The abstract FileSystem class is the original class to access Hadoop filesystems; non-abstract subclasses exist for all Hadoop-supported filesystems.
All operations that take a Path to this interface MUST support relative paths. In such a case, they must be resolved relative to the working directory defined by setWorkingDirectory().
For all clients, therefore, we also add the notion of a state component PWD: this represents the present working directory of the client. Changes to this state are not reflected in the filesystem itself: they are unique to the instance of the client.
Implementation Note: the static FileSystem get(URI uri, Configuration conf) method MAY return a pre-existing instance of a filesystem client class—a class that may also be in use in other threads. The implementations of FileSystem shipped with Apache Hadoop do not make any attempt to synchronize access to the working directory field.
All the requirements of a valid FileSystem are considered implicit preconditions and postconditions: all operations on a valid FileSystem MUST result in a new FileSystem that is also valid.
Get the status of a path
result = stat: FileStatus where:
if isFile(FS, p) :
stat.length = len(FS.Files[p])
stat.isdir = False
stat.blockSize > 0
elif isDir(FS, p) :
stat.length = 0
stat.isdir = True
elif isSymlink(FS, p) :
stat.length = 0
stat.isdir = False
stat.symlink = FS.Symlinks[p]
stat.hasAcl = hasACL(FS, p)
stat.isEncrypted = inEncryptionZone(FS, p)
stat.isErasureCoded = isErasureCoded(FS, p)
The returned FileStatus status of the path additionally carries details on ACL, encryption and erasure coding information. getFileStatus(Path p).hasAcl() can be queried to find if the path has an ACL. getFileStatus(Path p).isEncrypted() can be queried to find if the path is encrypted. getFileStatus(Path p).isErasureCoded() will tell if the path is erasure coded or not.
The function getHomeDirectory returns the home directory for the FileSystem and the current user account.
For some FileSystems, the path is ["/", "users", System.getProperty("user-name")].
However, for HDFS, the username is derived from the credentials used to authenticate the client with HDFS. This may differ from the local user account name.
It is the responsibility of the FileSystem to determine the actual home directory of the caller.
Lists entries under a path, path.
If path refers to a file and the filter accepts it, then that file’s FileStatus entry is returned in a single-element array.
If the path refers to a directory, the call returns a list of all its immediate child paths which are accepted by the filter —and does not include the directory itself.
A PathFilter filter is a class whose accept(path) returns true iff the path path meets the filter’s conditions.
if isFile(FS, path) and filter.accept(path) :
result = [ getFileStatus(path) ]
elif isFile(FS, path) and not filter.accept(P) :
result = []
elif isDir(FS, path):
result = [
getFileStatus(c) for c in children(FS, path) if filter.accepts(c)
]
Implicit invariant: the contents of a FileStatus of a child retrieved via listStatus() are equal to those from a call of getFileStatus() to the same path:
forall fs in listStatus(path) : fs == getFileStatus(fs.path)
Ordering of results: there is no guarantee of ordering of the listed entries. While HDFS currently returns an alphanumerically sorted list, neither the Posix readdir() nor Java’s File.listFiles() API calls define any ordering of returned values. Applications which require a uniform sort order on the results must perform the sorting themselves.
Null return: Local filesystems prior to 3.0.0 returned null upon access error. It is considered erroneous. Expect IOException upon access error.
By the time the listStatus() operation returns to the caller, there is no guarantee that the information contained in the response is current. The details MAY be out of date, including the contents of any directory, the attributes of any files, and the existence of the path supplied.
The state of a directory MAY change during the evaluation process.
After an entry at path P is created, and before any other changes are made to the filesystem, listStatus(P) MUST find the file and return its status.
After an entry at path P is deleted, and before any other changes are made to the filesystem, listStatus(P) MUST raise a FileNotFoundException.
After an entry at path P is created, and before any other changes are made to the filesystem, the result of listStatus(parent(P)) SHOULD include the value of getFileStatus(P).
