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#
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import collections.abc
import warnings
from typing import Iterator, List
import zarr.storage
from xcube.util.assertions import assert_instance, assert_true
[docs]
class CachedZarrStore(zarr.storage.Store):
"""A read-only Zarr store that is faster than
*store* because it uses a writable *cache* store.
The *cache* store is assumed to
read values for a given key much faster than *store*.
Note that iterating keys and containment checks are performed
on *store* only.
:param store: A Zarr store that is known
to be slow in reading values.
:param cache: A writable Zarr store that can
read values faster than *store*.
"""
_readable = True # Because the base class is readable
_listable = True # Because the base class is listable
_writeable = False # Because this is not yet supported
_erasable = False # Because this is not yet supported
def __init__(self,
store: collections.abc.MutableMapping,
cache: collections.abc.MutableMapping):
assert_instance(store, collections.abc.MutableMapping, name="store")
assert_instance(cache, collections.abc.MutableMapping, name="cache")
if not isinstance(store, zarr.storage.BaseStore):
store = zarr.storage.KVStore(store)
if not isinstance(cache, zarr.storage.BaseStore):
cache = zarr.storage.KVStore(cache)
assert_true(store.is_readable(), message='store must be readable')
assert_true(cache.is_readable(), message='cache must be readable')
assert_true(cache.is_writeable(), message='cache must be writable')
self._store = store
self._cache = cache
self._implement_op('listdir')
self._implement_op('getsize')
@property
def store(self) -> zarr.storage.BaseStore:
return self._store
@property
def cache(self) -> zarr.storage.BaseStore:
return self._cache
def _implement_op(self, op: str):
if hasattr(self._store, op):
assert hasattr(self, "_" + op)
setattr(self, op, getattr(self, "_" + op))
def _listdir(self, path: str = "") -> List[str]:
# noinspection PyUnresolvedReferences
return self._store.listdir(path=path)
def _getsize(self, path: str) -> None:
# noinspection PyBroadException
try:
# noinspection PyUnresolvedReferences
size = self._cache.getsize(path)
except BaseException:
size = -1
if size < 0:
# noinspection PyUnresolvedReferences
size = self._store.getsize(path)
return size
def __len__(self) -> int:
return len(self._store)
def __iter__(self) -> Iterator[str]:
return iter(self._store)
def __contains__(self, key: str):
return key in self._store
def __getitem__(self, key: str) -> bytes:
try:
return self._cache[key]
except KeyError:
pass
value = self._store[key]
# noinspection PyBroadException
try:
self._cache[key] = value
except BaseException as e:
warnings.warn(f"cache write failed for key {key!r}: {e}")
return value
def __setitem__(self, key: str, value: bytes) -> None:
raise NotImplementedError()
def __delitem__(self, key: str) -> None:
raise NotImplementedError()