class org.apache.hadoop.fs.FSDataInputStream

Class FSDataInputStream extends DataInputStream

The core behavior of FSDataInputStream is defined by, with extensions that add key assumptions to the system.

  1. The source is a local or remote filesystem.
  2. The stream being read references a finite array of bytes.
  3. The length of the data does not change during the read process.
  4. The contents of the data does not change during the process.
  5. The source file remains present during the read process.
  6. Callers may use to offsets within the array of bytes, with future reads starting at this offset.
  7. The cost of forward and backward seeks is low.
  8. There is no requirement for the stream implementation to be thread-safe.
  9. BUT, if a stream implements PositionedReadable, “positioned reads” MUST be thread-safe.

Files are opened via, which, if successful, returns:

result = FSDataInputStream(0, FS.Files[p])

The stream can be modeled as:

FSDIS = (pos, data[], isOpen)

with access functions:


Implicit invariant: the size of the data stream equals the size of the file as returned by FileSystem.getFileStatus(Path p)

forall p in dom(FS.Files[p]) :
len(data(FSDIS)) == FS.getFileStatus(p).length


The semantics of are defined in the interface definition within the JRE.

The operation MUST be idempotent; the following sequence is not an error:


Implementation Notes

  • Implementations SHOULD be robust against failure. If an inner stream is closed, it should be checked for being null first.

  • Implementations SHOULD NOT raise IOException exceptions (or any other exception) during this operation. Client applications often ignore these, or may fail unexpectedly.


FSDIS' = ((undefined), (undefined), False)


Return the current position. The outcome when a stream is closed is undefined.




result = pos(FSDIS)

Return the data at the current position.

  1. Implementations should fail when a stream is closed.
  2. There is no limit on how long read() may take to complete.




if ( pos < len(data) ):
   FSDIS' = (pos + 1, data, True)
   result = data[pos]
    result = -1[], offset, length)

Read length bytes of data into the destination buffer, starting at offset offset. The source of the data is the current position of the stream, as implicitly set in pos.


buffer != null else raise NullPointerException
length >= 0
offset < len(buffer)
length <= len(buffer) - offset
pos >= 0 else raise EOFException, IOException

Exceptions that may be raised on precondition failure are


Not all filesystems check the isOpen state.


if length == 0 :
  result = 0

else if pos > len(data):
  result = -1

  let l = min(length, len(data)-length) :
    buffer' = buffer where forall i in [0..l-1]:
       buffer'[o+i] = data[pos+i]
    FSDIS' = (pos+l, data, true)
    result = l

The API states that if the amount of data to be read (i.e. length) then the call must block until the amount of data available is greater than zero —that is, until there is some data. The call is not required to return when the buffer is full, or indeed block until there is no data left in the stream.

That is, rather than l being simply defined as min(length, len(data)-length), it strictly is an integer in the range 1..min(length, len(data)-length). While the caller may expect as much of the buffer as possible to be filled in, it is within the specification for an implementation to always return a smaller number, perhaps only ever 1 byte.

What is critical is that unless the destination buffer size is 0, the call must block until at least one byte is returned. Thus, for any data source of length greater than zero, repeated invocations of this read() operation will eventually read all the data.


Not all subclasses implement the Seek operation:

supported(FSDIS, else raise [UnsupportedOperationException, IOException]

If the operation is supported, the file SHOULD be open:


Some filesystems do not perform this check, relying on the read() contract to reject reads on a closed stream (e.g. RawLocalFileSystem).

A seek(0) MUST always succeed, as the seek position must be positive and less than the length of the Stream:

s > 0 and ((s==0) or ((s < len(data)))) else raise [EOFException, IOException]

Some FileSystems do not raise an exception if this condition is not met. They instead return -1 on any read() operation where, at the time of the read, len(data(FSDIS)) < pos(FSDIS).


FSDIS' = (s, data, True)

There is an implicit invariant: a seek to the current position is a no-op


Implementations may recognise this operation and bypass all other precondition checks, leaving the input stream unchanged.


This operation instructs the source to retrieve data[] from a different source from the current source. This is only relevant if the filesystem supports multiple replicas of a file and there is more than 1 replica of the data at offset offset.


Not all subclasses implement this operation, and instead either raise an exception or return False.

supported(FSDIS, Seekable.seekToNewSource) else raise [UnsupportedOperationException, IOException]

Examples: CompressionInputStream , HttpFSFileSystem

If supported, the file must be open:



The majority of subclasses that do not implement this operation simply fail.

if not supported(FSDIS, Seekable.seekToNewSource(s)):
    result = False

Examples: RawLocalFileSystem , HttpFSFileSystem

If the operation is supported and there is a new location for the data:

    FSDIS' = (pos, data', true)
    result = True

The new data is the original data (or an updated version of it, as covered in the Consistency section below), but the block containing the data at offset is sourced from a different replica.

If there is no other copy, FSDIS is not updated; the response indicates this:

    result = False

Outside of test methods, the primary use of this method is in the {{FSInputChecker}} class, which can react to a checksum error in a read by attempting to source the data elsewhere. If a new source can be found it attempts to reread and recheck that portion of the file.

interface PositionedReadable

The PositionedReadable operations supply “positioned reads” (“pread”). They provide the ability to read data into a buffer from a specific position in the data stream. Positioned reads equate to a at a particular offset followed by a[], offset, length), only there is a single method invocation, rather than seek then read, and two positioned reads can optionally run concurrently over a single instance of a FSDataInputStream stream.

