Prefect 3 - Error running task with a custom Docker container

[Lukem76]

I have a project with a simple python file that is containerized with a Dockerfile.
The dockerfile is:

FROM python:3.9-slim

WORKDIR /app

COPY . .

RUN pip install --no-cache-dir -r requirements.txt

CMD ["python", "prefect-test.py"]

prefect-test.py is a simple file that print a message. If I build the image locally with Docker Desktop and run it, it works. I already pushed it on docker hub in a public repo.

Now I want to start a Prefect server versione 3 (not 2, it's important). I created this Dokerfile:

FROM prefecthq/prefect:3-latest

WORKDIR /app

CMD ["prefect", "server", "start", "--host", "0.0.0.0"]

I build the image and ran it with:

docker build -t prefect-server .

docker run -d -p 4200:4200 --name prefect-server prefect-server

Then I access the GIU at http://localhost:4200

Everything works.

Now I want to create a work pool, start an agent and do a deploy. The target is to create a deploy file that run the Dockerimage that run the prefect-test.py file.

I create the work pool with:

prefect work-pool create --type docker my-work-pool

I started a worker:

prefect worker start --pool docker my-work-pool

Now I have to define the deployment. I created a file named deploy_buy.py in a different folder than the root where I have the Dockerfile and prefect-test.py file:

from prefect import flow

@flow(log_prints=True)
def buy():
print("Selling securities")

if __name__ == "__main__":
  buy.deploy(
	  name="my-deployment",
	  work_pool_name="my-work-pool",
	  image="docker.io/xxx/image"
  )

I copied it from official doc here: https://docs.prefect.io/v3/deploy/infrastructure-examples/docker

If I run the task, I see the message "Buying securities" in logs and at the end I see:
Finished in state Completed()
Process for flow run 'xxx' exited cleanly.
But the file prefect-test.py wasn't invoked, because I don't see any other log.

I used a different folder for deploy file because, if I don't do that, when I run deploy, it gives me an error saying that a Dockerfile is already in that folder. Executing in a different folder, it creates automatically a Dockerfile with this:

FROM prefecthq/prefect:3.1.4-python3.10
COPY . /opt/prefect/deploy/
WORKDIR /opt/prefect/deploy/

This is not clear, because that file doesn't contain anything about how to run my prefect-test.py file. For this reason I created another deploy file in the root of my project with this:

from prefect import flow
from prefect.docker import DockerImage

@flow(log_prints=True)
def buy():
    print("Selling securities")

if __name__ == "__main__":
    buy.deploy(
        name="my-deployment",
        work_pool_name="my-work-pool",
        image=DockerImage(
            name="docker.io/xxx/image",
            dockerfile="Dockerfile"
        ), 
        push=False
    )

I used dockerfile="Dockerfile" because I want Prefect to use my Dockerfile. If I remove that line and try to make a deploy, as I wrote above, Prefect tells me that I already have a Dockerfile in that folder. With that line, I get no error.
I don't want prefect create another build and push it, but I want it uses the image I already have on dockerhub. For this reason I put also:

push=False

If I deploy it and run the task, I get error:

timestamp,level,flow_run_id,task_run_id,message
	2024-12-01 15:06:40.326981+00:00,20,b89520d8-a1ca-4802-b2f0-c998e81c6029,,Worker 'DockerWorker e7ea2375-4f9b-4a61-a31e-a81e2b9c3ce0' submitting flow run 'b89520d8-a1ca-4802-b2f0-c998e81c6029'
	2024-12-01 15:06:41.402569+00:00,20,b89520d8-a1ca-4802-b2f0-c998e81c6029,,Completed submission of flow run 'b89520d8-a1ca-4802-b2f0-c998e81c6029'
	2024-12-01 15:06:44.210221+00:00,20,b89520d8-a1ca-4802-b2f0-c998e81c6029,,Opening process...
	2024-12-01 15:06:46.943062+00:00,20,b89520d8-a1ca-4802-b2f0-c998e81c6029,,Downloading flow code from storage at '.'
	2024-12-01 15:06:47.181580+00:00,40,b89520d8-a1ca-4802-b2f0-c998e81c6029,,"Flow could not be retrieved from deployment.
	Traceback (most recent call last):
	File ""<frozen importlib._bootstrap_external>"", line 850, in exec_module
	File ""<frozen importlib._bootstrap>"", line 228, in _call_with_frames_removed
	File ""deploy.py"", line 2, in <module>
	from prefect.docker import DockerImage
	ImportError: cannot import name 'DockerImage' from 'prefect.utilities.dockerutils' (/usr/local/lib/python3.9/site-packages/prefect/utilities/dockerutils.py)

