com.google.cloud.dataflow.sdk.io

Interface Source.Reader<T>

All Superinterfaces:

AutoCloseable

All Known Subinterfaces:

BoundedSource.BoundedReader<T>, UnboundedSource.UnboundedReader<OutputT>

All Known Implementing Classes:

AvroSource.AvroReader, BlockBasedSource.BlockBasedReader, BoundedSource.AbstractBoundedReader, ByteOffsetBasedSource.ByteOffsetBasedReader, CompressedSource.CompressedReader, DatastoreIO.DatastoreReader, FileBasedSource.FileBasedReader, Source.AbstractReader

Enclosing class:

Source<T>

public static interface Source.Reader<T>
extends AutoCloseable

The interface that readers of custom input sources must implement.

This interface is deliberately distinct from Iterator because the current model tends to be easier to program and more efficient in practice for iterating over sources such as files, databases etc. (rather than pure collections).

Reader implementations do not need to be thread-safe; they may only be accessed by a single thread at once.

Callers of Readers must obey the following access pattern:

• One call to start()
  • If start() returned true, any number of calls to getCurrent* methods
• Repeatedly, a call to advance(). This may be called regardless of what the previous start()/advance() returned.
  • If advance() returned true, any number of calls to getCurrent* methods

For example, if the reader is reading a fixed set of data:

 for (boolean available = reader.start(); available; available = reader.advance()) {
   T item = reader.getCurrent();
   Instant timestamp = reader.getCurrentTimestamp();
   ...
 }
 

If the set of data being read is continually growing:

 boolean available = reader.start();
 while (true) {
   if (available) {
     T item = reader.getCurrent();
     Instant timestamp = reader.getCurrentTimestamp();
     ...
     resetExponentialBackoff();
   } else {
     exponentialBackoff();
   }
   available = reader.advance();
 }
 

Note: this interface is a work-in-progress and may change.

Method Summary

Modifier and Type	Method and Description
boolean	advance() Advances the reader to the next valid record.
void	close() Closes the reader.
T	getCurrent() Returns the value of the data item that was read by the last start() or advance() call.
Source<T>	getCurrentSource() Returns a Source describing the same input that this Reader reads (including items already read).
org.joda.time.Instant	getCurrentTimestamp() Returns the timestamp associated with the current data item.
boolean	start() Initializes the reader and advances the reader to the first record.

Method Detail

start

boolean start()
       throws IOException

Initializes the reader and advances the reader to the first record.

This method should be called exactly once. The invocation should occur prior to calling advance() or getCurrent(). This method may perform expensive operations that are needed to initialize the reader.

Returns:

true if a record was read, false if there is no more input available.

Throws:

IOException

advance

boolean advance()
         throws IOException

Advances the reader to the next valid record.

Returns:

true if a record was read, false if there is no more input available.

Throws:

IOException

getCurrent

T getCurrent()
      throws NoSuchElementException

Returns the value of the data item that was read by the last start() or advance() call. The returned value must be effectively immutable and remain valid indefinitely.

Multiple calls to this method without an intervening call to advance() should return the same result.

Throws:

NoSuchElementException - if the reader is at the beginning of the input and start() or advance() wasn't called, or if the last start() or advance() returned false.

getCurrentTimestamp

org.joda.time.Instant getCurrentTimestamp()
                                   throws NoSuchElementException

Returns the timestamp associated with the current data item.

If the source does not support timestamps, this should return BoundedWindow.TIMESTAMP_MIN_VALUE.

Multiple calls to this method without an intervening call to advance() should return the same result.

Throws:

NoSuchElementException - if the reader is at the beginning of the input and start() or advance() wasn't called, or if the last start() or advance() returned false.

close

void close()
    throws IOException

Closes the reader. The reader cannot be used after this method is called.

Specified by:

close in interface AutoCloseable

Throws:

IOException

getCurrentSource

Source<T> getCurrentSource()

Returns a Source describing the same input that this Reader reads (including items already read).

A reader created from the result of getCurrentSource, if consumed, MUST return the same data items as the current reader.