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<@ULVA73B9P> Is it possible to preserve existing s...
# ask-marvin
d
@Marvin Is it possible to preserve existing schedules in the UI for flows when they are redeployed using`prefect deploy -a` and a 
prefect.yaml
file?
m
It looks like preserving schedules when using 
prefect deploy -a
is a topic with some complexity. While documentation excerpts show how to manage schedules within the 
prefect.yaml
file, they don't directly address preserving existing schedules upon redeployment. Here are key points: 1. Schedules in 
prefect.yaml
: You can define schedules directly in the 
prefect.yaml
file under the 
deployments
section. (Source: https://docs.prefect.io/latest/concepts/schedules/) 2. GitHub Discussion & PRs: A pull request (PR) on GitHub seems to add CLI commands and adapters to work with multiple schedules, potentially preserving prior schedules. However, reading through the linked issue might provide further insights. (Source: https://github.com/PrefectHQ/prefect/pull/11726) To ensure your schedules are preserved, you might want to add them directly into the 
prefect.yaml
before deploying, given the current version's capabilities and ongoing improvements. If you need more specific step-by-step guidance, feel free to ask!
d
@Marvin In that case, can I specify schedules using cron clocks in my python, and have then honoured during an automated deployment using 
prefect deploy
?
m
Yes, you can specify schedules using cron clocks in your Python code and have them honored during an automated deployment using 
prefect deploy
. Here's a simple example of how you can define a cron schedule in Python, within the 
prefect.yaml
file, and have it honored during deployment: 1. Define the Flow in Python:
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python
from prefect import flow

@flow(log_prints=True)
def my_workflow():
    print("This is my workflow")

if __name__ == "__main__":
    my_workflow()
2. Create a 
prefect.yaml
File: Add a 
schedules
section to the 
prefect.yaml
file to define your cron-based schedule.
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yaml
deployments:
  - name: my-deployment
    schedules:
      - cron: "0 12 * * *"  # Runs every day at noon
        timezone: "America/New_York"
        active: true
3. Automate Deployment: Use the 
prefect deploy -a
command to deploy your flow with the defined schedules.
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bash
prefect deploy -a
The Prefect scheduler will automatically honor the cron schedule specified in your 
prefect.yaml
file during the deployment. For more detailed information, refer to the Schedules section of the Prefect documentation: https://docs.prefect.io/latest/concepts/schedules/ Hope this helps! If you need any more details, feel free to ask.
d
@Marvin Not what I asked for, my 
prefect.yaml
is auto-generted in my ci/cd pipeline, I need to differentiate between each flow as only some have schedules and they're always different.
m
If you want to dynamically define and preserve schedules programmatically in your Python code and ensure they're deployed correctly within your CI/CD pipeline without manually editing the 
prefect.yaml
, you can do so directly in your deployment script. Here's how you can handle it: 1. Define the flow with the schedule in the Python script: You can use the 
CronSchedule
or other schedule classes directly within your deployment script.
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python
from prefect import flow, task
from prefect.deployments import Deployment
from prefect.client.schemas.schedules import CronSchedule

@flow
def my_workflow():
    print("This is my workflow")

if __name__ == "__main__":
    # Define the cron schedule
    cron_schedule = CronSchedule(cron="0 12 * * *", timezone="America/New_York")
    
    # Create a deployment with the schedule
    deployment = Deployment.build_from_flow(
        flow=my_workflow,
        name="my-scheduled-deployment",
        schedule=cron_schedule
    )
    
    # Apply the deployment
    deployment.apply()
2. Run your deployment script in your CI/CD pipeline: Ensure that your CI/CD pipeline runs this script. When you call 
deployment.apply()
, Prefect will handle creating and applying the 
prefect.yaml
configuration automatically, preserving your schedule definitions. By doing this, you keep everything programmatically defined and dynamic, eliminating the need for manual configuration in the 
prefect.yaml
file. By integrating this script into your CI/CD pipeline, your deployments will have their respective schedules correctly honored. For more detailed documentation, you can refer to the Prefect scheduling documentation: https://docs.prefect.io/latest/concepts/schedules/ I hope this helps! If you have any more questions, feel free to ask.
d
@Marvin Is it not possible to define these as a parameter in the 
@flow
decorator in python?
m
Based on the available documentation and examples, it does not appear that defining schedules as parameters within the 
@flow
decorator is supported in Prefect. Schedules need to be specified through deployment configurations or through the 
Deployment
class. Here's a recommended approach: 1. Define the Flow Without Schedule in Python:
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python
from prefect import flow

@flow
def my_workflow():
    print("This is my workflow")
2. Programmatically Add Schedules Using the Deployment Class:
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python
from prefect.deployments import Deployment
from prefect.client.schemas.schedules import CronSchedule

# Define the cron schedule
cron_schedule = CronSchedule(cron="0 12 * * *", timezone="America/New_York")

# Create a deployment with the schedule
deployment = Deployment.build_from_flow(
    flow=my_workflow,
    name="my-scheduled-deployment",
    schedule=cron_schedule
)

# Apply the deployment
deployment.apply()
Integrating this script into your CI/CD pipeline will allow you to dynamically define different schedules for each flow without needing to manually handle the 
prefect.yaml
file. For more detailed information, you can refer to the Prefect scheduling documentation: https://docs.prefect.io/latest/concepts/schedules/ I recommend following this approach to ensure that your schedules are correctly recognized and applied during deployment. If you have more questions or need further assistance, feel free to ask!
d
Thanks @Marvin, for completeness, I'll share my final solution. I added anchors to my prefect.yaml and used grep in bash to programmatically check for these before adding references to the deployment section later
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# prefect.yaml
...
definitions:
  ...
  schedules:
    - &deployment_name
        - cron: "0 6,22 * * *"
          timezone: "Europe/London"
          active: true
...
m
Great solution! Using YAML anchors and 
grep
in your CI/CD pipeline to manage and dynamically inject schedules is a clever way to handle this. Here’s a quick rundown of your approach for completeness: 1. Define Schedules with Anchors:
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yaml
# prefect.yaml
...
definitions:
  ...
  schedules:
    - &unique_schedule
        cron: "0 6,22 * * *"
        timezone: "Europe/London"
        active: true
...
2. Use Grep in Your CI/CD Pipeline: You can then use shell scripts to check for the presence of specific flows and inject necessary schedules or configurations accordingly.
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bash
# Example shell script to update prefect.yaml programmatically

if grep -q "deployment_name" prefect.yaml; then
  echo "Adding schedule to deployment..."
  # Add code to insert the anchor reference in the necessary place within your prefect.yaml
  sed -i '/deployments:/ a \ \ - <<: *unique_schedule' prefect.yaml
else
  echo "Deployment not found. Skipping schedule injection."
fi
This method allows you to maintain flexibility and programmatically control your configurations, which is excellent for managing multiple flows with different schedules. If there's anything else you need or further questions, feel free to reach out. Well done on the implementation!
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