After an entry at path P is created, and before any other changes are made to the filesystem, the result of listStatus(parent(P)) SHOULD NOT include the value of getFileStatus(P).
This is not a theoretical possibility, it is observable in HDFS when a directory contains many thousands of files.
Consider a directory "/d" with the contents:
a part-0000001 part-0000002 ... part-9999999
If the number of files is such that HDFS returns a partial listing in each response, then, if a listing listStatus("/d") takes place concurrently with the operation rename("/d/a","/d/z")), the result may be one of:
[a, part-0000001, ... , part-9999999] [part-0000001, ... , part-9999999, z] [a, part-0000001, ... , part-9999999, z] [part-0000001, ... , part-9999999]
While this situation is likely to be a rare occurrence, it MAY happen. In HDFS these inconsistent views are only likely when listing a directory with many children.
Other filesystems may have stronger consistency guarantees, or return inconsistent data more readily.
This is exactly equivalent to listStatus(Path, DEFAULT_FILTER) where DEFAULT_FILTER.accept(path) = True for all paths.
The atomicity and consistency constraints are as for listStatus(Path, DEFAULT_FILTER).
Enumerate all files found in the list of directories passed in, calling listStatus(path, filter) on each one.
As with listStatus(path, filter), the results may be inconsistent. That is: the state of the filesystem changed during the operation.
There are no guarantees as to whether paths are listed in a specific order, only that they must all be listed, and, at the time of listing, exist.
All paths must exist. There is no requirement for uniqueness.
forall p in paths : exists(fs, p) else raise FileNotFoundException
The result is an array whose entries contain every status element found in the path listings, and no others.
result = [listStatus(p, filter) for p in paths]
Implementations MAY merge duplicate entries; and/or optimize the operation by recoginizing duplicate paths and only listing the entries once.
The default implementation iterates through the list; it does not perform any optimizations.
The atomicity and consistency constraints are as for listStatus(Path, PathFilter).
Enumerate all files found in the list of directories passed in, calling listStatus(path, DEFAULT_FILTER) on each one, where the DEFAULT_FILTER accepts all path names.
Return an iterator enumerating the LocatedFileStatus entries under a path. This is similar to listStatus(Path) except that the return value is an instance of the LocatedFileStatus subclass of a FileStatus, and that rather than return an entire list, an iterator is returned.
This is actually a protected method, directly invoked by listLocatedStatus(Path path). Calls to it may be delegated through layered filesystems, such as FilterFileSystem, so its implementation MUST be considered mandatory, even if listLocatedStatus(Path path) has been implemented in a different manner. There are open JIRAs proposing making this method public; it may happen in future.
There is no requirement for the iterator to provide a consistent view of the child entries of a path. The default implementation does use listStatus(Path) to list its children, with its consistency constraints already documented. Other implementations may perform the enumeration even more dynamically. For example fetching a windowed subset of child entries, so avoiding building up large data structures and the transmission of large messages. In such situations, changes to the filesystem are more likely to become visible.
Callers MUST assume that the iteration operation MAY fail if changes to the filesystem take place between this call returning and the iteration being completely performed.
The operation generates a set of results, resultset, equal to the result of listStatus(path, filter):
if isFile(FS, path) and filter.accept(path) :
resultset = [ getLocatedFileStatus(FS, path) ]
elif isFile(FS, path) and not filter.accept(path) :
resultset = []
elif isDir(FS, path) :
resultset = [
getLocatedFileStatus(FS, c)
for c in children(FS, path) where filter.accept(c)
]
The operation getLocatedFileStatus(FS, path: Path): LocatedFileStatus is defined as a generator of a LocatedFileStatus instance ls where:
fileStatus = getFileStatus(FS, path) bl = getFileBlockLocations(FS, path, 0, fileStatus.len) locatedFileStatus = new LocatedFileStatus(fileStatus, bl)
The ordering in which the elements of resultset are returned in the iterator is undefined.
The atomicity and consistency constraints are as for listStatus(Path, PathFilter).
