org.apache.hadoop.fs

Interface PositionedReadable

All Known Implementing Classes:

FSDataInputStream, FSInputStream, HdfsDataInputStream

@InterfaceAudience.Public
 @InterfaceStability.Evolving
public interface PositionedReadable

Stream that permits positional reading. Implementations are required to implement thread-safe operations; this may be supported by concurrent access to the data, or by using a synchronization mechanism to serialize access. Not all implementations meet this requirement. Those that do not cannot be used as a backing store for some applications, such as Apache HBase. Independent of whether or not they are thread safe, some implementations may make the intermediate state of the system, specifically the position obtained in Seekable.getPos() visible.

Method Summary

Modifier and Type	Method and Description
default int	maxReadSizeForVectorReads() What is the largest size that we should group ranges together as?
default int	minSeekForVectorReads() What is the smallest reasonable seek?
int	read(long position, byte[] buffer, int offset, int length) Read up to the specified number of bytes, from a given position within a file, and return the number of bytes read.
void	readFully(long position, byte[] buffer) Read number of bytes equal to the length of the buffer, from a given position within a file.
void	readFully(long position, byte[] buffer, int offset, int length) Read the specified number of bytes, from a given position within a file.
default void	readVectored(List<? extends org.apache.hadoop.fs.FileRange> ranges, IntFunction<ByteBuffer> allocate) Read fully a list of file ranges asynchronously from this file.

Method Detail

read

int read(long position,
         byte[] buffer,
         int offset,
         int length)
  throws IOException

Read up to the specified number of bytes, from a given position within a file, and return the number of bytes read. This does not change the current offset of a file, and is thread-safe. Warning: Not all filesystems satisfy the thread-safety requirement.

Parameters:

position - position within file

buffer - destination buffer

offset - offset in the buffer

length - number of bytes to read

Returns:

actual number of bytes read; -1 means "none"

Throws:

IOException - IO problems.

readFully

void readFully(long position,
               byte[] buffer,
               int offset,
               int length)
        throws IOException

Read the specified number of bytes, from a given position within a file. This does not change the current offset of a file, and is thread-safe. Warning: Not all filesystems satisfy the thread-safety requirement.

Parameters:

position - position within file

buffer - destination buffer

offset - offset in the buffer

length - number of bytes to read

Throws:

IOException - IO problems.

EOFException - the end of the data was reached before the read operation completed

readFully

void readFully(long position,
               byte[] buffer)
        throws IOException

Read number of bytes equal to the length of the buffer, from a given position within a file. This does not change the current offset of a file, and is thread-safe. Warning: Not all filesystems satisfy the thread-safety requirement.

Parameters:

position - position within file

buffer - destination buffer

Throws:

IOException - IO problems.

EOFException - the end of the data was reached before the read operation completed

minSeekForVectorReads

default int minSeekForVectorReads()

What is the smallest reasonable seek?

Returns:

the minimum number of bytes

maxReadSizeForVectorReads

default int maxReadSizeForVectorReads()

What is the largest size that we should group ranges together as?

Returns:

the number of bytes to read at once

readVectored

default void readVectored(List<? extends org.apache.hadoop.fs.FileRange> ranges,
                          IntFunction<ByteBuffer> allocate)
                   throws IOException

Read fully a list of file ranges asynchronously from this file. The default iterates through the ranges to read each synchronously, but the intent is that FSDataInputStream subclasses can make more efficient readers. As a result of the call, each range will have FileRange.setData(CompletableFuture) called with a future that when complete will have a ByteBuffer with the data from the file's range.

The position returned by getPos() after readVectored() is undefined.

If a file is changed while the readVectored() operation is in progress, the output is undefined. Some ranges may have old data, some may have new and some may have both.

While a readVectored() operation is in progress, normal read api calls may block.

Parameters:

ranges - the byte ranges to read

allocate - the function to allocate ByteBuffer

Throws:

IOException - any IOE.