Consolidate zarr metadata into single key

docs/api/convenience.rst

@@ -10,3 +10,5 @@ Convenience functions (``zarr.convenience``)
 .. autofunction:: copy_all
 .. autofunction:: copy_store
 .. autofunction:: tree
+.. autofunction:: consolidate_metadata
+.. autofunction:: open_consolidated

docs/api/storage.rst

@@ -27,6 +27,8 @@ Storage (``zarr.storage``)
     .. automethod:: invalidate_values
     .. automethod:: invalidate_keys
 
+.. autoclass:: ConsolidatedMetadataStore
+
 .. autofunction:: init_array
 .. autofunction:: init_group
 .. autofunction:: contains_array

docs/release.rst

@@ -9,6 +9,13 @@ Release notes
 Enhancements
 ~~~~~~~~~~~~
 
+* Add "consolidated" metadata as an experimental feature: use
+  :func:`zarr.convenience.consolidate_metadata` to copy all metadata from the various
+  metadata keys within a dataset hierarchy under a single key, and
+  :func:`zarr.convenience.open_consolidated` to use this single key. This can greatly
+  cut down the number of calls to the storage backend, and so remove a lot of overhead
+  for reading remote data. By :user:`Martin Durant <martindurant>`, :issue:`268`.
+
 * Support has been added for structured arrays with sub-array shape and/or nested fields. By
   :user:`Tarik Onalan <onalant>`, :issue:`111`, :issue:`296`.
 

docs/tutorial.rst

@@ -778,9 +778,11 @@ chunk size, which will reduce the number of chunks and thus reduce the number of
 round-trips required to retrieve data for an array (and thus reduce the impact of network
 latency). Another option is to try to increase the compression ratio by changing
 compression options or trying a different compressor (which will reduce the impact of
-limited network bandwidth). As of version 2.2, Zarr also provides the
-:class:`zarr.storage.LRUStoreCache` which can be used to implement a local in-memory cache
-layer over a remote store. E.g.::
+limited network bandwidth).
+
+As of version 2.2, Zarr also provides the :class:`zarr.storage.LRUStoreCache`
+which can be used to implement a local in-memory cache layer over a remote
+store. E.g.::
 
     >>> s3 = s3fs.S3FileSystem(anon=True, client_kwargs=dict(region_name='eu-west-2'))
     >>> store = s3fs.S3Map(root='zarr-demo/store', s3=s3, check=False)
@@ -797,13 +799,51 @@ layer over a remote store. E.g.::
     b'Hello from the cloud!'
     0.0009490990014455747
 
-If you are still experiencing poor performance with distributed/cloud storage, please
-raise an issue on the GitHub issue tracker with any profiling data you can provide, as
-there may be opportunities to optimise further either within Zarr or within the mapping
-interface to the storage.
+If you are still experiencing poor performance with distributed/cloud storage,
+please raise an issue on the GitHub issue tracker with any profiling data you
+can provide, as there may be opportunities to optimise further either within
+Zarr or within the mapping interface to the storage.
 
 .. _tutorial_copy:
 
+Consolidating metadata
+~~~~~~~~~~~~~~~~~~~~~~
+
+(This is an experimental feature.)
+
+Since there is a significant overhead for every connection to a cloud object
+store such as S3, the pattern described in the previous section may incur
+significant latency while scanning the metadata of the dataset hierarchy, even
+though each individual metadata object is small.  For cases such as these, once
+the data are static and can be regarded as read-only, at least for the
+metadata/structure of the dataset hierarchy, the many metadata objects can be
+consolidated into a single one via
+:func:`zarr.convenience.consolidate_metadata`. Doing this can greatly increase
+the speed of reading the dataset metadata, e.g.::
+
+   >>> zarr.consolidate_metadata(store)  # doctest: +SKIP
+
+This creates a special key with a copy of all of the metadata from all of the
+metadata objects in the store.
+
+Later, to open a Zarr store with consolidated metadata, use
+:func:`zarr.convenience.open_consolidated`, e.g.::
+
+   >>> root = zarr.open_consolidated(store)  # doctest: +SKIP
+
+This uses the special key to read all of the metadata in a single call to the