The equivalent to listLocatedStatus(path, DEFAULT_FILTER), where DEFAULT_FILTER accepts all path names.
Create an iterator over all files in/under a directory, potentially recursing into child directories.
The goal of this operation is to permit large recursive directory scans to be handled more efficiently by filesystems, by reducing the amount of data which must be collected in a single RPC call.
The outcome is an iterator, whose output from the sequence of iterator.next() calls can be defined as the set iteratorset:
if not recursive:
iteratorset == listStatus(path)
else:
iteratorset = [
getLocatedFileStatus(FS, d)
for d in descendants(FS, path)
]
The function getLocatedFileStatus(FS, d) is as defined in listLocatedStatus(Path, PathFilter).
The atomicity and consistency constraints are as for listStatus(Path, PathFilter).
if s < 0 or l < 0 : raise {HadoopIllegalArgumentException, InvalidArgumentException}
If the filesystem is location aware, it must return the list of block locations where the data in the range [s:s+l] can be found.
if f == null :
result = null
elif f.getLen() <= s:
result = []
else result = [ locations(FS, b) for b in blocks(FS, p, s, s+l)]
Where
def locations(FS, b) = a list of all locations of a block in the filesystem def blocks(FS, p, s, s + l) = a list of the blocks containing data(FS, path)[s:s+l]
Note that that as length(FS, f) is defined as 0 if isDir(FS, f), the result of getFileBlockLocations() on a directory is []
If the filesystem is not location aware, it SHOULD return
[
BlockLocation(["localhost:9866"] ,
["localhost"],
["/default/localhost"]
0, f.getLen())
] ;
*A bug in Hadoop 1.0.3 means that a topology path of the same number of elements as the cluster topology MUST be provided, hence Filesystems SHOULD return that "/default/localhost" path. While this is no longer an issue, the convention is generally retained.
Get the “default” block size for a filesystem. This is often used during split calculations to divide work optimally across a set of worker processes.
result = integer > 0
Although there is no defined minimum value for this result, as it is used to partition work during job submission, a block size that is too small will result in badly partitioned workload, or even the JobSubmissionClient and equivalent running out of memory as it calculates the partitions.
Any FileSystem that does not actually break files into blocks SHOULD return a number for this that results in efficient processing. A FileSystem MAY make this user-configurable (the S3 and Swift filesystem clients do this).
Get the “default” block size for a path —that is, the block size to be used when writing objects to a path in the filesystem.
result = integer >= 0
The outcome of this operation is usually identical to getDefaultBlockSize(), with no checks for the existence of the given path.
Filesystems that support mount points may have different default values for different paths, in which case the specific default value for the destination path SHOULD be returned.
It is not an error if the path does not exist: the default/recommended value for that part of the filesystem MUST be returned.
This method is exactly equivalent to querying the block size of the FileStatus structure returned in getFileStatus(p). It is deprecated in order to encourage users to make a single call to getFileStatus(p) and then use the result to examine multiple attributes of the file (e.g. length, type, block size). If more than one attribute is queried, This can become a significant performance optimization —and reduce load on the filesystem.
Create a directory and all its parents
if exists(FS, p) and not isDir(FS, p) :
raise [ParentNotDirectoryException, FileAlreadyExistsException, IOException]
FS' where FS'.Directories' = FS.Directories + [p] + ancestors(FS, p) result = True
The condition exclusivity requirement of a FileSystem’s directories, files and symbolic links must hold.
The probe for the existence and type of a path and directory creation MUST be atomic. The combined operation, including mkdirs(parent(F)) MAY be atomic.
The return value is always true—even if a new directory is not created (this is defined in HDFS).
