issue_comments: 572355926

I tried to do serialization/deserialization by hand:

• logged in to one of the Dask worker, loaded zarr data locally using open_zarr, pickled the resulting dataset

python ds = xr.open_zarr("/sciserver/filedb02-01/ocean/LLC4320/SST") pickle.dump(ds, open("/home/dask/zarr.p", "wb"))

• copied the pickle file to the client, tried to unpickle it

ds = pickle.load(open("zarr.p", "rb"))

It failed with the same error:

``` UnpicklingErrorTraceback (most recent call last) <ipython-input-77-4809dc01c404> in <module> ----> 1 a = pickle.loads(s)

UnpicklingError: pickle data was truncated

import pickle, xarray pickle.load(open("zarr.p", "rb")) zarr = pickle.load(open("zarr.p", "rb"))

KeyErrorTraceback (most recent call last) ~/miniconda3/lib/python3.6/site-packages/zarr/hierarchy.py in init(self, store, path, read_only, chunk_store, cache_attrs, synchronizer) 109 mkey = self._key_prefix + group_meta_key --> 110 meta_bytes = store[mkey] 111 except KeyError:

~/miniconda3/lib/python3.6/site-packages/zarr/storage.py in getitem(self, key) 726 else: --> 727 raise KeyError(key) 728

KeyError: '.zgroup'

During handling of the above exception, another exception occurred:

ValueErrorTraceback (most recent call last) <ipython-input-83-cd9f4ae936eb> in <module> ----> 1 zarr = pickle.load(open("zarr.p", "rb"))

~/miniconda3/lib/python3.6/site-packages/zarr/hierarchy.py in setstate(self, state) 269 270 def setstate(self, state): --> 271 self.init(*state) 272 273 def _item_path(self, item):

~/miniconda3/lib/python3.6/site-packages/zarr/hierarchy.py in init(self, store, path, read_only, chunk_store, cache_attrs, synchronizer) 110 meta_bytes = store[mkey] 111 except KeyError: --> 112 err_group_not_found(path) 113 else: 114 meta = decode_group_metadata(meta_bytes)

~/miniconda3/lib/python3.6/site-packages/zarr/errors.py in err_group_not_found(path) 27 28 def err_group_not_found(path): ---> 29 raise ValueError('group not found at path %r' % path) 30 31

ValueError: group not found at path '' ```

I then tried the same thing with a NetCDF dataset, and it worked fine. Also, the pickle file for NetCDF was much smaller. So I guess in the case of zarr dataset there is some initialization code that tries to open the zarr files when the dataset object gets deserialized on the client, and of course it cannot, because there is no data on the client. That explains a lot... although I'm still not sure if xarray was ever intended to be used that way. Maybe I'm trying to do a completely wrong thing here?