hadoop/hadoop-common-project/hadoop-common/src/site/markdown/filesystem/filesystem.md

class org.apache.hadoop.fs.FileSystem

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 which ship with Apache Hadoop do not make any attempt to synchronize access to the working directory field.

Invariants

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.

Predicates and other state access operations

boolean exists(Path p)

def exists(FS, p) = p in paths(FS)

boolean isDirectory(Path p)

def isDirectory(FS, p)= p in directories(FS)

boolean isFile(Path p)

def isFile(FS, p) = p in files(FS)

FileStatus getFileStatus(Path p)

Get the status of a path

Preconditions

if not exists(FS, p) : raise FileNotFoundException

Postconditions

result = stat: FileStatus where:
    if isFile(FS, p) :
        stat.length = len(FS.Files[p])
        stat.isdir = False
    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]
    if inEncryptionZone(FS, p) :
        stat.isEncrypted = True
    else
        stat.isEncrypted = False

Path getHomeDirectory()

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.

Preconditions

Postconditions

result = p where valid-path(FS, p)

There is no requirement that the path exists at the time the method was called, or, if it exists, that it points to a directory. However, code tends to assume that not isFile(FS, getHomeDirectory()) holds to the extent that follow-on code may fail.

Implementation Notes

• The FTPFileSystem queries this value from the remote filesystem and may fail with a RuntimeException or subclass thereof if there is a connectivity problem. The time to execute the operation is not bounded.

FileStatus[] listStatus(Path path, PathFilter filter)

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.

Preconditions

Path path must exist:

if not exists(FS, path) : raise FileNotFoundException

Postconditions

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.

Atomicity and Consistency

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.

FileStatus[] listStatus(Path path)

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).

FileStatus[] listStatus(Path[] paths, PathFilter 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.

Preconditions

All paths must exist. There is no requirement for uniqueness.

forall p in paths :
  exists(fs, p) else raise FileNotFoundException

Postconditions

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).

FileStatus[] listStatus(Path[] paths)

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.

RemoteIterator[LocatedFileStatus] listLocatedStatus(Path path, PathFilter filter)

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.

Preconditions

Path path must exist:

exists(FS, path) : raise FileNotFoundException

Postconditions

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).

RemoteIterator[LocatedFileStatus] listLocatedStatus(Path path)

The equivalent to listLocatedStatus(path, DEFAULT_FILTER), where DEFAULT_FILTER accepts all path names.

RemoteIterator[LocatedFileStatus] listFiles(Path path, boolean recursive)

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.

Preconditions

exists(FS, path) else raise FileNotFoundException

Postconditions

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).

BlockLocation[] getFileBlockLocations(FileStatus f, int s, int l)

Preconditions

if s < 0 or l < 0 : raise {HadoopIllegalArgumentException, InvalidArgumentException}

• HDFS throws HadoopIllegalArgumentException for an invalid offset or length; this extends IllegalArgumentException.

Postconditions

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.

BlockLocation[] getFileBlockLocations(Path P, int S, int L)

Preconditions

if p == null : raise NullPointerException
if not exists(FS, p) : raise FileNotFoundException

Postconditions

result = getFileBlockLocations(getFileStatus(FS, P), S, L)

long getDefaultBlockSize()

Get the "default" block size for a filesystem. This often used during split calculations to divide work optimally across a set of worker processes.

Preconditions

Postconditions

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 either too many jobs being submitted for efficient work, or the JobSubmissionClient running out of memory.

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).

long getDefaultBlockSize(Path p)

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.

Preconditions

Postconditions

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.

long getBlockSize(Path p)

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.

Preconditions

if not exists(FS, p) :  raise FileNotFoundException

Postconditions

result == getFileStatus(P).getBlockSize()

The outcome of this operation MUST be identical to that contained in the FileStatus returned from getFileStatus(P).

State Changing Operations

boolean mkdirs(Path p, FsPermission permission)

Create a directory and all its parents

Preconditions

 if exists(FS, p) and not isDir(FS, p) :
     raise [ParentNotDirectoryException, FileAlreadyExistsException, IOException]

Postconditions

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).