FSDataOutputStream create(Path p,
FsPermission permission,
boolean overwrite,
int bufferSize,
short replication,
long blockSize,
Progressable progress) throws IOException;
The file must not exist for a no-overwrite create:
if not overwrite and isFile(FS, p) : raise FileAlreadyExistsException
Writing to or overwriting a directory must fail.
if isDir(FS, p) : raise {FileAlreadyExistsException, FileNotFoundException, IOException}
FileSystems may reject the request for other reasons, such as the FS being read-only (HDFS), the block size being below the minimum permitted (HDFS), the replication count being out of range (HDFS), quotas on namespace or filesystem being exceeded, reserved names, etc. All rejections SHOULD be IOException or a subclass thereof and MAY be a RuntimeException or subclass. For instance, HDFS may raise a InvalidPathException.
FS' where : FS'.Files'[p] == [] ancestors(p) is-subset-of FS'.Directories' result = FSDataOutputStream
The updated (valid) FileSystem must contains all the parent directories of the path, as created by mkdirs(parent(p)).
The result is FSDataOutputStream, which through its operations may generate new filesystem states with updated values of FS.Files[p]
Some implementations split the create into a check for the file existing from the actual creation. This means the operation is NOT atomic: it is possible for clients creating files with overwrite==true to fail if the file is created by another client between the two tests.
S3A, Swift and potentially other Object Stores do not currently change the FS state until the output stream close() operation is completed. This MAY be a bug, as it allows >1 client to create a file with overwrite==false, and potentially confuse file/directory logic
The Local FileSystem raises a FileNotFoundException when trying to create a file over a directory, hence it is listed as an exception that MAY be raised when this precondition fails.
Not covered: symlinks. The resolved path of the symlink is used as the final path argument to the create() operation
Make a FSDataOutputStreamBuilder to specify the parameters to create a file.
createFile(p) returns a FSDataOutputStreamBuilder only and does not make change on filesystem immediately. When build() is invoked on the FSDataOutputStreamBuilder, the builder parameters are verified and create(Path p) is invoked on the underlying filesystem. build() has the same preconditions and postconditions as create(Path p).
Implementations without a compliant call SHOULD throw UnsupportedOperationException.
Make a FSDataOutputStreamBuilder to specify the parameters to append to an existing file.
appendFile(p) returns a FSDataOutputStreamBuilder only and does not make change on filesystem immediately. When build() is invoked on the FSDataOutputStreamBuilder, the builder parameters are verified and append() is invoked on the underlying filesystem. build() has the same preconditions and postconditions as append().
Implementations without a compliant call SHOULD throw UnsupportedOperationException.
if not isFile(FS, p)) : raise [FileNotFoundException, IOException]
This is a critical precondition. Implementations of some FileSystems (e.g. Object stores) could shortcut one round trip by postponing their HTTP GET operation until the first read() on the returned FSDataInputStream. However, much client code does depend on the existence check being performed at the time of the open() operation. Implementations MUST check for the presence of the file at the time of creation. This does not imply that the file and its data is still at the time of the following read() or any successors.
result = FSDataInputStream(0, FS.Files[p])
The result provides access to the byte array defined by FS.Files[p]; whether that access is to the contents at the time the open() operation was invoked, or whether and how it may pick up changes to that data in later states of FS is an implementation detail.
The result MUST be the same for local and remote callers of the operation.
HDFS MAY throw UnresolvedPathException when attempting to traverse symbolic links
HDFS throws IOException("Cannot open filename " + src) if the path exists in the metadata, but no copies of any its blocks can be located; -FileNotFoundException would seem more accurate and useful.
Delete a path, be it a file, symbolic link or directory. The recursive flag indicates whether a recursive delete should take place —if unset then a non-empty directory cannot be deleted.
Except in the special case of the root directory, if this API call completed successfully then there is nothing at the end of the path. That is: the outcome is desired. The return flag simply tells the caller whether or not any change was made to the state of the filesystem.
Note: many uses of this method surround it with checks for the return value being false, raising exception if so. For example
if (!fs.delete(path, true)) throw new IOException("Could not delete " + path);
This pattern is not needed. Code SHOULD just call delete(path, recursive) and assume the destination is no longer present —except in the special case of root directories, which will always remain (see below for special coverage of root directories).
