Another question, which may be a rather naive one. What happens if the execution environment where a Prefect flow is deployed gets deleted? For example, an EC2 instance gets terminated for some reason. I'm guessing that because of the lack of a persistence layer, that flow is lost.

I need to process streaming data from Kafka and then send the result back to output. I was able to achieve my objective using the following approach. Wanted to check if this is the right way or is there a better way to process streaming data

def get_stream_data():
    consumer = KafkaConsumer("my-topic",
                             bootstrap_servers=['localhost:9092'],
                             value_deserializer=lambda m: json.loads(m.decode('ascii')),
                             consumer_timeout_ms=1000)

    messages = [messages.value for messages in consumer]
    if len(messages) == 0:
        raise prefect.engine.signals.SKIP("No data received")
    return messages

def process_itemwise(data):
    data["result"] = 1
    return data

def publish_result(data):
    producer = KafkaProducer(bootstrap_servers=['localhost:9092'],
                             value_deserializer=lambda m: json.dumps(m).encode('ascii')))
    producer.send("results", data)
    producer.flush()

with Flow("custom") as flow:
    data = get_stream_data()
    result = process_itemwise.map(data)
    publish_result.map(result)

while True:
    flow.run()

Does Prefect core support hyper parameter optimization libraries such as

Tune

?

Question on using an external Dask cluster (DaskExecutor) running in adaptive mode -- we notice that running a Flow that maps over elements (say 100 elements) only seems to use the number of Dask workers that existed at the time the flow run started. We would have expected the Dask cluster to begin scaling up workers (since it's in adaptive mode) and starting to run more Prefect tasks in parallel. I'm not sure we've really characterized this behavior well, but we've observed it enough that it seems to be a pattern. (We have a notebook that - using the k8s APIs - plots the number of requested workers, running workers, & nodes and could show some of what we're seeing if it would help.)

What is the best practice of decorating tasks. I have modules where I decorated funcs with @task. Then I import them in main.py for the Flow. But I'm having problem using sphinx to generate docs. No error messages just HTML output does not contain doc strings

Somebody set me straight on something silly I must be doing wrong. I have a really simple Flow that looks like this:

with Flow("ScaleDownDaskOffHours", environment=env, schedule=schedule) as flow:
    config = get_dask_scheduler_config()
    new_config = reset_config(config)
    save_results = save_new_config(new_config)
    save_results.set_upstream(None, flow)
    ifelse(new_config != config, save_results, None)

I'm getting the warning

PrefectWarning: One of the tasks passed to the switch condition has upstream dependencies: <Task: save_new_config>. Those upstream tasks could run even if the switch condition fails, which might cause unexpected results.

I thought the

save_results.set_upstream(None, flow)

line would take care of that but clearly not. What's the right way to address that?

Apologies for a naive question! I have a flow that is roughly doing:

filepaths = get_filepath(directory)

# return a list of file paths

outpaths = get_output_paths(filepaths)

# return a corresponding list of paths to write to after cleaning

dataframes = read_file.map(filepaths)

# read files into memory in parallel

cleaned_dfs = clean_dataframe.map(dataframes)

# clean dataframes in parallel

write_dataframe.map(cleaned_dfs, outpaths)

# Write dataframes to file in parallel I have 16 cores and ~400 files and am using the DaskExecutor. During execution, ALL files are read into memory before any of the cleaning child task starts, then all cleaning tasks are performed before any writing to file commences. This means the memory is being flooded as each file is ~2GB. It's kind of like a breadth-first approach, whereas I'd like it to run depth-first: read 16 files into memory at once, clean them and write to file, then a core can free the memory and read in the next file. Is there a way to achieve this, possibly with triggers? The only workaround I've been able to think up is to write a single big task that does the reading/cleaning/writing steps - but I feel like that's kind of not the point of Prefect. Huge thanks in advance if anyone can help!

Hello! Quick question. The Schedules doc say:

Prefect does not support sub-minute schedules.

Is this a hard limit? I have the need to run a Prefect Flow secondly for a minute (and then stop). 🙏

when using docker storage and/or remote executors, what’s considered the best practice for shipping code around? does it make sense to rely on cloudpickle to send any required non-third party code via the flow itself, or should everything run from the same base image that has the needed internal functions?

Good morning! Apologies if this is answered in the docs, I can't seem to find it. Prefect always seems to run in UTC, and so the

CronClock

needs to be set in UTC. Not a big deal, but is it possible to override the timezone? 🕐

Hi All, I'm trying to figure out what the best-practice would be within Prefect if within a task I need to send a job to a 3rd party appliance which cannot scale, so I need to not send too many concurrent jobs to it. All I can find in relation to this is the queued state I could put a given Task in. I assume then I'd be looking at building my own resource queuing system (using redis-or-the-like) to queue these jobs..

I am trying to run prefect flow on Dask cluster using kubernetes and AWS. I was able to import "prefect" and also print its version. But when I execute the flow, it says

ModuleNotFoundError: No module named 'prefect'

. I have installed all the dependencies that need to be installed by Dask-worker and Dask-jupyter using yaml file.

