Does anyone have a working example of specifying a `result h Prefect Community #ask-community

Does anyone have a working example of specifying a...

Joe Schmid

07/25/2019, 3:01 PM

Does anyone have a working example of specifying a

result_handler

and having

write()

read()

functions called? (Mainly interested in

result_handler

on a Task, but Flow-level or overall default is fine, too.) I can't seem to get my ResultHandler called and I'm sure it's something silly I'm doing. (I can post a simple example if it helps, but I suspect others have this working just fine.)

Adam Roderick

07/25/2019, 3:22 PM

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from prefect import task, Flow

def my_state_handler(task, old_state, new_state):
    type = new_state.type if False else None
    print(f'log this stuff! task: {task.name}, type: {type}, message: {new_state.message}, result: {new_state.result}')


@task(state_handlers=[my_state_handler])
def do_something(var = 'Nothing'):
    print(f'doing something: {var}')
    return var


with Flow('doing it', state_handlers=[my_state_handler]) as flow:
    t1 = do_something(1)
    t2 = do_something(t1+ 99)
    t3 = do_something(3)
    t4 = do_something(4)


flow_state = flow.run()
t1_state = flow_state.result[t1]
print(f't1_state: {t1_state}')

Adam Roderick

07/25/2019, 3:23 PM

@Joe Schmid oh I just realized this is a state handler not a result handler. I'll leave it in case it helps, but prob isn't the same thing

Chris White

07/25/2019, 3:28 PM

Hey @Joe Schmid! Good question - result handlers are never called unless Core is being run on Cloud; I’d be interested to hear if you want them to be called for testing purposes or for actual functionality that you need in Core

Joe Schmid

07/25/2019, 3:37 PM

Hi @Chris White, that makes sense now! (That they're only called when running on Cloud.) My thinking was that having control over storage could work well when we're preparing large feature vectors (hundreds or maybe even 1,000+ features for large numbers of rows). We currently (without Prefect) persist these to parquet files in an S3 bucket.

Joe Schmid

07/25/2019, 3:40 PM

I don't think we have to have this functionality in Core since we could just do this manually (write & read) in our Tasks, but it seemed more "Prefect-y" to just return data & let a ResultHandler store it and load it.

Chris White

07/25/2019, 3:45 PM

That makes sense! In an older version of Prefect the

checkpoint

flag on tasks would have the effect of calling result handlers regardless of whether it was being run on Cloud, but we ended up making this something that is only turned on in Cloud. However, if you want to see it in action, simply set this environment variable:

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PREFECT__CONTEXT__CLOUD=True

This should only have the effect of turning on “checkpointing” for tasks (tasks which are initialized with

checkpoint=True

)

Chris White

07/25/2019, 3:47 PM

Caveat: - in this case all Parameters must be JSON serializable This makes me wonder if we should just rename this context variable to “result_handlers” or something more generic, and elevate it to a first-class toggle in Prefect Core (and this toggle just happens to be always turned on in Cloud)

Joe Schmid

07/25/2019, 4:00 PM

I kinda like that option. (first-class toggle) We'll play with it a bit and try to have some more educated feedback for you. Thanks for the info!

Chris White

07/25/2019, 4:10 PM

Anytime!

Chris White

07/29/2019, 3:47 PM

Hey @Joe Schmid! -> https://github.com/PrefectHQ/prefect/pull/1283 was just merged, which allows you to set an environment variable / config option