Subscribers don't reconnect to the subscription when there is no traffic

[Artee]

I am using StreamingPull mechanism to receive messages from the Pub/Sub.

Here is only 2 overriding for default SubscriberFactory settings:

@Bean
    public SubscriberFactory subscriberFactory(
            GcpProjectIdProvider id,
            TransportChannelProvider channelProvider,
            CredentialsProvider credentialsProvider) throws IOException {
        PubSubConfiguration config = new PubSubConfiguration();
        config.initialize(id.getProjectId());
        config.getSubscriber().setParallelPullCount(2); //overriding1
        config.getSubscriber().setMaxDurationPerAckExtension(0); //overriding2
        DefaultSubscriberFactory factory = new DefaultSubscriberFactory(id, config);
        factory.setChannelProvider(channelProvider);
        factory.setPullEndpoint(url());
        factory.setCredentialsProvider(credentialsProvider);
        return factory;
    }

The second overriding helps me to resolve this issue: https://stackoverflow.com/questions/77711978/my-subscriber-receive-a-message-from-gcp-subscription-w...

I don't have any other overriding, but when there is no trafic I get this picute of number_of_open_streaming_pull subscription metric:

I know that Pub/Sub prefers to avoid a long-running sticky connection and client library should reopens a StreamingPull connection.. It works well when there is an active traffic. Any advice I can achive a stable reopens even when there is no traffic?

I found some issues about this:
spring-attic/spring-cloud-gcp#1005 - resolved as internal pub/sub issue
spring-attic/spring-cloud-gcp#2552 - adviced to use Synchronous Pull, but I want to go further with Asynchronous pull because for my app lower latency and higher throughput are very important.

We use spring-cloud-gcp-pubsub:5.0

[ms4446]

The behavior you're experiencing with Pub/Sub, where subscribers do not always reliably reconnect to the subscription when there's no traffic, can be challenging, especially when relying on the StreamingPull mechanism for real-time message processing. While StreamingPull connections are designed to be long-lived, factors like configuration, internal service behavior, and network conditions can affect reconnection stability.

Given you've addressed a separate acknowledgment configuration issue, here are several additional techniques to improve the stability of your StreamingPull connections, even during low-traffic periods:

Implementing Heartbeat Messages

• Design Consideration: Craft heartbeat messages with a unique attribute or payload pattern that clearly distinguishes them from regular application messages. This could be a specific key-value pair in the message attributes.

• Publishing Strategy: Set a regular interval for publishing heartbeat messages that balances maintaining the connection's liveliness without introducing unnecessary overhead. A typical interval might range from every few minutes to every hour, depending on your application's tolerance for connection startup latency.

• Subscriber Handling: Update your subscriber logic to immediately recognize and discard heartbeat messages, ensuring they do not impact your application's normal processing flow.

Adjusting Subscriber Settings

• Flow Control Optimization: Experiment with maxOutstandingElementCount and maxOutstandingRequestBytes settings to find an optimal configuration that prevents your subscriber from being overwhelmed by messages while minimizing idle connection times.

• Guidance and Experimentation: Consult the latest Google Cloud Pub/Sub documentation for recommendations on subscriber configurations. Conduct thorough testing under various traffic conditions to understand the impact of different settings on connection stability and message processing efficiency.

Implementing Connection Reinitialization Logic

• Inactivity Detection: Develop a mechanism to monitor for signs of connection inactivity or unexpected disconnections. This could involve tracking the timestamp of the last received message and comparing it against a predefined inactivity threshold.

• Reconnection Procedure: Design a reconnection procedure that can safely restart the subscriber or establish a new StreamingPull connection without risking message duplication or loss. This procedure should include steps for gracefully shutting down the current connection, ensuring all pending acknowledgments are handled, and then initiating a new connection.

Keeping Dependencies Updated

• Update Schedule: Implement a regular schedule for reviewing and updating your project's dependencies, including spring-cloud-gcp-pubsub, to benefit from the latest performance improvements and bug fixes.

• Monitoring Changes: Pay close attention to the release notes and change logs for the libraries you use, especially for updates that might affect connection management, performance, or reliability.

Enhancing Logging and Monitoring

• Detailed Logging: Enhance your logging strategy to capture detailed information about the lifecycle of your StreamingPull connections, including connection establishment, disconnection, reconnection attempts, and any errors or warnings.

• Custom Metrics: Develop custom metrics to monitor the health and behavior of your Pub/Sub connections. Track metrics such as connection uptime, number of reconnections over time, and the duration of any disconnections to identify patterns and potential areas for improvement.

Addressing StreamingPull connection stability in Pub/Sub, especially during low-traffic periods, requires a multifaceted approach. By implementing heartbeat messages, optimizing subscriber settings, establishing reconnection logic, keeping dependencies up to date, enhancing logging and monitoring, and engaging with the community and support, you can significantly improve the reliability and efficiency of your Pub/Sub messaging system. Continuous monitoring, testing, and adjustment based on observed system behavior and performance metrics will be crucial for maintaining an effective Pub/Sub implementation.