+backend storage.
+
+Note that, the hierarchy could still be opened in the normal way and altered,
+causing the consolidated metadata to become out of sync with the real state of
+the dataset hierarchy. In this case,
+:func:`zarr.convenience.consolidate_metadata` would need to be called again.
+
+To protect against consolidated metadata accidentally getting out of sync, the
+root group returned by :func:`zarr.convenience.open_consolidated` is read-only
+for the metadata, meaning that no new groups or arrays can be created, and
+arrays cannot be resized. However, data values with arrays can still be updated.
+
 Copying/migrating data
 ----------------------
 

requirements_dev.txt

@@ -46,6 +46,7 @@ python-dateutil==2.7.3
 readme-renderer==22.0
 requests==2.19.1
 requests-toolbelt==0.8.0
+setuptools-scm==3.1.0
 s3fs==0.1.6
 s3transfer==0.1.13
 scandir==1.9.0

zarr/__init__.py

@@ -12,6 +12,7 @@
 from zarr.sync import ThreadSynchronizer, ProcessSynchronizer
 from zarr.codecs import *
 from zarr.convenience import (open, save, save_array, save_group, load, copy_store,
-                              copy, copy_all, tree)
+                              copy, copy_all, tree, consolidate_metadata,
+                              open_consolidated)
 from zarr.errors import CopyError, MetadataError, PermissionError
 from zarr.version import version as __version__

zarr/attrs.py

@@ -4,8 +4,8 @@
 from collections import MutableMapping
 
 
-from zarr.compat import text_type
 from zarr.errors import PermissionError
+from zarr.meta import parse_metadata
 
 
 class Attributes(MutableMapping):
@@ -43,7 +43,7 @@ def _get_nosync(self):
         except KeyError:
             d = dict()
         else:
-            d = json.loads(text_type(data, 'ascii'))
+            d = parse_metadata(data)
         return d
 
     def asdict(self):

zarr/compat.py

@@ -19,6 +19,8 @@ class PermissionError(Exception):
     def OrderedDict_move_to_end(od, key):
         od[key] = od.pop(key)
 
+    from collections import Mapping
+
 
 else:  # pragma: py2 no cover
 
@@ -29,3 +31,5 @@ def OrderedDict_move_to_end(od, key):
 
     def OrderedDict_move_to_end(od, key):
         od.move_to_end(key)
+
+    from collections.abc import Mapping

zarr/convenience.py

@@ -15,28 +15,34 @@
 from zarr.errors import err_path_not_found, CopyError
 from zarr.util import normalize_storage_path, TreeViewer, buffer_size
 from zarr.compat import PY2, text_type
+from zarr.meta import ensure_str, json_dumps
 
 
 # noinspection PyShadowingBuiltins
-def open(store, mode='a', **kwargs):
+def open(store=None, mode='a', **kwargs):
     """Convenience function to open a group or array using file-mode-like semantics.
 
     Parameters
     ----------
-    store : MutableMapping or string
+    store : MutableMapping or string, optional
         Store or path to directory in file system or name of zip file.
     mode : {'r', 'r+', 'a', 'w', 'w-'}, optional
         Persistence mode: 'r' means read only (must exist); 'r+' means
         read/write (must exist); 'a' means read/write (create if doesn't
         exist); 'w' means create (overwrite if exists); 'w-' means create
         (fail if exists).
     **kwargs
-        Additional parameters are passed through to :func:`zarr.open_array` or
-        :func:`zarr.open_group`.
+        Additional parameters are passed through to :func:`zarr.creation.open_array` or
+        :func:`zarr.hierarchy.open_group`.
+
+    Returns
+    -------
+    z : :class:`zarr.core.Array` or :class:`zarr.hierarchy.Group`
+        Array or group, depending on what exists in the given store.
 
     See Also
     --------
-    zarr.open_array, zarr.open_group
+    zarr.creation.open_array, zarr.hierarchy.open_group
 
     Examples
     --------
@@ -68,7 +74,8 @@ def open(store, mode='a', **kwargs):
 
     path = kwargs.get('path', None)
     # handle polymorphic store arg
-    store = normalize_store_arg(store, clobber=(mode == 'w'))
+    clobber = mode == 'w'
+    store = normalize_store_arg(store, clobber=clobber)
     path = normalize_storage_path(path)
 
     if mode in {'w', 'w-', 'x'}:
