docs for config

docs/user-guide/extending.rst

@@ -68,8 +68,6 @@ useful for providing specialized implementations, such as GPU-based codecs. In c
 multiple codecs, the :mod:`zarr.core.config` mechanism can be used to select the preferred
 implementation. 
 
-TODO: Link to documentation of :mod:`zarr.core.config`
-
 .. note::
     This sections explains how custom codecs can be created for Zarr version 3. For Zarr
     version 2, codecs should subclass the 

docs/user-guide/v3_migration.rst

@@ -138,7 +138,7 @@ Configuration
 ~~~~~~~~~~~~~
 
 There is a new configuration system based on `donfig <https://github.com/pytroll/donfig>`_, 
-which can be accessed via :data:`zarr.config`. 
+which can be accessed via :mod:`zarr.core.config`. 
 Configuration values can be set using code like the following:
 
 .. code-block:: python

src/zarr/core/config.py

@@ -1,3 +1,64 @@
+"""
+The config module is responsible for managing the configuration of zarr
+and is based on the `donfig <https://github.com/pytroll/donfig>`_ Python library.
+
+Configuration values can be set using code like the following:
+
+.. code-block:: python
+
+    import zarr
+    zarr.config.set({"array.order": "F"})
+
+Alternatively, configuration values can be set using environment variables, e.g.
+``ZARR_ARRAY__ORDER=F``.
+
+The configuration can also be read from a YAML file in standard locations.
+For more information, see the
+`donfig documentation <https://donfig.readthedocs.io/en/latest/>`_.
+
+Configuration options include the following:
+
+- Default Zarr format ``default_zarr_version``
+- Default array order in memory ``array.order``
+- Async and threading options, e.g. ``async.concurrency`` and ``threading.max_workers``
+- Selections of implementations of codecs, codec pipelines and buffers
+
+For selecting custom implementations of codecs, pipelines, buffers and ndbuffers,
+first register the implementations in the registry and then select them in the config.
+For example, an implementation of the bytes codec in a class "custompackage.NewBytesCodec",
+requires the value of ``codecs.bytes.name`` to be "custompackage.NewBytesCodec".
+
+This is the current default configuration:
+
+.. code-block:: python
+
+    {
+        "default_zarr_version": 3,
+        "array": {"order": "C"},
+        "async": {"concurrency": 10, "timeout": None},
+        "threading": {"max_workers": None},
+        "json_indent": 2,
+        "codec_pipeline": {
+            "path": "zarr.core.codec_pipeline.BatchedCodecPipeline",
+            "batch_size": 1,
+        },
+        "codecs": {
+            "blosc": "zarr.codecs.blosc.BloscCodec",
+            "gzip": "zarr.codecs.gzip.GzipCodec",
+            "zstd": "zarr.codecs.zstd.ZstdCodec",
+            "bytes": "zarr.codecs.bytes.BytesCodec",
+            "endian": "zarr.codecs.bytes.BytesCodec",
+            "crc32c": "zarr.codecs.crc32c_.Crc32cCodec",
+            "sharding_indexed": "zarr.codecs.sharding.ShardingCodec",
+            "transpose": "zarr.codecs.transpose.TransposeCodec",
+            "vlen-utf8": "zarr.codecs.vlen_utf8.VLenUTF8Codec",
+            "vlen-bytes": "zarr.codecs.vlen_utf8.VLenBytesCodec",
+        },
+        "buffer": "zarr.core.buffer.cpu.Buffer",
+        "ndbuffer": "zarr.core.buffer.cpu.NDBuffer",
+    }
+"""
+
 from __future__ import annotations
 
 from typing import Any, Literal, cast