interface PathCapabilities

The PathCapabilities interface provides a way to programmatically query the operations offered under a given path by an instance of FileSystem, FileContext or other implementing class.

public interface PathCapabilities {
  boolean hasPathCapability(Path path, String capability)
      throws IOException;
}

There are a number of goals here:

  1. Allow callers to probe for optional filesystem operations without actually having to invoke them.
  2. Allow filesystems with their own optional per-instance features to declare whether or not they are active for the specific instance.
  3. Allow for filesystem connectors which work with object stores to expose the fundamental difference in semantics of these stores (e.g: files not visible until closed, file rename being O(data)), directory rename being non-atomic, etc.

Available Capabilities

Capabilities are defined as strings and split into “Common Capabilites” and non-standard ones for a specific store.

The common capabilities are all defined under the prefix fs.capability.

Consult the javadocs for org.apache.hadoop.fs.CommonPathCapabilities for these.

Individual filesystems MAY offer their own set of capabilities which can be probed for. These MUST begin with fs. + the filesystem scheme + .capability. For example fs.s3a.capability.select.sql;

boolean hasPathCapability(path, capability)

Probe for the instance offering a specific capability under the given path.

Postconditions

if fs_supports_the_feature(path, capability):
  return True
else:
  return False

Return: True, iff the specific capability is available.

A filesystem instance MUST NOT return True for any capability unless it is known to be supported by that specific instance. As a result, if a caller probes for a capability then it can assume that the specific feature/semantics are available.

If the probe returns False then it can mean one of:

  1. The capability is unknown.
  2. The capability is known, and known to be unavailable on this instance.
  3. The capability is known but this local class does not know if it is supported under the supplied path.

This predicate is intended to be low cost. If it requires remote calls other than path/link resolution, it SHOULD conclude that the availability of the feature is unknown and return False.

The predicate MUST also be side-effect free.

Validity of paths There is no requirement that the existence of the path must be checked; the parameter exists so that any filesystem which relays operations to other filesystems (e.g viewfs) can resolve and relay it to the nested filesystem. Consider the call to be relatively lightweight.

Because of this, it may be that while the filesystem declares that it supports a capability under a path, the actual invocation of the operation may fail for other reasons.

As an example, while a filesystem may support append() under a path, if invoked on a directory, the call may fail.

That is for a path root = new Path("/"): the capabilities call may succeed

fs.hasCapabilities(root, "fs.capability.append") == true

But a subsequent call to the operation on that specific path may fail, because the root path is a directory:

fs.append(root)

Similarly, there is no checking that the caller has the permission to perform a specific operation: just because a feature is available on that path does not mean that the caller can execute the operation.

The hasCapabilities(path, capability) probe is therefore declaring that the operation will not be rejected as unsupported, not that a specific invocation will be permitted on that path by the caller.

Duration of availability

As the state of a remote store changes,so may path capabilities. This may be due to changes in the local state of the filesystem (e.g. symbolic links or mount points changing), or changes in its functionality (e.g. a feature becoming availaible/unavailable due to operational changes, system upgrades, etc.)

Capabilities which must be invoked to determine availablity

Some operations may be known by the client connector, and believed to be available, but may actually fail when invoked due to the state and permissons of the remote store —state which is cannot be determined except by attempting side-effecting operations.

A key example of this is symbolic links and the local filesystem. The filesystem declares that it supports this unless symbolic links are explicitly disabled —when invoked they may actually fail.

Implementors Notes

Implementors MUST NOT return true for any capability which is not guaranteed to be supported. To return true indicates that the implementation/deployment of the filesystem does, to the best of the knowledge of the filesystem client, offer the desired operations and semantics queried for.

For performance reasons, implementations SHOULD NOT check the path for existence, unless it needs to resolve symbolic links in parts of the path to determine whether a feature is present. This is required of FileContext and viewfs.

Individual filesystems MUST NOT unilaterally define new fs.capability-prefixed capabilities. Instead they MUST do one of the following:

  • Define and stabilize new cross-filesystem capability flags (preferred), and so formally add a new fs.capability value.
  • Use the scheme of the filesystem to as a prefix for their own options, e.g fs.hdfs.