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Added note about refresh and POSIX behavior #648

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Added note about refresh and POSIX behavior #648

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8 changes: 7 additions & 1 deletion gcsfs/core.py
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Original file line number Diff line number Diff line change
Expand Up @@ -210,7 +210,13 @@ class GCSFileSystem(asyn.AsyncFileSystem):
GCSFileSystem maintains a per-implied-directory cache of object listings and
fulfills all object information and listing requests from cache. This implied, for example, that objects
created via other processes *will not* be visible to the GCSFileSystem until the cache
refreshed. Calls to GCSFileSystem.open and calls to GCSFile are not effected by this cache.
refreshed. Calls to GCSFileSystem.open and calls to GCSFile are not affected by this cache.

*Note that caching creates divergences from the behavior of POSIX utilities such as 'ls'*. In a POSIX
filesystem, ls always returns the current state of a directory; however, if `refresh = False` (the
default), ls returns the state of the cache. `refresh = False` is the default because (unlike in a POSIX filesystem)
listing directory contents of a GCS bucket can be expensive. Developers are advised to set `refresh = True`
if exact POSIX behavior is desired.
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I wouldn't presuppose here what the user's expectations are. Let me have a go, see what you think.

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*Note that caching creates divergences from the behavior of POSIX utilities such as 'ls'*. In a POSIX
filesystem, ls always returns the current state of a directory; however, if `refresh = False` (the
default), ls returns the state of the cache. `refresh = False` is the default because (unlike in a POSIX filesystem)
listing directory contents of a GCS bucket can be expensive. Developers are advised to set `refresh = True`
if exact POSIX behavior is desired.
Note that directory listings are cached by default, because fetching those listings can be expensive. This is
contrary to local filesystem behaviour. The cache will be cleared if writing from this instance, but it can
become stale and return incorrect results if the storage is written to from another process/machine.
If you anticipate this possibility, you can set the use_listings_cache and listings_expiry_time arguments
to configure the caching, call `.invalidate_cache()` when required, or pass `refresh=True` to the
various listing methods.


In the default case the cache is never expired. This may be controlled via the ``cache_timeout``
GCSFileSystem parameter or via explicit calls to ``GCSFileSystem.invalidate_cache``.
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