	The above exception was the direct cause of the following exception:

	Traceback (most recent call last):
	File ""/usr/local/lib/python3.9/site-packages/prefect/engine.py"", line 414, in retrieve_flow_then_begin_flow_run
	flow = await load_flow_from_flow_run(flow_run, client=client)
	File ""/usr/local/lib/python3.9/site-packages/prefect/client/utilities.py"", line 51, in with_injected_client
	return await fn(*args, **kwargs)
	File ""/usr/local/lib/python3.9/site-packages/prefect/deployments/deployments.py"", line 264, in load_flow_from_flow_run
	flow = await run_sync_in_worker_thread(load_flow_from_entrypoint, str(import_path))
	File ""/usr/local/lib/python3.9/site-packages/prefect/utilities/asyncutils.py"", line 91, in run_sync_in_worker_thread
	return await anyio.to_thread.run_sync(
	File ""/usr/local/lib/python3.9/site-packages/anyio/to_thread.py"", line 33, in run_sync
	return await get_asynclib().run_sync_in_worker_thread(
	File ""/usr/local/lib/python3.9/site-packages/anyio/_backends/_asyncio.py"", line 877, in run_sync_in_worker_thread
	return await future
	File ""/usr/local/lib/python3.9/site-packages/anyio/_backends/_asyncio.py"", line 807, in run
	result = context.run(func, *args)
	File ""/usr/local/lib/python3.9/site-packages/prefect/flows.py"", line 1539, in load_flow_from_entrypoint
	flow = import_object(entrypoint)
	File ""/usr/local/lib/python3.9/site-packages/prefect/utilities/importtools.py"", line 201, in import_object
	module = load_script_as_module(script_path)
	File ""/usr/local/lib/python3.9/site-packages/prefect/utilities/importtools.py"", line 164, in load_script_as_module
	raise ScriptError(user_exc=exc, path=path) from exc
	prefect.exceptions.ScriptError: Script at 'deploy.py' encountered an exception: ImportError(""cannot import name 'DockerImage' from 'prefect.utilities.dockerutils' (/usr/local/lib/python3.9/site-packages/prefect/utilities/dockerutils.py)"")"
	2024-12-01 15:06:47.600011+00:00,20,b89520d8-a1ca-4802-b2f0-c998e81c6029,,Process for flow run 'nimble-wildcat' exited cleanly.

This is strange because that import is the same of the one in the example on Prefect v3 docs.

Any ideas?

Thanks

[zzstoatzz]

hi @Lukem76!

I notice you say

I don't want prefect create another build and push it, but I want it uses the image I already have on dockerhub. For this reason I put also:

If this is the case, all you need to do is update your .deploy() call like this

if __name__ == "__main__":
    buy.deploy(
        name="my-deployment",
        work_pool_name="my-work-pool",
        job_variables={"image": "your-registry/your-image:your-tag"},
        push=False
    )

to tell prefect use your image that you already have built on dockerhub, no DockerImage class required

if you do want to build your own image, keep reading

i am not yet sure about the ImportError you are seeing, but i have produced a self-contained, working example that will hopefully help you here, which uses this Dockerfile.

FROM prefecthq/prefect:3-latest

WORKDIR /opt/prefect

COPY examples/run_flows_on_docker/buy.py examples/run_flows_on_docker/buy.py

There are numerous valid ways to write your dockerfile, but I did it like this because I was running my script from the root of the repo and wanted to bake my flow into the image to reproduce your situation

» python examples/run_flows_on_docker/buy.py

[Lukem76]

Hi zzstoatzz and thanks.

I was trying to use DockerImage import because I was running deploy from the root of my project where I already have Dockerfile. Running it without using DockerImage where I can use dockerfile="Dockerfile", gives me the error "You already have a Dockerfile" or something similar.
And if I try to use dockerfile="Dockerfile" without using DockerImage it gives me error because syntax is not correct.
Is it a correct approach to move the deploy file under another folder where the deploy can create its own temporary Dockerfile without the problem of finding an already existent one?
Thanks

[Lukem76]

I tried also to change deploy file like this:

`from prefect import flow
import subprocess

@flow
def buy():
    subprocess.run(["python", "prefect-test.py"])

if __name__ == "__main__":
    buy.deploy(
        name="yyy",
        work_pool_name="yyy",
        image="docker.io/xxx/image",
        push=False
    )`

It seems to work now.
but I had to add subprocess.run(["python", "prefect-test.py"]) in def buy.