The interface declares positioned reads thread-safe (some of the implementations do not follow this guarantee).

Any positional read run concurrent with a stream operation — e.g., Seekable.getPos(), and — MUST run in isolation; there must not be mutual interference.

Concurrent positional reads and stream operations MUST be serializable; one may block the other so they run in series but, for better throughput and ‘liveness’, they SHOULD run concurrently.

Given two parallel positional reads, one at pos1 for len1 into buffer dest1, and another at pos2 for len2 into buffer dest2, AND given a concurrent, stream read run after a seek to pos3, the resultant buffers MUST be filled as follows, even if the reads happen to overlap on the underlying stream:

// Positioned read #1
read(pos1, dest1, ... len1) -> dest1[0..len1 - 1] =
  [data(FS, path, pos1), data(FS, path, pos1 + 1) ... data(FS, path, pos1 + len1 - 1]

// Positioned read #2
read(pos2, dest2, ... len2) -> dest2[0..len2 - 1] =
  [data(FS, path, pos2), data(FS, path, pos2 + 1) ... data(FS, path, pos2 + len2 - 1]

// Stream read
read(dest3, ... len3) -> dest3[0..len3 - 1] =
  [data(FS, path, pos3), data(FS, path, pos3 + 1) ... data(FS, path, pos3 + len3 - 1]

Note that implementations are not required to be atomic; the intermediate state of the operation (the change in the value of getPos()) may be visible.

Implementation preconditions

Not all FSDataInputStream implementations support these operations. Those that do not implement do not implement the PositionedReadable interface.

supported(FSDIS, else raise [UnsupportedOperationException, IOException]

This could be considered obvious: if a stream is not Seekable, a client cannot seek to a location. It is also a side effect of the base class implementation, which uses

Implicit invariant: for all PositionedReadable operations, the value of pos is unchanged at the end of the operation

pos(FSDIS') == pos(FSDIS)

Failure states

For any operations that fail, the contents of the destination buffer are undefined. Implementations may overwrite part or all of the buffer before reporting a failure.

int, buffer, offset, length)

Read as much data as possible into the buffer space allocated for it.


position >= 0 else raise [EOFException, IOException, IllegalArgumentException, RuntimeException]
len(buffer) - offset >= length else raise [IndexOutOfBoundException, RuntimeException]
length >= 0
offset >= 0


The amount of data read is the less of the length or the amount of data available from the specified position:

let available = min(length, len(data)-position)
buffer'[offset..(offset+available-1)] = data[position..position+available -1]
result = available
  1. A return value of -1 means that the stream had no more available data.
  2. An invocation with length==0 implicitly does not read any data; implementations may short-cut the operation and omit any IO. In such instances, checks for the stream being at the end of the file may be omitted.
  3. If an IO exception occurs during the read operation(s), the final state of buffer is undefined.

void PositionedReadable.readFully(position, buffer, offset, length)

Read exactly length bytes of data into the buffer, failing if there is not enough data available.


position >= 0 else raise [EOFException, IOException, IllegalArgumentException, RuntimeException]
length >= 0
offset >= 0
len(buffer) - offset >= length else raise [IndexOutOfBoundException, RuntimeException]
(position + length) <= len(data) else raise [EOFException, IOException]

If an IO exception occurs during the read operation(s), the final state of buffer is undefined.

If there is not enough data in the input stream to satisfy the requests, the final state of buffer is undefined.


The buffer from offset offset is filled with the data starting at position

buffer'[offset..(offset+length-1)] = data[position..(position + length -1)]

PositionedReadable.readFully(position, buffer)

The semantics of this are exactly equivalent to

readFully(position, buffer, 0, len(buffer))

That is, the buffer is filled entirely with the contents of the input source from position position


  • All readers, local and remote, of a data stream FSDIS provided from a are expected to receive access to the data of FS.Files[p] at the time of opening.
  • If the underlying data is changed during the read process, these changes MAY or MAY NOT be visible.
  • Such changes that are visible MAY be partially visible.

At time t0

FSDIS0 = FS'read(p) = (0, data0[])

At time t1

FS' = FS' where FS'.Files[p] = data1

From time t >= t1, the value of FSDIS0 is undefined.

It may be unchanged == data0

forall l in len( == data0[l]

It may pick up the new data == data1

forall l in len( == data1[l]

It may be inconsistent, such that a read of an offset returns data from either of the datasets

forall l in len(
  ( == data0[l]) or ( == data1[l]))

That is, every value read may be from the original or updated file.

It may also be inconsistent on repeated reads of same offset, that is at time t2 > t1:

r2 =

While at time t3 > t2:

r3 =

It may be that r3 != r2. (That is, some of the data my be cached or replicated, and on a subsequent read, a different version of the file’s contents are returned).

Similarly, if the data at the path p, is deleted, this change MAY or MAY not be visible during read operations performed on FSDIS0.