A directory with children and recursive == False cannot be deleted
if isDir(FS, p) and not recursive and (children(FS, p) != {}) : raise IOException
(HDFS raises PathIsNotEmptyDirectoryException here.)
If the file does not exist the filesystem state does not change
if not exists(FS, p):
FS' = FS
result = False
The result SHOULD be False, indicating that no file was deleted.
A path referring to a file is removed, return value: True
if isFile(FS, p) :
FS' = (FS.Directories, FS.Files - [p], FS.Symlinks)
result = True
Deleting an empty root does not change the filesystem state and may return true or false.
if isDir(FS, p) and isRoot(p) and children(FS, p) == {} :
FS ' = FS
result = (undetermined)
There is no consistent return code from an attempt to delete the root directory.
Implementations SHOULD return true; this avoids code which checks for a false return value from overreacting.
Deleting an empty directory that is not root will remove the path from the FS and return true.
if isDir(FS, p) and not isRoot(p) and children(FS, p) == {} :
FS' = (FS.Directories - [p], FS.Files, FS.Symlinks)
result = True
Deleting a root path with children and recursive==True can do one of two things.
The POSIX model assumes that if the user has the correct permissions to delete everything, they are free to do so (resulting in an empty filesystem).
if isDir(FS, p) and isRoot(p) and recursive :
FS' = ({["/"]}, {}, {}, {})
result = True
HDFS never permits the deletion of the root of a filesystem; the filesystem must be taken offline and reformatted if an empty filesystem is desired.
if isDir(FS, p) and isRoot(p) and recursive :
FS' = FS
result = False
HDFS has the notion of Protected Directories, which are declared in the option fs.protected.directories. Any attempt to delete such a directory or a parent thereof raises an AccessControlException. Accordingly, any attempt to delete the root directory SHALL, if there is a protected directory, result in such an exception being raised.
This specification does not recommend any specific action. Do note, however, that the POSIX model assumes that there is a permissions model such that normal users do not have the permission to delete that root directory; it is an action which only system administrators should be able to perform.
Any filesystem client which interacts with a remote filesystem which lacks such a security model, MAY reject calls to delete("/", true) on the basis that it makes it too easy to lose data.
Deleting a non-root path with children recursive==true removes the path and all descendants
if isDir(FS, p) and not isRoot(p) and recursive :
FS' where:
not isDir(FS', p)
and forall d in descendants(FS, p):
not isDir(FS', d)
not isFile(FS', d)
not isSymlink(FS', d)
result = True
Deleting a file MUST be an atomic action.
Deleting an empty directory MUST be an atomic action.
A recursive delete of a directory tree MUST be atomic.
In terms of its specification, rename() is one of the most complex operations within a filesystem .
In terms of its implementation, it is the one with the most ambiguity regarding when to return false versus raising an exception.
Rename includes the calculation of the destination path. If the destination exists and is a directory, the final destination of the rename becomes the destination + the filename of the source path.
let dest = if (isDir(FS, src) and d != src) :
d + [filename(src)]
else :
d
All checks on the destination path MUST take place after the final dest path has been calculated.
Source src must exist:
exists(FS, src) else raise FileNotFoundException
dest cannot be a descendant of src:
if isDescendant(FS, src, dest) : raise IOException
This implicitly covers the special case of isRoot(FS, src).
dest must be root, or have a parent that exists:
isRoot(FS, dest) or exists(FS, parent(dest)) else raise IOException
The parent path of a destination must not be a file:
if isFile(FS, parent(dest)) : raise IOException
This implicitly covers all the ancestors of the parent.
There must not be an existing file at the end of the destination path:
if isFile(FS, dest) : raise FileAlreadyExistsException, IOException
Renaming a directory onto itself is no-op; return value is not specified.