Implementation Notes: Local FileSystem

The local FileSystem does not raise an exception if mkdirs(p) is invoked on a path that exists and is a file. Instead the operation returns false.

if isFile(FS, p):
   FS' = FS
   result = False

FSDataOutputStream create(Path, ...)

FSDataOutputStream create(Path p,
      FsPermission permission,
      boolean overwrite,
      int bufferSize,
      short replication,
      long blockSize,
      Progressable progress) throws IOException;

Preconditions

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.

Postconditions

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]

Implementation Notes

• 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.

• S3N, 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 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

FSDataOutputStream append(Path p, int bufferSize, Progressable progress)

Implementations MAY throw UnsupportedOperationException.

Preconditions

if not exists(FS, p) : raise FileNotFoundException

if not isFile(FS, p) : raise [FileNotFoundException, IOException]

Postconditions

FS
result = FSDataOutputStream

Return: FSDataOutputStream, which can update the entry FS.Files[p] by appending data to the existing list.

FSDataInputStream open(Path f, int bufferSize)

Implementations MAY throw UnsupportedOperationException.

Preconditions

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.

Postconditions

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 implementation notes

1. HDFS MAY throw UnresolvedPathException when attempting to traverse symbolic links

2. 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.

boolean delete(Path p, boolean recursive)

Preconditions

A directory with children and recursive == false cannot be deleted

if isDir(FS, p) and not recursive and (children(FS, p) != {}) : raise IOException

Postconditions

Nonexistent path

If the file does not exist the FS state does not change

if not exists(FS, p):
    FS' = FS
    result = False

The result SHOULD be False, indicating that no file was deleted.

Simple File

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

Empty root directory

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.

Empty (non-root) directory

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

Recursive delete of root directory

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

In contrast, HDFS never permits the deletion of the root of a filesystem; the filesystem can be taken offline and reformatted if an empty filesystem is desired.

if isDir(FS, p) and isRoot(p) and recursive :
    FS' = FS
    result = False

Recursive delete of non-root directory

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

Atomicity

• 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.

Implementation Notes

• S3N, Swift, FTP and potentially other non-traditional FileSystems implement delete() as recursive listing and file delete operation. This can break the expectations of client applications -and means that they cannot be used as drop-in replacements for HDFS.

boolean rename(Path src, Path d)

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

Preconditions

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

Postconditions

Renaming a directory onto itself

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 self

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 onto a nonexistent path

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

Renaming a directory onto a directory

If src is a directory then all its children will then exist under dest, while the path src and its descendants will no longer not 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

Renaming into a path where the parent path does not exist

  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, S3N

The outcome is as a normal rename, with the additional (implicit) feature that the parent directores of the destination also exist

exists(FS', parent(dest))

*Other Filesystems (including Swift) *

Other filesystems strictly reject the operation, raising a FileNotFoundException

Concurrency requirements

• 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.

Implementation Notes

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.

void concat(Path p, Path sources[])

Joins multiple blocks together to create a single file. This is a little-used operation currently implemented only by HDFS.

Implementations MAY throw UnsupportedOperationException

Preconditions

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

Postconditions

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.

boolean truncate(Path p, long newLength)

Truncate file p to the specified newLength.

Implementations MAY throw UnsupportedOperationException.

Preconditions

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.

Postconditions

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.

Concurrency

If an input stream is open when truncate() occurs, the outcome of read operations related to the part of the file being truncated is undefined.

interface RemoteIterator

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

1. Iterator's optional void remove() method is not supported.
2. For those methods which are supported, IOException exceptions may be raised.

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.

boolean hasNext()

Returns true if-and-only-if a subsequent single call to next() would return an element rather than raise an exception.

Preconditions

Postconditions

result = True ==> next() will succeed.
result = False ==> next() will raise an exception

Multiple calls to hasNext(), without any intervening next() calls, MUST return the same value.

boolean has1 = iterator.hasNext();
boolean has2 = iterator.hasNext();
assert has1 == has2;

E next()

Return the next element in the iteration.

Preconditions

hasNext() else raise java.util.NoSuchElementException

Postconditions

result = the next element in the iteration

Repeated calls to next() return subsequent elements in the sequence, until the entire sequence has been returned.

Concurrency and the Remote Iterator

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.

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.