Zarr support (backend, in esmvalcore.preprocessor._io.py)

environment.yml

@@ -5,6 +5,7 @@ channels:
   - nodefaults
 
 dependencies:
+  - aiohttp
   - cartopy
   - cf-units
   - cftime
@@ -18,6 +19,7 @@ dependencies:
   - fire
   - geopy
   - humanfriendly
+  - intake-esm
   - iris >=3.12.2  # https://github.com/SciTools/iris/issues/6417
   - iris-esmf-regrid >=0.11.0
   - iris-grib >=0.20.0  # github.com/ESMValGroup/ESMValCore/issues/2535
@@ -46,6 +48,7 @@ dependencies:
   - shapely >=2.0.0
   - xarray
   - yamale
+  - zarr >3
   # Python packages needed for building docs
   - autodocsumm >=0.2.2
   - ipython <9.0  # github.com/ESMValGroup/ESMValCore/issues/2680

esmvalcore/preprocessor/_io.py

@@ -9,7 +9,9 @@
 from itertools import groupby
 from pathlib import Path
 from typing import TYPE_CHECKING, Any
+from urllib.parse import urlparse
 
+import fsspec
 import iris
 import ncdata
 import xarray as xr
@@ -75,6 +77,7 @@ def _restore_lat_lon_units(
 def load(
     file: str | Path | Cube | CubeList | xr.Dataset | ncdata.NcData,
     ignore_warnings: list[dict[str, Any]] | None = None,
+    backend_kwargs: dict[str, Any] | None = None,
 ) -> CubeList:
     """Load Iris cubes.
 
@@ -83,10 +86,19 @@ def load(
     file:
         File to be loaded. If ``file`` is already a loaded dataset, return it
         as a :class:`~iris.cube.CubeList`.
+        File as ``Path`` object could be a Zarr store.
     ignore_warnings:
         Keyword arguments passed to :func:`warnings.filterwarnings` used to
         ignore warnings issued by :func:`iris.load_raw`. Each list element
         corresponds to one call to :func:`warnings.filterwarnings`.
+    backend_kwargs:
+        Dict to hold info needed by storage backend e.g. to access
+        a PRIVATE S3 bucket containing object stores (e.g. netCDF4 files);
+        needed by ``fsspec`` and its extensions e.g. ``s3fs``, so
+        most of the times this will include ``storage_options``. Note that Zarr
+        files are opened via ``http`` extension of ``fsspec``, so no need
+        for ``storage_options`` in that case (ie anon/anon). Currently only used
+        in Zarr file opening.
 
     Returns
     -------
@@ -102,7 +114,19 @@ def load(
 
     """
     if isinstance(file, (str, Path)):
-        cubes = _load_from_file(file, ignore_warnings=ignore_warnings)
+        extension = (
+            file.suffix
+            if isinstance(file, Path)
+            else os.path.splitext(file)[1]
+        )
+        if "zarr" not in extension:
+            cubes = _load_from_file(file, ignore_warnings=ignore_warnings)
+        else:
+            cubes = _load_zarr(
+                file,
+                ignore_warnings=ignore_warnings,
+                backend_kwargs=backend_kwargs,
+            )
     elif isinstance(file, Cube):
         cubes = CubeList([file])
     elif isinstance(file, CubeList):
@@ -134,6 +158,53 @@ def load(
     return cubes
 
 
+def _load_zarr(
+    file: str | Path | Cube | CubeList | xr.Dataset | ncdata.NcData,
+    ignore_warnings: list[dict[str, Any]] | None = None,
+    backend_kwargs: dict[str, Any] | None = None,
+) -> CubeList:
+    # case 1: Zarr store is on remote object store
+    # file's URI will always be either http or https
+    if urlparse(str(file)).scheme in ["http", "https"]:
+        # basic test that opens the Zarr/.zmetadata file for Zarr2
+        # or Zarr/zarr.json for Zarr3
+        fs = fsspec.filesystem("http")
+        valid_zarr = True
+        try:
+            fs.open(str(file) + "/zarr.json", "rb")  # Zarr3
+        except Exception:  # noqa: BLE001
+            try:
+                fs.open(str(file) + "/.zmetadata", "rb")  # Zarr2
+            except Exception:  # noqa: BLE001
+                valid_zarr = False
+        # we don't want to catch any specific aiohttp/fsspec exception
+        # bottom line is that that file has issues, so raise
+        if not valid_zarr:
+            msg = (
+                f"File '{file}' can not be opened as Zarr file at the moment."
+            )
+            raise ValueError(msg)
+
+        time_coder = xr.coders.CFDatetimeCoder(use_cftime=True)
+        zarr_xr = xr.open_dataset(
+            file,
+            consolidated=True,
+            decode_times=time_coder,
+            engine="zarr",
+            backend_kwargs=backend_kwargs,
+        )
+    # case 2: Zarr store is local to the file system
+    else:
+        zarr_xr = xr.open_dataset(
+            file,
+            consolidated=False,
+            engine="zarr",
+            backend_kwargs=backend_kwargs,
+        )
+
+    return dataset_to_iris(zarr_xr, ignore_warnings=ignore_warnings)
+
+
 def _load_from_file(
     file: str | Path,
     ignore_warnings: list[dict[str, Any]] | None = None,