In POSIX the result is False; in HDFS the result is True.
if isDir(FS, src) and src == dest :
FS' = FS
result = (undefined)
Renaming a file to itself is a no-op; the result is True.
if isFile(FS, src) and src == dest :
FS' = FS
result = True
Renaming a file where the destination is a directory moves the file as a child of the destination directory, retaining the filename element of the source path.
if isFile(FS, src) and src != dest:
FS' where:
not exists(FS', src)
and exists(FS', dest)
and data(FS', dest) == data (FS, dest)
result = True
If src is a directory then all its children will then exist under dest, while the path src and its descendants will no longer exist. The names of the paths under dest will match those under src, as will the contents:
if isDir(FS, src) isDir(FS, dest) and src != dest :
FS' where:
not exists(FS', src)
and dest in FS'.Directories]
and forall c in descendants(FS, src) :
not exists(FS', c))
and forall c in descendants(FS, src) where isDir(FS, c):
isDir(FS', dest + childElements(src, c)
and forall c in descendants(FS, src) where not isDir(FS, c):
data(FS', dest + childElements(s, c)) == data(FS, c)
result = True
not exists(FS, parent(dest))
There is no consistent behavior here.
HDFS
The outcome is no change to FileSystem state, with a return value of false.
FS' = FS; result = False
Local Filesystem
The outcome is as a normal rename, with the additional (implicit) feature that the parent directories of the destination also exist.
exists(FS', parent(dest))
*Other Filesystems (including Swift) *
Other filesystems strictly reject the operation, raising a FileNotFoundException
The core operation of rename()—moving one entry in the filesystem to another—MUST be atomic. Some applications rely on this as a way to coordinate access to data.
Some FileSystem implementations perform checks on the destination FileSystem before and after the rename. One example of this is ChecksumFileSystem, which provides checksummed access to local data. The entire sequence MAY NOT be atomic.
Files open for reading, writing or appending
The behavior of rename() on an open file is unspecified: whether it is allowed, what happens to later attempts to read from or write to the open stream
Renaming a directory onto itself
The return code of renaming a directory onto itself is unspecified.
Destination exists and is a file
Renaming a file atop an existing file is specified as failing, raising an exception.
Local FileSystem : the rename succeeds; the destination file is replaced by the source file.
HDFS : The rename fails, no exception is raised. Instead the method call simply returns false.
Missing source file
If the source file src does not exist, FileNotFoundException should be raised.
HDFS fails without raising an exception; rename() merely returns false.
FS' = FS result = false
The behavior of HDFS here should not be considered a feature to replicate. FileContext explicitly changed the behavior to raise an exception, and the retrofitting of that action to the DFSFileSystem implementation is an ongoing matter for debate.
Joins multiple blocks together to create a single file. This is a little-used operation currently implemented only by HDFS.
Implementations without a compliant call SHOULD throw UnsupportedOperationException.
if not exists(FS, p) : raise FileNotFoundException if sources==[] : raise IllegalArgumentException
All sources MUST be in the same directory:
for s in sources: if parent(S) != parent(p) raise IllegalArgumentException
All block sizes must match that of the target:
for s in sources: getBlockSize(FS, S) == getBlockSize(FS, p)
No duplicate paths:
not (exists p1, p2 in (sources + [p]) where p1 == p2)
HDFS: All source files except the final one MUST be a complete block:
for s in (sources[0:length(sources)-1] + [p]): (length(FS, s) mod getBlockSize(FS, p)) == 0
FS' where: (data(FS', T) = data(FS, T) + data(FS, sources[0]) + ... + data(FS, srcs[length(srcs)-1])) and for s in srcs: not exists(FS', S)
HDFS’s restrictions may be an implementation detail of how it implements concat by changing the inode references to join them together in a sequence. As no other filesystem in the Hadoop core codebase implements this method, there is no way to distinguish implementation detail from specification.
Truncate file p to the specified newLength.
Implementations without a compliant call SHOULD throw UnsupportedOperationException.
if not exists(FS, p) : raise FileNotFoundException if isDir(FS, p) : raise [FileNotFoundException, IOException] if newLength < 0 || newLength > len(FS.Files[p]) : raise HadoopIllegalArgumentException
HDFS: The source file MUST be closed. Truncate cannot be performed on a file, which is open for writing or appending.