@@ -1069,3 +1076,110 @@ def copy_all(source, dest, shallow=False, without_attrs=False, log=None,
         _log_copy_summary(log, dry_run, n_copied, n_skipped, n_bytes_copied)
 
     return n_copied, n_skipped, n_bytes_copied
+
+
+def consolidate_metadata(store, metadata_key='.zmetadata'):
+    """
+    Consolidate all metadata for groups and arrays within the given store
+    into a single resource and put it under the given key.
+
+    This produces a single object in the backend store, containing all the
+    metadata read from all the zarr-related keys that can be found. After
+    metadata have been consolidated, use :func:`open_consolidated` to open
+    the root group in optimised, read-only mode, using the consolidated
+    metadata to reduce the number of read operations on the backend store.
+
+    Note, that if the metadata in the store is changed after this
+    consolidation, then the metadata read by :func:`open_consolidated`
+    would be incorrect unless this function is called again.
+
+    .. note:: This is an experimental feature.
+
+    Parameters
+    ----------
+    store : MutableMapping or string
+        Store or path to directory in file system or name of zip file.
+    metadata_key : str
+        Key to put the consolidated metadata under.
+
+    Returns
+    -------
+    g : :class:`zarr.hierarchy.Group`
+        Group instance, opened with the new consolidated metadata.
+
+    See Also
+    --------
+    open_consolidated
+
+    """
+    import json
+
+    store = normalize_store_arg(store)
+
+    def is_zarr_key(key):
+        return (key.endswith('.zarray') or key.endswith('.zgroup') or
+                key.endswith('.zattrs'))
+
+    out = {
+        'zarr_consolidated_format': 1,
+        'metadata': {
+            key: json.loads(ensure_str(store[key]))
+            for key in store if is_zarr_key(key)
+        }
+    }
+    store[metadata_key] = json_dumps(out).encode()
+    return open_consolidated(store, metadata_key=metadata_key)
+
+
+def open_consolidated(store, metadata_key='.zmetadata', mode='r+'):
+    """Open group using metadata previously consolidated into a single key.
+
+    This is an optimised method for opening a Zarr group, where instead of
+    traversing the group/array hierarchy by accessing the metadata keys at
+    each level, a single key contains all of the metadata for everything.
+    For remote data sources where the overhead of accessing a key is large
+    compared to the time to read data.
+
+    The group accessed must have already had its metadata consolidated into a
+    single key using the function :func:`consolidate_metadata`.
+
+    This optimised method only works in modes which do not change the
+    metadata, although the data may still be written/updated.
+
+    Parameters
+    ----------
+    store : MutableMapping or string
+        Store or path to directory in file system or name of zip file.
+    metadata_key : str
+        Key to read the consolidated metadata from. The default (.zmetadata)
+        corresponds to the default used by :func:`consolidate_metadata`.
+    mode : {'r', 'r+'}, optional
+        Persistence mode: 'r' means read only (must exist); 'r+' means
+        read/write (must exist) although only writes to data are allowed,
+        changes to metadata including creation of new arrays or group
+        are not allowed.
+
+    Returns
+    -------
+    g : :class:`zarr.hierarchy.Group`
+        Group instance, opened with the consolidated metadata.
+
+    See Also
+    --------
+    consolidate_metadata
+
+    """
+
+    from .storage import ConsolidatedMetadataStore
+
+    # normalize parameters
+    store = normalize_store_arg(store)
+    if mode not in {'r', 'r+'}:
+        raise ValueError("invalid mode, expected either 'r' or 'r+'; found {!r}"
+                         .format(mode))
+
+    # setup metadata sotre
+    meta_store = ConsolidatedMetadataStore(store, metadata_key=metadata_key)
+
+    # pass through
+    return open(store=meta_store, chunk_store=store, mode=mode)

zarr/core.py

@@ -165,6 +165,9 @@ def _load_metadata_nosync(self):