Hello, everyone! I'm new to Prefect and Python as well. Correct me if I'm wrong: tasks inside a flow will run in sequence. If a task fails the downstream ones will not be executed. On my tests, the execution flow is not deterministic. In the code below I expect the following output:

==== START ====
=== starting the flow
== generate called
= status: [1, 2, 3]
== task runner called with 1
== task runner called with 2
== task runner called with 3
==== END ====

Code:

@task
def generate_task_list() -> list:
    print("== generate called")
    return [1, 2, 3]


@task(max_retries=3, retry_delay=timedelta(seconds=0))
def task_runner(x: int) -> None:
    print('== task runner called with {}'.format(x))


@task
def print_status(text: any) -> None:
    print('= status: {}'.format(text))


with Flow("random-mapping") as f:
    print_status('=== starting the flow')
    values = generate_task_list()
    print_status(values)
    final = task_runner.map(x=values)

print('==== START ====')
f.run()
print('==== END ====')

By running this multiple times I get different results but never what I expected. What am I missing? Thank you!

is there a simple way to use a secret in a custom task? so far I’ve been using a

RemoteEnvironment

and my existing cluster already loads

GOOGLE_APPLICATION_CREDENTIALS

from a k8s secret; for a

DaskKubernetesEnvironment

I guess I need to pull it from a prefect

Secret

instead. do I just need to add

os.environ['GOOGLE_APPLICATION_CREDENTIALS'] = Secret(...).get()

everywhere or is there an easier way?

Is there possible to run

tasks

inside

tasks

and

flows

inside

tasks

? I was able to call a

flow

inside a task but got a

ValueError: Could not infer an active Flow context

error every time I tried to send parameters to the

flows

from the

tasks

. Got the same error when calling a

task

within a

task

.

Hi guys, good morning. 🙂 I'm starting to use prefect, and have a quick newbie question: What kind of scenario would I use task class instead of the task decorator? Just trying to understand it better.

I would like to try running my prefect flow on a dask-kubernetes cluster. I saw that it is possible but I am not able to grasp how. Can you share an example of running a prefect flow on a dask-kubernetes cluster? Should I create a custom worker (pod/docker) for the cluster with the appropriate libraries? How does my prefect flow end up executing on the dask-worker if the worker pod/docker does not contain my code?

Hello folks. I feel like I'm missing something pretty basic. I have the following code that runs

CreateRidAlert

task twice and throws warnings:

create_alert = CreateAlert()
create_rid_alert = CreateRidAlert()

with Flow('Alert Flow') as flow:
    alert_json = Parameter(name="alert_json", required=True)
    alert = create_alert(alert_json)
    rid_alert = create_rid_alert(alert)
    ifelse(has_rid(alert), rid_alert, None)
    rid = extract_rid(rid_alert)

I'd like to remove the

rid_alert = create_rid_alert(alert)

line but then I don't know how to wire up the

ifelse

line with a result to pass to

extract_rid

for data flow. This may not be a clear question.

Quick question about scheduling: is there a way to gracefully cancel a running (infinite) flow that uses a schedule? Also, I need to manage several of them in my application, so I plan to write some sort of management class for runs. Except, maybe something like this already exists, does anybody know?

Hi folks, what is the best way to reference and mutate a variable between `flow.run()`s? Should it be a

Parameter

? My actual use case is to have a dict which is a copy of one of my database tables which the incoming data needs to refer to to find its foreign key each time. If I find a new name that I don’t have in my dict I’ll need to update the table in the database and mutate my dictionary, then reference the mutated version in subsequent `flow.run()`s. I’m only new to Prefect but I have to say I love your work, thank you!

Hi, are there ways to invoke a non-Python step in Prefect?

Would the Docker task be the one to use?

It seems that Prefect doesn't support multiple return, or some Python syntax? ----------------------------------- @task def hello(x): a=1 b=2 return a, b with Flow('test flow') as flow: a, b = hello(1) #<---- 'NoneType' object is not callable hello(a) hello(b) ______________________________

Hi everyone, quick question: I am building my doc with Sphinx and wanted to refer to the Prefect docs with intersphinx. This is relatively easy, as long as there is a public

objects.inv

but I cannot find it for Prefect… Is this supported ?

A Flow's parameters need to be passed to at least one task's run() function, or error would be raised, right? __________________________________________________ from prefect import task, Flow, Parameter from prefect.core.task import Task class print_plus_one(Task): def __init__(self, x): super().__init__() self.x = x def run(self): print(self.x + 1) with Flow('Parameterized Flow') as flow: x = Parameter('x') print_plus_one(x)() flow.run(parameters=dict(x=1)) # prints 2 flow.run(parameters=dict(x=100)) # prints 101 __________________________________________________ ValueError: Flow.run received the following unexpected parameters: x

This isn't 100% Prefect related but thought it would be relevant for some users. We have a need for different types of Dask workers, e.g. high memory, GPU, etc. This would work really well with Prefect's support for task tagging to specify Dask worker resources: https://docs.prefect.io/api/unreleased/engine/executors.html#daskexecutor We use dask-kubernetes (on AWS EKS) which currently supports just a single k8s pod spec for all Dask workers. I'm about to start on a potential PR in dask-kubernetes that would allow for different worker types, but only in non-adaptive mode. Before I dive in, I thought I'd ask if anyone knows of existing work in this area of creating and scaling different Dask worker types, especially on k8s. I see this old issue: https://github.com/dask/distributed/issues/2118 but haven't found much else. (I've also asked on the Dask users gitter channel.)

Quick question, can you

task.map()

over a Pandas DataFrame? So you could get each row as a Series for each task call. Or does it only work with generic iterables? Thank you.

Can't quite figure this out from the docs: is there a way to limit the number of tasks in a map that are run in parallel?

Seeing something a bit odd - unsure if I'm using the library wrong. When using

with raise_on_exception():
   state = flow.run(parameters)

I get raised exceptions on conditional misses - like

switch

statements missing. The

raise signals.SKIP('Provided value "{}" did not match "{}"'.format(value, self.value)

call ends up firing and stopping execution

this is on the most recent release