pyproject.toml

@@ -32,6 +32,7 @@ dynamic = [
     "version",
 ]
 dependencies = [
+    "aiohttp",
     "cartopy",
     "cf-units",
     "dask[array,distributed]>=2025",  # Core/issues/2503
@@ -44,6 +45,7 @@ dependencies = [
     "fire",
     "geopy",
     "humanfriendly",
+    "intake-esm",
     "iris-grib>=0.20.0",  # github.com/ESMValGroup/ESMValCore/issues/2535
     "isodate>=0.7.0",
     "jinja2",
@@ -68,6 +70,7 @@ dependencies = [
     "stratify>=0.3",
     "xarray",
     "yamale",
+    "zarr>3",
 ]
 description = "A community tool for pre-processing data from Earth system models in CMIP and running analysis scripts"
 license = {text = "Apache License, Version 2.0"}

tests/integration/preprocessor/_io/test_zarr.py

@@ -0,0 +1,224 @@
+"""
+Integration tests for :func:`esmvalcore.preprocessor._io._load_zarr`.
+
+This is a dedicated test module for Zarr files IO; we have identified
+a number of issues with Zarr IO so it deserves its own test module.
+
+We have a permanent bucket: esmvaltool-zarr at CEDA's object store
+"url": "https://uor-aces-o.s3-ext.jc.rl.ac.uk/esmvaltool-zarr",
+where will host a number of test files. Bucket is anon/anon
+(read/GET-only, but PUT can be allowed). Bucket operations are done
+via usual MinIO client (mc command) e.g. ``mc list``, ``mc du`` etc.
+
+Further performance investigations are being run with a number of tests
+that look at ncdata at https://github.com/valeriupredoi/esmvaltool_zarr_tests
+also see https://github.com/pp-mo/ncdata/issues/139
+"""
+
+from importlib.resources import files as importlib_files
+from pathlib import Path
+
+import cf_units
+import pytest
+
+from esmvalcore.preprocessor._io import load
+
+
+@pytest.mark.parametrize("input_type", [str, Path])
+def test_load_zarr2_local(input_type):
+    """Test loading a Zarr2 store from local FS."""
+    zarr_path = (
+        Path(importlib_files("tests"))
+        / "sample_data"
+        / "zarr-sample-data"
+        / "example_field_0.zarr2"
+    )
+
+    cubes = load(input_type(zarr_path))
+
+    assert len(cubes) == 1
+    cube = cubes[0]
+    assert cube.var_name == "q"
+    assert cube.standard_name == "specific_humidity"
+    assert cube.long_name is None
+    assert cube.units == cf_units.Unit("1")
+    coords = cube.coords()
+    coord_names = [coord.standard_name for coord in coords]
+    assert "longitude" in coord_names
+    assert "latitude" in coord_names
+
+
+def test_load_zarr2_remote():
+    """Test loading a Zarr2 store from a https Object Store."""
+    zarr_path = (
+        "https://uor-aces-o.s3-ext.jc.rl.ac.uk/"
+        "esmvaltool-zarr/example_field_0.zarr2"
+    )
+
+    # with "dummy" storage options
+    cubes = load(
+        zarr_path,
+        ignore_warnings=None,
+        backend_kwargs={"storage_options": {}},
+    )
+
+    assert len(cubes) == 1
+    cube = cubes[0]
+    assert cube.var_name == "q"
+    assert cube.standard_name == "specific_humidity"
+    assert cube.long_name is None
+    assert cube.units == cf_units.Unit("1")
+    coords = cube.coords()
+    coord_names = [coord.standard_name for coord in coords]
+    assert "longitude" in coord_names
+    assert "latitude" in coord_names
+
+    # without storage_options
+    cubes = load(zarr_path)
+
+    assert len(cubes) == 1
+    cube = cubes[0]
+    assert cube.var_name == "q"
+    assert cube.standard_name == "specific_humidity"
+    assert cube.long_name is None
+    assert cube.units == cf_units.Unit("1")
+    coords = cube.coords()
+    coord_names = [coord.standard_name for coord in coords]
+    assert "longitude" in coord_names
+    assert "latitude" in coord_names
+
+
+def test_load_zarr3_remote():
+    """Test loading a Zarr3 store from a https Object Store."""
+    zarr_path = (
+        "https://uor-aces-o.s3-ext.jc.rl.ac.uk/"
+        "esmvaltool-zarr/example_field_0.zarr3"
+    )
+
+    # with "dummy" storage options
+    cubes = load(
+        zarr_path,
+        ignore_warnings=None,
