WIP: Add rechunking example

examples/rechunking/Dockerfile_virtualizarr

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+# Python 3.11
+FROM python:3.11-slim-buster
+
+
+RUN apt-get update \
+    # Install aws-lambda-cpp build dependencies
+    && apt-get install -y \
+      g++ \
+      make \
+      cmake \
+      unzip \
+    # cleanup package lists, they are not used anymore in this image
+    && rm -rf /var/lib/apt/lists/* \
+    && apt-cache search linux-headers-generic
+
+ARG FUNCTION_DIR="/function"
+
+# Copy function code
+RUN mkdir -p ${FUNCTION_DIR}
+
+# Update pip
+# NB botocore/boto3 are pinned due to https://github.com/boto/boto3/issues/3648
+#    using versions from https://github.com/aio-libs/aiobotocore/blob/72b8dd5d7d4ef2f1a49a0ae0c37b47e5280e2070/setup.py
+#    due to s3fs dependency
+RUN pip install --upgrade --ignore-installed pip wheel six setuptools \
+    && pip install --upgrade --no-cache-dir --ignore-installed \
+        awslambdaric \
+        botocore==1.29.76 \
+        boto3==1.26.76 \
+        redis \
+        httplib2 \
+        requests \
+        numpy \
+        scipy \
+        pandas \
+        pika \
+        kafka-python \
+        cloudpickle \
+        ps-mem \
+        tblib
+
+# Set working directory to function root directory
+WORKDIR ${FUNCTION_DIR}
+
+# Add Lithops
+COPY lithops_lambda.zip ${FUNCTION_DIR}
+RUN unzip lithops_lambda.zip \
+    && rm lithops_lambda.zip \
+    && mkdir handler \
+    && touch handler/__init__.py \
+    && mv entry_point.py handler/
+
+# Put your dependencies here, using RUN pip install... or RUN apt install...
+
+COPY requirements.txt requirements.txt
+RUN pip install --no-cache-dir -r requirements.txt
+
+ENTRYPOINT [ "/usr/local/bin/python", "-m", "awslambdaric" ]
+CMD [ "handler.entry_point.lambda_handler" ]

examples/rechunking/README.md

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+# Example rechunking workflow
+
+1. Set up a Python environment
+```bash
+conda create --name virtualizarr-rechunk -y python=3.11
+conda activate virtualizarr-rechunk
+pip install -r requirements.txt
+```
+
+1. Set up cubed executor by following https://github.com/cubed-dev/cubed/blob/main/examples/lithops/aws/README.md
+```bash
+export CUBED_CONFIG=$(pwd)
+
+1. Build a runtime image for Cubed
+```bash
+export LITHOPS_CONFIG_FILE=$(pwd)/config.aws
+lithops runtime build -b aws_lambda -f Dockerfile_virtualizarr virtualizarr-runtime
+```
+
+1. Run the script
+```bash
+python cubed-rechunk.py
+```
+
+## Cleaning up
+To rebuild the Litops image, delete the existing one by running
+```bash
+lithops runtime delete -b aws_lambda -d virtualizarr-runtime
+```

examples/rechunking/cubed-rechunk.py

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+# Example rechunking flow using VirtualiZarr, cubed, and xarray-cubed.
+# Inspired by Pythia's cookbook: https://projectpythia.org/kerchunk-cookbook
+# This script proves the concept but requires further optimization for
+# production. Please, suggest improvements.
+
+import fsspec
+import lithops
+import xarray as xr
+
+from virtualizarr import open_virtual_dataset
+
+fs_read = fsspec.filesystem("s3", anon=True, skip_instance_cache=True)
+files_paths = fs_read.glob("s3://wrf-se-ak-ar5/ccsm/rcp85/daily/2060/*")
+file_pattern = sorted(["s3://" + f for f in files_paths])
+
+# truncate file_pattern while debugging
+file_pattern = file_pattern[:4]
+
+print(f"{len(file_pattern)} file paths were retrieved.")
+
+
+def map_references(fil):
+    """ Open a virtual datasets from a list of file paths.
+    """
+    vds = open_virtual_dataset(fil,
+                               indexes={},
+                               loadable_variables=['Time'],
+                               cftime_variables=['Time'],
+                               )
+    return vds
+
+
+def reduce_references(results):
+    """ Combine virtual datasets into a single dataset.
+
+    """
+    combined_vds = xr.combine_nested(
+        results,
+        concat_dim=['Time'],
+        coords='minimal',
+        compat='override',
+    )
+    # possibly write parquet to s3 here
+    return combined_vds
+
+
+fexec = lithops.FunctionExecutor()  # config=lambda_config
+
+futures = fexec.map_reduce(
+    map_references,
+    file_pattern,
+    reduce_references,
+    spawn_reducer=100
+    )
+
+ds = futures.get_result()
+
+ds.virtualize.to_kerchunk('combined.json', format='json')
+
+# In jupyter, open_dataset seems to cache the json, such that
+# changes aren't propogated until the kernel is restarted.
+combined_ds = xr.open_dataset('combined.json',
+                              engine="kerchunk",
+                              chunks={},
+                              chunked_array_type='cubed',
+                              )
+
+combined_ds['Time'].attrs = {}  # to_zarr complains about attrs
+
+rechunked_ds = combined_ds.chunk(
+    chunks={'Time': 5, 'south_north': 25, 'west_east': 32}
+)
+
+rechunked_ds.to_zarr('rechunked.zarr',
+                     mode='w',
+                     encoding={},  # TODO
+                     consolidated=True,
+                     safe_chunks=False,
+                     )

examples/rechunking/cubed.yaml

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+spec:
+  work_dir: "s3://cubed-$USER-temp"
+  allowed_mem: "2GB"
+  executor_name: "lithops"
+  executor_options:
+    runtime: "virtualizarr-runtime"
+    runtime_memory: 2000

examples/rechunking/requirements.txt

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+boto
+cftime
+cubed
+cubed-xarray
+h5py
+kerchunk
+lithops
+s3fs
+virtualizarr
+xarray