Zarr consolidated

ci/requirements-py36-zarr-dev.yml

@@ -18,4 +18,4 @@ dependencies:
   - pip:
     - coveralls
     - pytest-cov
-    - git+https://github.com/alimanfoo/zarr.git
+    - git+https://github.com/zarr-developers/zarr.git

doc/io.rst

@@ -635,6 +635,31 @@ For example:
     Not all native zarr compression and filtering options have been tested with
     xarray.
 
+Consolidated Metadata
+~~~~~~~~~~~~~~~~~~~~~
+
+Xarray needs to read all of the zarr metadata when it opens a dataset.
+In some storage mediums, such as with cloud object storage (e.g. amazon S3),
+this can introduce significant overhead, because two separate HTTP calls to the
+object store must be made for each variable in the dataset.
+With version 2.3, zarr will support a feature called *consolidated metadata*,
+which allows all metadata for the entire dataset to be stored with a single
+key (by default called ``.zmetadata``). This can drastically speed up
+opening the store. (For more information on this feature, consult the
+`zarr docs <https://zarr.readthedocs.io/en/latest/tutorial.html#consolidating-metadata>`_.)
+
+If you have zarr version 2.3 or greater, xarray can write and read stores
+with consolidated metadata. To write consolidated metadata, pass the
+``consolidate=True`` option to the
+:py:attr:`Dataset.to_zarr <xarray.Dataset.to_zarr>` method::
+
+    ds.to_zarr('foo.zarr', consolidate=True)
+
+To read a consolidated store, pass the ``consolidated=True`` option to
+:py:func:`~xarray.open_zarr`::
+
+    ds = xr.open_zarr('foo.zarr', consolidated=True)
+
 .. _io.cfgrib:
 
 GRIB format via cfgrib
@@ -678,7 +703,7 @@ Formats supported by PseudoNetCDF
 ---------------------------------
 
 xarray can also read CAMx, BPCH, ARL PACKED BIT, and many other file
-formats supported by PseudoNetCDF_, if PseudoNetCDF is installed. 
+formats supported by PseudoNetCDF_, if PseudoNetCDF is installed.
 PseudoNetCDF can also provide Climate Forecasting Conventions to
 CMAQ files. In addition, PseudoNetCDF can automatically register custom
 readers that subclass PseudoNetCDF.PseudoNetCDFFile. PseudoNetCDF can

doc/whats-new.rst

@@ -36,6 +36,8 @@ Breaking changes
 Enhancements
 ~~~~~~~~~~~~
 
+- Ability to read and write consolidated metadata in zarr stores.
+  By `Ryan Abernathey <https://github.com/rabernat>`_.
 - :py:class:`CFTimeIndex` uses slicing for string indexing when possible (like
   :py:class:`pandas.DatetimeIndex`), which avoids unnecessary copies.
   By `Stephan Hoyer <https://github.com/shoyer>`_
@@ -56,15 +58,15 @@ Breaking changes
   - ``Dataset.T`` has been removed as a shortcut for :py:meth:`Dataset.transpose`.
     Call :py:meth:`Dataset.transpose` directly instead.
   - Iterating over a ``Dataset`` now includes only data variables, not coordinates.
-    Similarily, calling ``len`` and ``bool`` on a ``Dataset`` now  
+    Similarily, calling ``len`` and ``bool`` on a ``Dataset`` now
     includes only data variables.
   - ``DataArray.__contains__`` (used by Python's ``in`` operator) now checks
-    array data, not coordinates. 
+    array data, not coordinates.
   - The old resample syntax from before xarray 0.10, e.g.,
     ``data.resample('1D', dim='time', how='mean')``, is no longer supported will
     raise an error in most cases. You need to use the new resample syntax
     instead, e.g., ``data.resample(time='1D').mean()`` or
-    ``data.resample({'time': '1D'}).mean()``. 
+    ``data.resample({'time': '1D'}).mean()``.
 
 
 - New deprecations (behavior will be changed in xarray 0.12):
@@ -101,13 +103,13 @@ Breaking changes
     than by default trying to coerce them into ``np.datetime64[ns]`` objects.
     A :py:class:`~xarray.CFTimeIndex` will be used for indexing along time
     coordinates in these cases.
-  - A new method :py:meth:`~xarray.CFTimeIndex.to_datetimeindex` has been added 
+  - A new method :py:meth:`~xarray.CFTimeIndex.to_datetimeindex` has been added
     to aid in converting from a  :py:class:`~xarray.CFTimeIndex` to a
     :py:class:`pandas.DatetimeIndex` for the remaining use-cases where
     using a :py:class:`~xarray.CFTimeIndex` is still a limitation (e.g. for
     resample or plotting).
   - Setting the ``enable_cftimeindex`` option is now a no-op and emits a
-    ``FutureWarning``. 
+    ``FutureWarning``.
 
 Enhancements
 ~~~~~~~~~~~~
@@ -194,7 +196,7 @@ Bug fixes
   the dates must be encoded using cftime rather than NumPy (:issue:`2272`).
   By `Spencer Clark <https://github.com/spencerkclark>`_.
 
