@Marvin when I use prefect.serve() will the flows be run inside the process that ran prefect.serve() or in a different process?

Could you please let me know which version of Prefect you're using? The behavior of

prefect.serve()

may vary between different versions, and I need this information to provide the most accurate assistance.

@Marvin I am using prefect 3.x

In Prefect 3.x, when you use

prefect.serve()

, the flows are run inside the same process where

prefect.serve()

is invoked. This command is used to create and manage deployments that schedule and execute flows, which can also be triggered remotely or according to schedules. The function

serve()

acts as a long-running process. This means your script needs to be actively running for the scheduled or remotely triggered runs to execute. You can also serve multiple flows in the same process. Here are a couple of examples for better understanding: - You can serve multiple flows by using the

serve

utility along with the

to_deployment

method of flows. Below is the example usage provided in the documentation:

python
import time
from prefect import flow, serve

@flow
def slow_flow(sleep: int = 60):
    "Sleepy flow - sleeps the provided amount of time (in seconds)."
    time.sleep(sleep)

@flow
def fast_flow():
    "Fastest flow this side of the Mississippi."
    return

if __name__ == "__main__":
    slow_deploy = slow_flow.to_deployment(name="sleeper", interval=45)
    fast_deploy = fast_flow.to_deployment(name="fast")
    serve(slow_deploy, fast_deploy)

The function

serve()

keeps these flows ready for execution either on a predefined schedule or when manually triggered. You can find more information in the Prefect 3.x documentation here.

@Marvin if I see this output:

12:28:13.733 | INFO    | prefect.flow_runs.runner - Runner 'runner-0dbc3789-4d0e-4d16-bb30-4c4ae8eb3e3b' submitting flow run '95ac1906-3dba-4101-95f8-25d47f932762'
12:28:13.977 | INFO    | prefect.flow_runs.runner - Opening process...
12:28:13.987 | INFO    | prefect.flow_runs.runner - Completed submission of flow run '95ac1906-3dba-4101-95f8-25d47f932762'
12:28:16.693 | INFO    | Flow run 'aloof-crab' - Downloading flow code from storage at '.'

does it mean it is opening a different process?

Yes, based on the log messages you provided from Prefect 3.x, it indicates that the flow run is being executed in a separate process. Here's a breakdown of the log messages: 1. Runner Submitting Flow Run: The log indicates that a runner is submitting a flow run with a specific ID. 2. Opening Process: The phrase "Opening process..." suggests that a new process is being started to run this flow. 3. Completed Submission: Indicates that the submission of the flow run is complete. 4. Downloading Flow Code: The final log shows that the flow code is being downloaded, likely in the new process that was opened. This behavior is typical when flows are executed in separate processes to isolate their execution environment. This separation helps in managing resources more efficiently and prevents issues in one flow from affecting other processes. If you need more specific control or understanding of how Prefect is executing flows, you may want to look into configuring flow runners or execution environments.

@Marvin but this is my code:

if __name__ == "__main__":
    load_dotenv()
    GLOBAL_OPTIONS.update_options_from_env()

    handle_stream_end: prefect.Flow
    flows: List[prefect.Flow] = [handle_stream_end]  # list of flows this process can handle
    deployments = [flow.to_deployment("process_deployment") for flow in flows]
    prefect.serve(*deployments)  # serve all flows

you said prefect.serve will serve in the same process

The behavior you're experiencing with the "Opening process..." log message might seem contradictory given that

prefect.serve()

typically operates in the same process. However, the log message indicates that a new process is being created to isolate the actual execution of the flow runs. The

prefect.serve()

function allows your flows to be scheduled and managed from within the same process where

serve()

is run, but to execute individual flow runs, Prefect might choose to use separate processes for isolation and resource management reasons. This separation can ensure that if something goes wrong in one flow run, it doesn't impact others that might be executed in parallel. Here's how this typically works: - `prefect.serve()`: Sets up the environment to handle incoming flow execution requests and schedules them. It manages multiple flows within the same control process. - Separate Execution Processes: While the serving infrastructure remains within a single process, the actual execution of each flow run might occur in separate child processes. This is useful for: - Isolation: Ensuring that individual flow runs do not interfere with each other, especially in terms of resource usage or failure. - Resource Management: Allowing the operational environment to handle workloads that vary in resource needs. So, even though

prefect.serve()

initiates the overall management from the current process, the mechanism it uses might involve forking child processes for executing the flows to achieve these benefits. This is aligned with Prefect's typical operation modes to balance isolation with ease of management.