Node-level initializations for UDF

[jaychia]

Is your feature request related to a problem? Please describe.

When running a UDF that performs some initializations (for example, commonly - downloading and caching a model on disk), there can be a problem where multiple workers attempt to do so at the same time and end up thrashing each other.

We should allow for a mechanism to perform node-level initializations in a UDF, that execute once-per-node. Alternatively, if the user is provided with a unique worker ID through the daft.context, then they could also do more intelligent things such as using a different cache folder for each stateful UDF's initializations.

[jaychia]

This is possible already without any Daft-provided functionality. Users can utilize a library such as https://github.com/tox-dev/py-filelock to perform node-level locking.

from filelock import FileLock

class MyUDF:

    def __init__(self):
        with FileLock("/tmp/.myudf.lock"):
            download_model_to_disk()
        self.model = load_model()

Documentation will be added in an FAQ section for UDFs.

Closing!