-- Chunked datasets can now roundtrip to Zarr storage continually 
+- Chunked datasets can now roundtrip to Zarr storage continually
   with `to_zarr` and ``open_zarr`` (:issue:`2300`).
   By `Lily Wang <https://github.com/lilyminium>`_.
 

xarray/backends/api.py

@@ -861,7 +861,7 @@ def save_mfdataset(datasets, paths, mode='w', format=None, groups=None,
 
 
 def to_zarr(dataset, store=None, mode='w-', synchronizer=None, group=None,
-            encoding=None, compute=True):
+            encoding=None, compute=True, consolidate=False):
     """This function creates an appropriate datastore for writing a dataset to
     a zarr ztore
 
@@ -876,16 +876,20 @@ def to_zarr(dataset, store=None, mode='w-', synchronizer=None, group=None,
     _validate_dataset_names(dataset)
     _validate_attrs(dataset)
 
-    store = backends.ZarrStore.open_group(store=store, mode=mode,
+    zstore = backends.ZarrStore.open_group(store=store, mode=mode,
                                           synchronizer=synchronizer,
-                                          group=group)
+                                          group=group,
+                                          consolidate_on_close=consolidate)
 
     writer = ArrayWriter()
     # TODO: figure out how to properly handle unlimited_dims
-    dump_to_store(dataset, store, writer, encoding=encoding)
+    dump_to_store(dataset, zstore, writer, encoding=encoding)
     writes = writer.sync(compute=compute)
 
-    if not compute:
+    if compute:
+        _finalize_store(writes, zstore)
+    else:
         import dask
-        return dask.delayed(_finalize_store)(writes, store)
-    return store
+        return dask.delayed(_finalize_store)(writes, zstore)
+
+    return zstore

xarray/backends/zarr.py

@@ -224,7 +224,8 @@ class ZarrStore(AbstractWritableDataStore):
     """
 
     @classmethod
-    def open_group(cls, store, mode='r', synchronizer=None, group=None):
+    def open_group(cls, store, mode='r', synchronizer=None, group=None,
+                   consolidated=False, consolidate_on_close=False):
         import zarr
         min_zarr = '2.2'
 
@@ -234,15 +235,27 @@ def open_group(cls, store, mode='r', synchronizer=None, group=None):
                                       "installation "
                                       "http://zarr.readthedocs.io/en/stable/"
                                       "#installation" % min_zarr)
-        zarr_group = zarr.open_group(store=store, mode=mode,
-                                     synchronizer=synchronizer, path=group)
-        return cls(zarr_group)
 
-    def __init__(self, zarr_group):
+        if consolidated or consolidate_on_close:
+            if LooseVersion(zarr.__version__) <= '2.2': # pragma: no cover
+                raise NotImplementedError("Zarr version 2.3 or greater is "
+                                          "required by for consolidated "
+                                          "metadata.")
+
+        open_kwargs = dict(mode=mode, synchronizer=synchronizer, path=group)
+        if consolidated:
+            # TODO: an option to pass the metadata_key keyword
+            zarr_group = zarr.open_consolidated(store, **open_kwargs)
+        else:
+            zarr_group = zarr.open_group(store, **open_kwargs)
+        return cls(zarr_group, consolidate_on_close)
+
+    def __init__(self, zarr_group, consolidate_on_close=False):
         self.ds = zarr_group
         self._read_only = self.ds.read_only
         self._synchronizer = self.ds.synchronizer
         self._group = self.ds.path
+        self._consolidate_on_close = consolidate_on_close
 
     def open_store_variable(self, name, zarr_array):
         data = indexing.LazilyOuterIndexedArray(ZarrArrayWrapper(name, self))
@@ -333,11 +346,16 @@ def store(self, variables, attributes, *args, **kwargs):
     def sync(self):
         pass
 
+    def close(self):
+        if self._consolidate_on_close:
+            import zarr
+            zarr.consolidate_metadata(self.ds.store)
+
 
 def open_zarr(store, group=None, synchronizer=None, auto_chunk=True,
               decode_cf=True, mask_and_scale=True, decode_times=True,
               concat_characters=True, decode_coords=True,
-              drop_variables=None):
+              drop_variables=None, consolidated=False):
     """Load and decode a dataset from a Zarr store.
 
     .. note:: Experimental
@@ -383,10 +401,13 @@ def open_zarr(store, group=None, synchronizer=None, auto_chunk=True,
     decode_coords : bool, optional
         If True, decode the 'coordinates' attribute to identify coordinates in
         the resulting dataset.
-    drop_variables: string or iterable, optional
+    drop_variables : string or iterable, optional
         A variable or list of variables to exclude from being parsed from the
         dataset. This may be useful to drop variables with problems or
         inconsistent values.
+    consolidated : bool, optional
+        Whether to open the store using zarr's consolidated metadata
+        capability. Only works for stores that have already been consolidated.
 