+        backend_kwargs={"storage_options": {}},
+    )
+
+    assert len(cubes) == 1
+    cube = cubes[0]
+    assert cube.var_name == "q"
+    assert cube.standard_name == "specific_humidity"
+    assert cube.long_name is None
+    assert cube.units == cf_units.Unit("1")
+    coords = cube.coords()
+    coord_names = [coord.standard_name for coord in coords]
+    assert "longitude" in coord_names
+    assert "latitude" in coord_names
+
+
+def test_load_zarr3_cmip6_metadata():
+    """
+    Test loading a Zarr3 store from a https Object Store.
+
+    This test loads just the metadata, no computations.
+
+    This is an actual CMIP6 dataset (Zarr built from netCDF4 via Xarray)
+    - Zarr store on disk: 243 MiB
+    - compression: Blosc
+    - Dimensions: (lat: 128, lon: 256, time: 2352, axis_nbounds: 2)
+    - chunking: time-slices; netCDF4.Dataset.chunking() = [1, 128, 256]
+
+    Test takes 8-9s (median: 8.5s) and needs max Res mem: 1GB
+    """
+    zarr_path = (
+        "https://uor-aces-o.s3-ext.jc.rl.ac.uk/"
+        "esmvaltool-zarr/pr_Amon_CNRM-ESM2-1_02Kpd-11_r1i1p2f2_gr_200601-220112.zarr3"
+    )
+
+    # with "dummy" storage options
+    cubes = load(
+        zarr_path,
+        ignore_warnings=None,
+        backend_kwargs={"storage_options": {}},
+    )
+
+    assert len(cubes) == 1
+    cube = cubes[0]
+    assert cube.var_name == "pr"
+    assert cube.standard_name == "precipitation_flux"
+    assert cube.long_name == "Precipitation"
+    assert cube.units == cf_units.Unit("kg m-2 s-1")
+    assert cube.has_lazy_data()
+
+
+def test_load_zarr_remote_not_zarr_file():
+    """
+    Test loading a Zarr store from a https Object Store.
+
+    This fails due to the file being loaded is not a Zarr file.
+    """
+    zarr_path = (
+        "https://uor-aces-o.s3-ext.jc.rl.ac.uk/"
+        "esmvaltool-zarr/example_field_0.zarr17"
+    )
+
+    msg = (
+        "File 'https://uor-aces-o.s3-ext.jc.rl.ac.uk/"
+        "esmvaltool-zarr/example_field_0.zarr17' can not "
+        "be opened as Zarr file at the moment."
+    )
+    with pytest.raises(ValueError, match=msg):
+        load(zarr_path)
+
+
+def test_load_zarr_remote_not_file():
+    """
+    Test loading a Zarr store from a https Object Store.
+
+    This fails due to non-existing file.
+    """
+    zarr_path = (
+        "https://uor-aces-o.s3-ext.jc.rl.ac.uk/"
+        "esmvaltool-zarr/example_field_0.zarr22"
+    )
+
+    msg = (
+        "File 'https://uor-aces-o.s3-ext.jc.rl.ac.uk/"
+        "esmvaltool-zarr/example_field_0.zarr22' can not "
+        "be opened as Zarr file at the moment."
+    )
+    with pytest.raises(ValueError, match=msg):
+        load(zarr_path)
+
+
+def test_load_zarr_local_not_file():
+    """
+    Test loading something that has a zarr extension.
+
+    But file doesn't exist (on local FS).
+    """
+    zarr_path = "esmvaltool-zarr/example_field_0.zarr22"
+
+    # "Unable to find group" or "No group found"
+    # Zarr keeps changing the exception string so matching
+    # is bound to fail the test
+    with pytest.raises(FileNotFoundError):
+        load(zarr_path)
+
+
+def test_load_zarr_local_not_zarr_file():
+    """
+    Test loading something that has a zarr extension.
+
+    But file is plaintext (on local FS).
+    """
+    zarr_path = (
+        Path(importlib_files("tests"))
+        / "sample_data"
+        / "zarr-sample-data"
+        / "example_field_0.zarr17"
+    )
+
+    # "Unable to find group" or "No group found"
+    # Zarr keeps changing the exception string so matching
+    # is bound to fail the test
+    with pytest.raises(FileNotFoundError):
+        load(zarr_path)

tests/sample_data/zarr-sample-data/example_field_0.zarr17

@@ -0,0 +1 @@
+This is not a Zarr file. Go grab lunch!

tests/sample_data/zarr-sample-data/example_field_0.zarr2/.zattrs

@@ -0,0 +1,3 @@
+{
+    "Conventions": "CF-1.12"
+}

tests/sample_data/zarr-sample-data/example_field_0.zarr2/.zgroup

@@ -0,0 +1,3 @@
+{
+    "zarr_format": 2
+}