Google Cloud Pub/Sub is a powerful messaging service designed for high-throughput, asynchronous messaging between services. It’s especially effective in event-driven architectures, where systems need to communicate reliably and at scale. However, managing message acknowledgment efficiently becomes crucial when dealing with long-running processing tasks. A common issue that developers encounter is the dreaded ack_deadline_exceeded error, particularly when messages are not acknowledged within the default time window. This article explores how such issues arise and how adopting a flow control and acknowledgment extension pattern can help prevent so-called message “redelivery storms.”

TLDR

Long-running subscribers in Google Cloud Pub/Sub often trigger ack_deadline_exceeded errors due to delayed processing, leading to message redelivery. This can overwhelm subscribers and degrade system performance. Implementing flow control and dynamic acknowledgment deadline extensions can prevent this problem by avoiding backlogs and giving more time for message processing. This combination ensures reliable message delivery without system overload or inefficient retries.

Understanding ack_deadline_exceeded in Pub/Sub

In Google Cloud Pub/Sub, when a subscriber receives a message, it has a limited time—known as the acknowledgment deadline—to process and acknowledge that message. By default, this deadline is 10 seconds. If the subscriber doesn’t send an acknowledgment (ACK) by that deadline, the message is considered unprocessed and gets redelivered.

This behavior is generally beneficial for ensuring message reliability, but it can cause unintended consequences in certain situations. In long-running or high-load subscriber systems, if messages are not acknowledged quickly enough, they start getting redelivered. This creates a domino effect or “redelivery storm” where:

  • Subscribers get loaded with duplicate messages.
  • System performance degrades.
  • Processing throughput declines due to repetitive work.
  • Downstream systems may be overwhelmed with duplicate requests or actions.

The Double-Edged Sword of Message Redelivery

While redelivery ensures that no message goes unprocessed, it can turn into a bottleneck if not managed properly. The core issue is that Pub/Sub does not inherently know how long each message takes to process. It has no visibility into your application’s logic, so it treats a message as unacknowledged and reschedules it for delivery—even if the subscriber is still working on it.

If your subscriber is under-resourced or receives a high volume of long-running tasks, redelivery begins to compound. This intensifies resource contention, creating a vicious cycle of retries, partial restores, and congested workers. Even with exponential backoff in place, it may not be enough to prevent this feedback loop from spiraling out of control.

Flow Control: The First Line of Defense

Flow control in Google Cloud Pub/Sub is used to limit the number of messages or the amount of memory used by messages being processed by the subscriber at any one time. It prevents the local message queue in the subscriber’s environment from being overwhelmed by slowing or pausing message delivery when the limits are reached.

There are typically three main thresholds you can configure:

  • Max Outstanding Messages: Limits the number of messages held in memory awaiting processing.
  • Max Outstanding Bytes: Caps the total memory consumption of unprocessed messages.
  • Max Concurrency: Governs how many threads or async workers handle message processing concurrently.

By setting conservative flow control limits, you ensure that each message has a fair opportunity to be processed fully without flooding the system. However, flow control alone doesn’t fully solve cases where each message takes longer than the default acknowledgment deadline of 10 seconds.

The Role of Acknowledgment Deadline Extension

To tackle long-running processing tasks, Pub/Sub provides automatic and manual acknowledgment deadline extension capabilities. When enabled, the subscriber client will periodically extend the deadline for messages currently being processed, preventing them from being redelivered prematurely.

There are two options here:

  1. Automatically managed extensions: Let the Pub/Sub client library periodically extend acknowledgment deadlines on behalf of the application.
  2. Custom extensions: Manually manage and extend deadlines based on your business logic, typically through a background loop or monitoring thread.

Automatic extension is preferable in most cases because it requires less maintenance and reduces operational complexity. However, custom logic can be useful in highly specialized or asynchronous processing environments.

Combining Flow Control and Ack Extension: The Winning Combo

Flow control limits how many messages are actively processed, while acknowledgment extensions ensure each message has enough time to complete. Combined, they allow systems to:

  • Process fewer messages concurrently.
  • Extend processing time intelligently without triggering redeliveries.
  • Maintain system stability even under high load.
  • Prevent duplicated work caused by unintentional retries.

For example, suppose a subscriber is configured with a maximum of 10 outstanding messages and automatic acknowledgment extensions. Even if each task takes 30-60 seconds to complete, the system can extend deadlines as needed while keeping the workload capped. This approach virtually eliminates message storms and unbounded retries.

Real-World Example: Image Processing Service

Consider a microservice that processes user-uploaded images. Each image takes 40-60 seconds to resize and store across multiple formats. If this service receives Pub/Sub messages every few seconds and processes each in parallel without limits or extensions, it will quickly overwhelm itself. Within a minute, Pub/Sub begins resending messages that are still being processed. The system becomes backed up, and the performance drops significantly.

By applying:

  • Flow control to process only 5 messages at a time, and
  • Auto-extend settings to push acknowledgment deadlines up to 10 minutes,

the system processes the queue steadily without retries or bottlenecks—ensuring every image gets processed just once.

Conclusion

The ack_deadline_exceeded error in Google Cloud Pub/Sub is a symptom of deeper architectural and configuration imbalances, particularly in systems with long-running processing tasks. While the default settings may suffice for simple pipelines, scalable and resilient systems require careful resource and acknowledgment management.

Combining flow control with acknowledgment deadline extensions offers a powerful pattern that prevents message redelivery storms. It’s a best practice for high-reliability, distributed applications in cloud environments and ensures that message-driven systems stay performant and responsive under load.

FAQs

What is ack_deadline_exceeded in Pub/Sub?
It’s an error that occurs when a subscriber doesn’t acknowledge a message within the specified time limit. The message is then marked for redelivery.
How can I avoid message redelivery in long-running tasks?
By using flow control to manage how many messages you process at a time and allowing automatic acknowledgment deadline extensions during processing.
Is it better to use auto or manual ack deadline extension?
In most cases, auto extension is sufficient and reduces complexity. However, manual control may be appropriate in systems with custom timing logic.
Will redelivered messages be processed more than once?
Yes, unless you implement idempotent processing logic. Redelivered messages are treated as new messages by Pub/Sub unless otherwise handled in your code.
Can flow control be dynamically adjusted?
Currently, flow control settings are typically static, but you can restart or reconfigure subscribers to change limits based on observed load.