     Returns
     -------
@@ -423,7 +444,7 @@ def maybe_decode_store(store, lock=False):
     mode = 'r'
     zarr_store = ZarrStore.open_group(store, mode=mode,
                                       synchronizer=synchronizer,
-                                      group=group)
+                                      group=group, consolidated=consolidated)
     ds = maybe_decode_store(zarr_store)
 
     # auto chunking needs to be here and not in ZarrStore because variable

xarray/core/dataset.py

@@ -1222,7 +1222,7 @@ def to_netcdf(self, path=None, mode='w', format=None, group=None,
                          compute=compute)
 
     def to_zarr(self, store=None, mode='w-', synchronizer=None, group=None,
-                encoding=None, compute=True):
+                encoding=None, compute=True, consolidate=False):
         """Write dataset contents to a zarr group.
 
         .. note:: Experimental
@@ -1244,9 +1244,16 @@ def to_zarr(self, store=None, mode='w-', synchronizer=None, group=None,
             Nested dictionary with variable names as keys and dictionaries of
             variable specific encodings as values, e.g.,
             ``{'my_variable': {'dtype': 'int16', 'scale_factor': 0.1,}, ...}``
-        compute: boolean
-            If true compute immediately, otherwise return a
+        compute: bool, optional
+            If True compute immediately, otherwise return a
             ``dask.delayed.Delayed`` object that can be computed later.
+        consolidate: bool, optional
+            If True, apply zarr's `consolidate_metadata` function to the store
+            after writing.
+
+        References
+        ----------
+        https://zarr.readthedocs.io/
         """
         if encoding is None:
             encoding = {}
@@ -1256,7 +1263,8 @@ def to_zarr(self, store=None, mode='w-', synchronizer=None, group=None,
                              "and 'w-'.")
         from ..backends.api import to_zarr
         return to_zarr(self, store=store, mode=mode, synchronizer=synchronizer,
-                       group=group, encoding=encoding, compute=compute)
+                       group=group, encoding=encoding, compute=compute,
+                       consolidate=consolidate)
 
     def __unicode__(self):
         return formatting.dataset_repr(self)
@@ -1380,7 +1388,7 @@ def _validate_indexers(self, indexers):
         """ Here we make sure
         + indexer has a valid keys
         + indexer is in a valid data type
-        + string indexers are cast to the appropriate date type if the 
+        + string indexers are cast to the appropriate date type if the
           associated index is a DatetimeIndex or CFTimeIndex
         """
         from .dataarray import DataArray
@@ -1963,7 +1971,7 @@ def _validate_interp_indexer(x, new_x):
                                'Instead got\n{}'.format(new_x))
             else:
                 return (x, new_x)
-            
+
         variables = OrderedDict()
         for name, var in iteritems(obj._variables):
             if name not in indexers:

xarray/tests/test_backends.py

@@ -1,6 +1,7 @@
 from __future__ import absolute_import, division, print_function
 
 import contextlib
+from distutils.version import LooseVersion
 import itertools
 import math
 import os.path
@@ -1320,6 +1321,19 @@ def roundtrip_append(self, data, save_kwargs={}, open_kwargs={},
                          allow_cleanup_failure=False):
         pytest.skip("zarr backend does not support appending")
 
+
+    def test_roundtrip_consolidated(self):
+        # my understanding is that pytest.mark.skipif is broken here
+        import zarr
+        if LooseVersion(zarr.__version__) <= '2.2':
+            pytest.skip('zarr version 2.3 or greater needed for consolidated')
+        expected = create_test_data()
+        with self.roundtrip(expected,
+                            save_kwargs={'consolidate': True},
+                            open_kwargs={'consolidated': True}) as actual:
+            self.check_dtypes_roundtripped(expected, actual)
+            assert_identical(expected, actual)
+
     def test_auto_chunk(self):
         original = create_test_data().chunk()
 

xarray/tests/test_distributed.py

@@ -1,5 +1,6 @@
 """ isort:skip_file """
 from __future__ import absolute_import, division, print_function
+from distutils.version import LooseVersion
 import os
 import sys
 import pickle
@@ -130,6 +131,19 @@ def test_dask_distributed_zarr_integration_test(loop):
                     assert isinstance(restored.var1.data, da.Array)
                     computed = restored.compute()
                     assert_allclose(original, computed)
+            # repeat with consolidated metadata and delayed compute
+            import zarr
+            if LooseVersion(zarr.__version__) <= '2.2':
+                return
+            with create_tmp_file(allow_cleanup_failure=ON_WINDOWS,
+                                 suffix='.zarrc') as filename:
+                futures = original.to_zarr(filename, consolidate=True,
+                                           compute=False)
+                futures.compute()
+                with xr.open_zarr(filename, consolidated=True) as restored:
+                    assert isinstance(restored.var1.data, da.Array)
+                    computed = restored.compute()
+                    assert_allclose(original, computed)
 
 
 @requires_rasterio