FS' where:
len(FS.Files[p]) = newLength
Return: true, if truncation is finished and the file can be immediately opened for appending, or false otherwise.
HDFS: HDFS returns false to indicate that a background process of adjusting the length of the last block has been started, and clients should wait for it to complete before they can proceed with further file updates.
The RemoteIterator interface is used as a remote-access equivalent to java.util.Iterator, allowing the caller to iterate through a finite sequence of remote data elements.
The core differences are
public interface RemoteIterator<E> {
boolean hasNext() throws IOException;
E next() throws IOException;
}
The basic view of the interface is that hasNext() being true implies that next() will successfully return the next entry in the list:
while hasNext(): next()
Equally, a successful call to next() implies that had hasNext() been invoked prior to the call to next(), it would have been true.
boolean elementAvailable = hasNext();
try {
next();
assert elementAvailable;
} catch (NoSuchElementException e) {
assert !elementAvailable
}
The next() operator MUST iterate through the list of available results, even if no calls to hasNext() are made.
That is, it is possible to enumerate the results through a loop which only terminates when a NoSuchElementException exception is raised.
try {
while (true) {
process(iterator.next());
}
} catch (NoSuchElementException ignored) {
// the end of the list has been reached
}
The output of the iteration is equivalent to the loop
while (iterator.hasNext()) {
process(iterator.next());
}
As raising exceptions is an expensive operation in JVMs, the while(hasNext()) loop option is more efficient. (see also Concurrency and the Remote Iterator for a dicussion on this topic).
Implementors of the interface MUST support both forms of iterations; authors of tests SHOULD verify that both iteration mechanisms work.
The iteration is required to return a finite sequence; both forms of loop MUST ultimately terminate. All implementations of the interface in the Hadoop codebase meet this requirement; all consumers assume that it holds.
Returns true if-and-only-if a subsequent single call to next() would return an element rather than raise an exception.
The primary use of RemoteIterator in the filesystem APIs is to list files on (possibly remote) filesystems. These filesystems are invariably accessed concurrently; the state of the filesystem MAY change between a hasNext() probe and the invocation of the next() call.
During iteration through a RemoteIterator, if the directory is deleted on remote filesystem, then hasNext() or next() call may throw FileNotFoundException.
Accordingly, a robust iteration through a RemoteIterator would catch and discard NoSuchElementException exceptions raised during the process, which could be done through the while(true) iteration example above, or through a hasNext()/next() sequence with an outer try/catch clause to catch a NoSuchElementException alongside other exceptions which may be raised during a failure (for example, a FileNotFoundException)
try {
while (iterator.hasNext()) {
process(iterator.next());
}
} catch (NoSuchElementException ignored) {
// the end of the list has been reached
}
It is notable that this is not done in the Hadoop codebase. This does not imply that robust loops are not recommended —more that the concurrency problems were not considered during the implementation of these loops.
The StreamCapabilities provides a way to programmatically query the capabilities that OutputStream, InputStream, or other FileSystem class supports.
public interface StreamCapabilities {
boolean hasCapability(String capability);
}
Return true if the OutputStream, InputStream, or other FileSystem class has the desired capability.
The caller can query the capabilities of a stream using a string value. Here is a table of possible string values:
| String | Constant | Implements | Description |
|---|---|---|---|
| hflush | HFLUSH | Syncable | Flush out the data in client’s user buffer. After the return of this call, new readers will see the data. |
| hsync | HSYNC | Syncable | Flush out the data in client’s user buffer all the way to the disk device (but the disk may have it in its cache). Similar to POSIX fsync. |
| in:readahead | READAHEAD | CanSetReadahead | Set the readahead on the input stream. |
| dropbehind | DROPBEHIND | CanSetDropBehind | Drop the cache. |
| in:unbuffer | UNBUFFER | CanUnbuffer | Reduce the buffering on the input stream. |