             if config is None:
                 self._compressor = None
             else:
+                # temporary workaround for
+                # https://github.com/zarr-developers/numcodecs/issues/78
+                config = dict(config)
                 self._compressor = get_codec(config)
 
             # setup filters

zarr/creation.py

@@ -346,15 +346,15 @@ def array(data, **kwargs):
     return z
 
 
-def open_array(store, mode='a', shape=None, chunks=True, dtype=None, compressor='default',
-               fill_value=0, order='C', synchronizer=None, filters=None,
-               cache_metadata=True, cache_attrs=True, path=None, object_codec=None,
-               **kwargs):
+def open_array(store=None, mode='a', shape=None, chunks=True, dtype=None,
+               compressor='default', fill_value=0, order='C', synchronizer=None,
+               filters=None, cache_metadata=True, cache_attrs=True, path=None,
+               object_codec=None, chunk_store=None, **kwargs):
     """Open an array using file-mode-like semantics.
 
     Parameters
     ----------
-    store : MutableMapping or string
+    store : MutableMapping or string, optional
         Store or path to directory in file system or name of zip file.
     mode : {'r', 'r+', 'a', 'w', 'w-'}, optional
         Persistence mode: 'r' means read only (must exist); 'r+' means
@@ -391,6 +391,8 @@ def open_array(store, mode='a', shape=None, chunks=True, dtype=None, compressor=
         Array path within store.
     object_codec : Codec, optional
         A codec to encode object arrays, only needed if dtype=object.
+    chunk_store : MutableMapping or string, optional
+        Store or path to directory in file system or name of zip file.
 
     Returns
     -------
@@ -426,7 +428,10 @@ def open_array(store, mode='a', shape=None, chunks=True, dtype=None, compressor=
     # a : read/write if exists, create otherwise (default)
 
     # handle polymorphic store arg
-    store = normalize_store_arg(store, clobber=(mode == 'w'))
+    clobber = mode == 'w'
+    store = normalize_store_arg(store, clobber=clobber)
+    if chunk_store is not None:
+        chunk_store = normalize_store_arg(chunk_store, clobber=clobber)
     path = normalize_storage_path(path)
 
     # API compatibility with h5py
@@ -448,7 +453,7 @@ def open_array(store, mode='a', shape=None, chunks=True, dtype=None, compressor=
         init_array(store, shape=shape, chunks=chunks, dtype=dtype,
                    compressor=compressor, fill_value=fill_value,
                    order=order, filters=filters, overwrite=True, path=path,
-                   object_codec=object_codec)
+                   object_codec=object_codec, chunk_store=chunk_store)
 
     elif mode == 'a':
         if contains_group(store, path=path):
@@ -457,7 +462,7 @@ def open_array(store, mode='a', shape=None, chunks=True, dtype=None, compressor=
             init_array(store, shape=shape, chunks=chunks, dtype=dtype,
                        compressor=compressor, fill_value=fill_value,
                        order=order, filters=filters, path=path,
-                       object_codec=object_codec)
+                       object_codec=object_codec, chunk_store=chunk_store)
 
     elif mode in ['w-', 'x']:
         if contains_group(store, path=path):
@@ -468,14 +473,15 @@ def open_array(store, mode='a', shape=None, chunks=True, dtype=None, compressor=
             init_array(store, shape=shape, chunks=chunks, dtype=dtype,
                        compressor=compressor, fill_value=fill_value,
                        order=order, filters=filters, path=path,
-                       object_codec=object_codec)
+                       object_codec=object_codec, chunk_store=chunk_store)
 
     # determine read only status
     read_only = mode == 'r'
 
     # instantiate array
     z = Array(store, read_only=read_only, synchronizer=synchronizer,
-              cache_metadata=cache_metadata, cache_attrs=cache_attrs, path=path)
+              cache_metadata=cache_metadata, cache_attrs=cache_attrs, path=path,
+              chunk_store=chunk_store)
 
     return z