Afar

One man's magic is another man's engineering
Robert A. Heinlein

Installation

afar may be installed with pip:

pip install afar

or with conda:

conda install -c conda-forge afar

What is it?

afar allows you to run code on a remote Dask cluster using context managers and IPython magics. For example:

import afar
from dask.distributed import Client
client = Client()

with afar.run, remotely:
    import dask_cudf
    df = dask_cudf.read_parquet("s3://...")
    result = df.sum().compute()

Outside the context, result is a Dask Future whose data resides on a worker. result.result() is necessary to copy the data locally.

By default, only the last assignment is saved. One can specify which variables to save:

with afar.run("one", "two"), remotely:
    one = 1
    two = one + 1

one and two are now both Futures. They can be used directly in other afar.run contexts:

with afar.run as data, remotely:
    three = one + two

assert three.result() == 3
assert data["three"].result() == 3

data above is a dictionary of variable names to Futures. It may be necessary at times to get the data from here. Alternatively, you may pass a mapping to afar.run to use as the data.

run = afar.run(data={"four": 4})
with run, remotely:
    seven = three + four
assert run.data["seven"].result() == 7

If you want to automatically gather the data locally (to avoid calling .result()), use afar.get instead of afar.run:

with afar.get, remotely:
    five = two + three
assert five == 5

Interactivity in Jupyter

There are several enhancements when using afar in Jupyter Notebook or Qt console, JupyterLab, or any IPython-based frontend.

The rich repr of the final expression will be displayed if it's not an assignment:

with afar.run, remotely:
    three + seven
# displays 10!

Printing is captured and displayed locally and asynchronously:

with afar.run, remotely:
    print(three)
    print(seven, file=sys.stderr)
# 3
# 7

These are done asynchronously using ipywidgets.

Magic!

First load afar magic extension:

%load_ext afar

Now you can use afar as line or cell magic. %%afar is like with afar.run, remotely:. It can optionally accept a list of variable names to save:

%%afar x, y
x = 1
y = x + 1

and

z = %afar x + y

Custom Magic

You can create your own IPython magic like %afar that has arguments baked in:

afar.new_magic("on_gpus", where=remotely(resources={"GPU": 1}))

then:

%%on_gpus
import dask_cudf
df = dask_cudf.read_parquet("s3://...")
result = df.sum().compute()

Is this a good idea?

I don't know, but it sure is a joy to use 😃 !

For motivation, see https://github.com/dask/distributed/issues/4003

It's natural to be skeptical of unconventional syntax. And magic. afar is both unconventional and magical, yet it also works well and is surprisingly fun! Why not give it a try to see what you think?

We're still exploring the usability of afar and want to hear what you think. As you're learning afar, please ask yourself questions such as:

can we spell anything better?
does this offer opportunities?
what is surprising?
what is lacking?

Here's an example of an opportunity:

on_gpus = afar.remotely(resources={"GPU": 1})

with afar.run, on_gpus:
    ...

This now works! Keyword arguments to remotely will be passed to client.submit.

I don't know about you, but I think this is starting to look and feel kinda nice, and it could probably be even better :)

Caveats and Gotchas

Repeatedly copying data

afar automatically gets the data it needs--and only the data it needs--from the outer scope and sends it to the Dask cluster to compute on. Since we don't know whether local data has been modified between calls to afar, we serialize and send local variables every time we use run or get. This is generally fine: it works, it's safe, and is usually fast enough. However, if you do this frequently with large-ish data, the performance could suffer, and you may be using more memory on your local machine than necessary.

With Dask, a common pattern is to send data to the cluster with scatter and get a Future back. This works:

A = np.arange(10**7)
A = client.scatter(A)
with afar.run, remotely:
    B = A + 1
# A and B are now both Futures; their data is on the cluster

Another option is to pass data to run:

run = afar.run(data={"A": np.arange(10**7)})
with afar.run, remotely:
    B = A + 1
# run.data["A"] and B are now both Futures; their data is on the cluster

Here's a nifty trick to use if you're in an IPython notebook: use data=globals()!

run = afar.run(data=globals())
A = np.arange(10**7)
with run, remotely:
    B = A + 1
# A and B are now both Futures; their data is on the cluster

Mutating remote data

As with any Dask workload, one should be careful to not modify remote data that may be reused.

Mutating local data

Similarly, code run remotely isn't able to mutate local variables. For example:

d = {}
with afar.run, remotely:
    d['key'] = 'value'
# d == {}

eriknw / afar

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