issue_comments

@rabernat Agreed, let's open a new issue for that.

I have a preliminary implementation up in https://github.com/pydata/xarray/pull/1517

Indeed, there's no need to load the entire dataset into memory first. I think open_mfdataset is the model to emulate here -- it's parallelism that just works.

I'm not quite sure how to do this transparently in groupby operations yet. The problem is that you do want to apply some groupby operations on dask arrays without loading the entire group into memory, if there are only a few groups on a large datasets and the function itself is written in terms of dask operations. I think we will probably need some syntax to disambiguate that scenario.

I think #964 provides a viable path forward here.

Previously, I was imagining the user provides an function that maps xarray.DataArray -> xarray.DataArray. Such functions are tricky to parallelize with dask.array because need to run them to figure out the result dimensions/coordinates.

In contrast, with a user defined function ndarray -> ndarray, it's fairly straightforward to parallelize these with dask array (e.g., using dask.array.elemwise or dask.array.map_blocks). Then we could add the metadata back in afterwards with #964.

In principle, we could do this automatically -- especially if dask had a way to parallelize arbitrary NumPy generalized universal functions. Then the user could write something like xarray.apply(func, data, signature=signature, dask_array='auto') to automatically parallelize func over their data. In fact, I had this in some previous commits for #964, but took it out for now, just to reduce scope for the change.

With the single machine version of dask, we need to run one block first to infer the appropriate metadata for constructing the combined dataset.

Potentially a better approach would be to optionally leverage dask.distributed, which has the ability to run computation at the same time as graph construction. map_blocks could then kick off a bunch of map tasks to execute in parallel, and only worry about reassembling the blocks in a reduce after the results have come in.