DaskR prototype with rpy2 and reticulate

[mrocklin]

@quasiben has been playing with Dask and R with reticulate and rpy2 with the objective of providing Dask's concurrent.futures API to R (from which they could presumably build other systems). This is somewhat tricky because you have both a Python and R session living side-by-side and need to move things between them from time to time (hopefully infrequently) using reticulate or rpy2. There are a variety of ways to do this, I thought I'd lay out the way that makes the most sense to me.

• From R we need ways to get proxy objects in Python that point to our local objects in R without doing any clever/lossy conversion (like R dataframes to Pandas dataframes). Below I use rpy2 as this proxy object, but anything would do.

• From within Python we need ways to serialize and deserialize these proxy objects, presumably this involves calling R's serialize function on the R side and then bringing those bytes back to Python, and then doing the reverse with deserialize.

  On the Dask side we need to implement the following interface for rpy2 objects: https://distributed.readthedocs.io/en/latest/serialization.html#dask-serialization-family

  @dask_serialize.register(rpy.whatever_an_object_type_is)
  def serialize_rpy2(obj):
      r_bytes = rpy2.call("serialize", obj)  # call R `serialize` function on pointed-to-object
      py_bytes = rpy2.get_from_R(r_bytes)  # bring bytes back home to python
      header = {}
      frames = [py_bytes]
      return header, frames
  
  @dask_deserialize.register(rpy2.whatever_an_object_type_is)
  def deserialize_rpy2(header, frames):
      [py_bytes] = frames
      r_bytes = rpy2.send_to_R(py_bytes)
      obj = rpy2.call("deserialize", r_bytes)
      return obj

• Also on the Python side we need to implement the sizeof protocol` to quickly estimate how large an object is in bytes

  from dask.sizeof import sizeof
  @sizeof.register(rpy2.whatever_type_objects_are)
  def sizeof(x) -> int:
      return number_of_bytes

• We'll need to develop a basic API in R. If we're starting with the concurrent.futures interface (which seems simplest) then this probably means submit, gather, as_completed, wait, and map (is anything else critical?). We should check in with people familiar with R though to learn if there is a more native API that R users would find more familiar.

[mrocklin]

For size? https://stat.ethz.ch/R-manual/R-devel/library/utils/html/object.size.html

[dhirschfeld]

Maybe arrow/feather will (one day) provide a better way to interop with R.

It seems a fully-functional arrow implementation is still a ways off though:
apache/arrow#2489

[mrocklin]

Arrow would be important here if we wanted to run pandas code on R dataframes or R code on pandas dataframes. That isn't the case here. This issue is restricted to just calling R code on R objects (dataframes and otherwise). I don't think that Arrow is useful in this particular setting.

[mrocklin]

RPy2 has issues with threads. https://bitbucket.org/rpy2/rpy2/issues/449/rpy2-blocks-other-python-threads