@Marvin How do I pass the prefect run context to a code sequence which is being executed in a different thread?

To pass Prefect run context to a code sequence which is being executed in a different thread, you can use the

prefect.context

module to access the current run context with

prefect.context.get_run_context()

. If you are within a flow or task run, this function will provide the context, and if a task run context is available, it will be returned even if a flow run context is also available. Here's a snippet from the Prefect documentation that explains how to access the run context, including explicit access to the flow run or task run context:

python
from prefect.context import FlowRunContext, TaskRunContext

flow_run_ctx = FlowRunContext.get()
task_run_ctx = TaskRunContext.get()

Note that

get_run_context

,

FlowRunContext.get()

, and

TaskRunContext.get()

might return

None

if the context is not available, but they will not raise an error. To use this in a thread, make sure to capture the context outside of the thread and then pass it as an argument to the target function being executed by the thread. This is necessary because Prefect contexts are not inherently thread-safe and may not be available directly within a new thread. Here is an example based on the information provided in the documentation:

python
import threading
from prefect import flow
from prefect.context import get_run_context

@flow
def main_flow():
    # Get the current run context before creating a thread
    flow_context = get_run_context()

    # Function that will be run in a thread, takes the context as an argument
    def thread_function(ctx):
        # Use the provided context here
        pass
    
    # Create and start a thread, passing the captured context
    thread = threading.Thread(target=thread_function, args=(flow_context,))
    thread.start()
    thread.join()

# Run the flow
main_flow()

Below are the relevant links to the Prefect documentation and a related GitHub issue: - Runtime Context in Prefect documentation - GitHub issue regarding logging context with DaskTaskRunner and Task.map - Concurrency limits in Prefect documentation (Note: this is less related but found in the search) Remember that this context passing strategy is for manual thread management. Prefect also has native support for concurrency using `TaskRunner`s like

ConcurrentTaskRunner

, which manages the context for you when it runs tasks concurrently.

@Marvin If I am passing the run context into my thread, is there a way for me to then set the run context again inside of this thread?

Unfortunately, there are no results explicitly describing a process for setting or modifying the Prefect run context from within a new thread. The Prefect documentation does not detail a method for re-setting or modifying the run context once within a thread. The Runtime Context documentation suggests that contexts are meant to be read and not written to, as there are getters but no setters mentioned. The Prefect runtime context is built around the concept of being associated with the execution of flow runs and task runs that occur within Prefect's execution model. It appears that the context is designed to be accessed rather than modified, particularly because altering the context in distributed environments could result in unexpected behavior or conflicts. This GitHub issue on concurrent flows suggests that Prefect does not currently support certain operations in a multithreaded context, but there are no results related to modifying the context in a thread. Given the absence of details on setting the context within threads, it would seem that to maintain context consistency, Prefect's design might actively prevent modification of the context from within threads. If you need thread-specific variables or context, you might have to implement thread-local storage within your application code, separate from the Prefect context system. Thread-local storage allows you to have data that is global to a thread but not shared between threads. You may also open a discussion or issue in the Prefect GitHub repository to inquire about best practices for managing context within threaded applications or request future enhancements to the library.