messages on the topic, which is not ideal.

many thanks

[ms4446]

The returnImmediately parameter in the Google Cloud Pub/Sub .NET client API's Pull and PullAsync methods was deprecated primarily due to its inefficiency. This parameter, when set to true, could lead to frequent empty responses and increased network traffic, as it would return immediately even if no messages were available. While the potential for a race condition exists in any asynchronous messaging system, it wasn't the primary reason for the deprecation of returnImmediately.

To efficiently receive messages without the need for the returnImmediately parameter, it's recommended to use the SubscriberClient class provided by the Google Cloud Pub/Sub .NET client library. This class abstracts the details of the streaming pull mechanism, which maintains a long-lived connection to continuously stream messages to the client as they become available.

Here's an example of how to use the SubscriberClient for receiving messages:

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

public class SubscriberExample
{
    public static async Task Main(string subscriptionId)
    {
        SubscriberClient subscriber = await SubscriberClient.CreateAsync(subscriptionId);

        // Subscribe to messages
        await subscriber.StartAsync((PubsubMessage message, CancellationToken cancel) =>
        {
            // Handle message
            Console.WriteLine($"Received message {message.MessageId} published at {message.PublishTime.ToDateTime()}");
            Console.WriteLine($"Text: '{message.Data.ToStringUtf8()}'");

            // Acknowledge the message
            return Task.FromResult(SubscriberClient.Reply.Ack);
        });
    }
}

In this example, the SubscriberClient handles the details of establishing a streaming connection to the Pub/Sub service. Messages are delivered to the client as they become available, and the client acknowledges each message after processing it. This approach is more efficient than repeatedly polling with the Pull method and avoids the issues associated with the returnImmediately parameter.

By using the SubscriberClient and its streaming pull mechanism, you can achieve efficient and continuous message processing, ensuring that messages are received as soon as they are published to the topic. This method is particularly advantageous in high-volume messaging scenarios.

[raphambi]

Hi ms4446,

Thanks for the response. I understand this, but it doesn't answer the question. We actually want to have the semantics of non blocking pull of a single message of the queue and returning immediately with a single message or not, not processing multiple messages, even if the queue has more than one message.

Our client is not long lived and would not be repeatedly polling either, so perhaps not a normal or usual scenario. Nevertheless, it would be helpful if you could at least suggest if the required behaviour is possible, as the above, while agreed is right for most subscriber scenarios, does not fit what is required here. 

many thanks

[ms4446]

If you're looking for a way to perform a non-blocking, single-message pull from a Pub/Sub queue and return immediately whether or not a message is available, you're dealing with a specific use case that the standard Pub/Sub client libraries may not directly support. However, you can still achieve this behavior with a custom approach.

Here's a suggestion on how you might implement this:

1. Use the Pull Method with MaxMessages Set to 1: You can use the Pull or PullAsync method with the maxMessages parameter set to 1. This will ensure that the method returns after receiving at most one message.

2. Implement a Short Timeout: Since you want the method to return immediately if no messages are available, you'll need to implement a short timeout. You've already tried setting a timeout using CallSettings.FromExpiration(Expiration.FromTimeout(TimeSpan.FromSeconds(1))), but as you mentioned, this leads to a DeadlineExceeded exception. Instead, you can implement a timeout in your application logic. For example, you can start a timer before calling Pull or PullAsync and check the timer after the call returns. If the timer has exceeded your desired timeout, you can assume that no messages are available and return from your method.

3. Handle Exceptions and Timeouts Appropriately: In your implementation, make sure to handle any exceptions that might be thrown by the Pull or PullAsync method, including the DeadlineExceeded exception. If an exception occurs, you can decide whether to retry the pull request or handle it as a failure based on your application's requirements.

4. Asynchronous Handling: If you're using PullAsync, ensure that your timeout logic works correctly with asynchronous code. You might need to use tasks and cancellation tokens to manage the asynchronous calls and timeouts effectively.

Here's a basic outline of what this might look like in code:

public async Task<PubsubMessage> TryPullSingleMessageAsync(string subscriptionName, TimeSpan timeout)
{
    var subscriberClient = await SubscriberServiceApiClient.CreateAsync();
    var stopwatch = Stopwatch.StartNew();

    try
    {
        var response = await subscriberClient.PullAsync(
            subscriptionName,
            returnImmediately: false,
            maxMessages: 1
        );

        if (response.ReceivedMessages.Count > 0)
        {
            return response.ReceivedMessages[0].Message;
        }
    }
    catch (RpcException e) when (e.StatusCode == StatusCode.DeadlineExceeded)
    {
        // Handle DeadlineExceeded exception if necessary
    }

    if (stopwatch.Elapsed > timeout)
    {
        // Handle timeout scenario
    }

    return null; // or appropriate response indicating no message was received
}