Is this the correct approach?

[Lukem76]

Maybe I'm missing something because if I delete my project Dockerfile and put this deploy file:

from prefect import flow
import subprocess

@flow
def buy():
    subprocess.run(["python", "prefect-test.py"])

if __name__ == "__main__":
    buy.deploy(
        name="my-deployment",
        work_pool_name="my-work-pool",
        image="docker.io/user/repo:image",
        push=False
    )

in the root of my project and run python deploy and finally I run the task, it works

But if I leave my Dockerfile in the root and move the deploy file in a subfolder and run python deploy from there, it takes only few seconds to finish (while before it took longer), and when I run the task, it does nothing. It seems that the final image is empty this way

I don't understand which is the right way to do this.
I think also doc is a little bit confusing about this.

I don't want to delete my dockerfile because it's a separate project that could be run on another environment.
What am I missing?

P.S.: using job_variables={"image": "your-registry/your-image:your-tag"} it gives me error of "image" directive missing

Thanks

[zzstoatzz]

hi @Lukem76 - sorry if I confused with my example, but you generally shouldn't have to use subprocess.run to invoke prefect flows - I was just using it to create a self-contained MRE of your situation. Can you explain why you are using subprocess.run above?

I would generally recommend writing your own Dockerfile, you can use the same one for many deployments if you want, or define one per project (i recommend the latter)

in addition to the docs, there is an example here which should be roughly what I'd recommend.

P.S.: using job_variables={"image": "your-registry/your-image:your-tag"} it gives me error of "image" directive missing

It's easiest to help with actual stack traces and the code you used to produce that stack trace

[Lukem76]

Hi @zzstoatzz

Sorry if I was not clear in my explanation but I'm new to prefect and I'm really interested in using it.

This is my situation:
I have an already containerizer project that is a simple python file "prefect-test.py" that print a simple message (this is only an example, I could have different projects like this that do different things).
Its Dockerfile is:

FROM python:3.9-slim

WORKDIR /app

COPY . .

RUN pip install --no-cache-dir -r requirements.txt

CMD ["python", "prefect-test.py"]

I need to schedule a job in Prefect v3 in order to run it every hour. I want Prefect to run my container, I don't want to change my python file to adapt it to Prefect (adding decorators and so on...). My original project can't be changed.

What I can do is to create a deploy file with something that can execute my container inside Prefect.

How do I have to write my deploy file exactly?
Using a deploy file like this

from prefect import flow
import subprocess

@flow
def buy():
    WHAT DO I HAVE TO PLACE HERE???

if __name__ == "__main__":
    buy.deploy(
        name="my-deployment",
        work_pool_name="my-pool",
        image="xxx/yyy",
        push=False
    )

It's not clear for me what to insert inside "def buy():"
I tried with subprocess.run passing "prefect-test.py" because chatGPT suggested me :).
Do I have to insert the name of a function inside prefect-test.py file?

Using subprocess.run(["python", "prefect-test.py"]), deleting my project Dockerfile and running python deploy.py placed in the root, the deployment seems to work because I can run it and see the log printed by my prefect-test.py.

As I wrote before, If I create a deploy.py file inside the root of my project without deleting my project Dockerfile and run "python deploy.py", it tells me that I have already a Dockerfile (the file of my original project)

Using in the deploy.py file something like this:

image=DockerImage(
            name="my_image",
            tag="deploy-guide",
            dockerfile="Dockerfile"
)

forcing this way to use my original project Dockerfile, then it gives me an error:
prefect.exceptions.ScriptError: Script at 'deploy.py' encountered an exception: ImportError("cannot import name 'DockerImage' from 'prefect.utilities.dockerutils' (/usr/local/lib/python3.9/site-packages/prefect/utilities/dockerutils.py)").

I also tried to update python version in my image, using 3.11 but the error is the same.

In the github ecample you lined me there are too many examplese an I don't know whato to see.

I wrote you all the code I currently have except prefect-test.py because it only print a simple message. I don't have anything else to show you.

Thanks