@Marvin, what might cause a flow which almost always runs to completion without issue, to randomly run forever in Prefect 2?

If your Prefect 2 flow, which usually completes successfully, randomly runs indefinitely, there could be several potential causes based on Prefect 2.x behavior and issues reported by the community: 1. Concurrency Limits and Queue Management: If there are concurrency limits in your work pool that are reached, it might prevent your flow from executing as expected. This is particularly relevant if you have a setup with different priority levels for scheduling. Check if concurrency limits or priority settings in your work pool are affecting execution (source from documentation). 2. Handling of Subflows: If your main flow run starts subflows asynchronously, the subflows might continue to run even if the main flow run has completed or failed. This can make the main flow appear to run indefinitely, especially if it waits for these subflows. Consider implementing logic to cancel subflows when a parent flow enters a terminal state (source from GitHub issue). 3. Agent Disconnections or Failures: If a flow run's agent goes down unexpectedly, the flow might remain in a "running" state. This could occur if the agent process is interrupted, leaving the flow without a way to transition to a completed state (source from GitHub issue). To resolve this issue, review these aspects in your flow and deployment setup. You may need to adjust concurrency settings, improve handling of subflows, or ensure agent stability to prevent such scenarios. If the issue persists, looking into logs for any error messages or unexpected events during flow execution could provide additional insights.

@Marvin Would updating to Prefect 3 address this issue?

Updating to Prefect 3 might offer some improvements that could indirectly address issues with flows running indefinitely, but it's not guaranteed to solve this specific problem entirely. Here are some relevant points from Prefect 3's capabilities and recent release notes: 1. Pause and Resume Features: Prefect 3 has introduced features to pause and resume flow runs, which can offer more control over flow execution. Although these features might not directly prevent a flow from running indefinitely, they provide better control when managing long or complex workflows (source). 2. Handling of Flow and Task Failures: Prefect 3 introduces changes in how flow and task failures are handled. Task failures within a flow do not automatically cause the flow run to fail unless they affect the flow’s return value or raise an uncaught exception. This might help in scenarios where unforeseen task behavior causes a flow to hang. You may need to explicitly handle task failures to ensure critical tasks don't cause issues (source). 3. Bug Fixes and Improvements: Prefect 3 continues to refine settings and fix bugs that could contribute to better overall stability and prevent unusual run behavior, as seen in the latest release notes. If you're considering an upgrade to Prefect 3, it might provide improvements that enhance flow management and stability. However, it would be wise to ensure that your flows and task management logic are robust and address the potential issues highlighted earlier. Always test significant updates in a controlled environment before full deployment.

When these flow runs hang, sometimes it appears that the flow code never gets executed at all, as the first line of code is to log a status message, which doesn't appear in the log for that hung run. What could be causing this, @Marvin ?

The issue of a flow run appearing to hang without executing any tasks or logging any status messages can be caused by several underlying factors, particularly in Prefect 2.x. Here are some potential causes: 1. Agent Configuration and Infrastructure Issues: If there's a problem with the infrastructure on which Prefect agents are running (e.g., Docker, Kubernetes, etc.), it might prevent the flow from being executed properly. Make sure that the agents are correctly configured and actively monitoring for tasks. Reference documentation on different infrastructure options can provide more insights (source). 2. Concurrency Limits and Prioritization: The queuing mechanism may have concurrency or prioritization settings that delay or block certain flow runs, particularly if your setup involves multiple queues and varying priorities. Ensure that your work queues are not maxed out and that prioritization does not lead to indefinite delays (source). 3. Deployment Configuration Issues: Errors in the deployment configuration, particularly involving templating and parameter evaluation, might prevent a flow from starting properly. This includes issues with parameter placeholders in templates not being correctly evaluated or triggering errors that prevent flow initiation (source). 4. Logging not Set Up Correctly: If logging is not configured properly, you might not see expected log messages. Double-check that the logging setup is valid and that any logging configuration files or environments are correctly specified in your setup. It would be beneficial to thoroughly check your deployment, agent, and infrastructure setup to ensure all components are correctly configured and operational. You might also want to enable more verbose logging or use debugging features to catch and diagnose these initiation issues as they occur.

I'm not using an agent, but rather running a python script as a service which is using the 'serve' method @Marvin

Using the

serve

method in Prefect to run a flow as a service could lead to scenarios where the flow doesn't start or execute properly due to several factors: 1. Synchronization and Execution Issues: There have been updates and fixes related to how the

serve

method synchronizes and handles shutdowns. These updates ensure that the

serve

method behaves more consistently, particularly in response to interruptions like a KeyboardInterrupt. Ensuring you are using a version of Prefect where these issues are addressed might help (source). 2. Flow Timeout Misconfigurations: There were issues with flow timeouts being initiated incorrectly, specifically starting before upstream dependencies were fully resolved. This could lead to scenarios where the flow seems to hang as it waits for dependencies or conditions that are not yet satisfied (source). If you face issues with the

serve

method where the flow does not start as expected, ensure that you have updated to the latest Prefect version and have reviewed any timeout and dependency configurations that might affect flow execution. These areas might need adjustment to prevent such hanging issues.