com.google.cloud.dataflow.sdk.io

Class BoundedSource.BoundedReader<T>

java.lang.Object

com.google.cloud.dataflow.sdk.io.Source.Reader<T>

com.google.cloud.dataflow.sdk.io.BoundedSource.BoundedReader<T>

All Implemented Interfaces:

AutoCloseable

Direct Known Subclasses:

DatastoreIO.DatastoreReader, OffsetBasedSource.OffsetBasedReader

Enclosing class:

BoundedSource<T>

@Experimental(value=SOURCE_SINK)
public abstract static class BoundedSource.BoundedReader<T>
extends Source.Reader<T>

A Reader that reads a bounded amount of input and supports some additional operations, such as progress estimation and dynamic work rebalancing.

Boundedness

Once Source.Reader.start() or Source.Reader.advance() has returned false, neither will be called again on this object.

Thread safety

All methods will be run from the same thread except splitAtFraction(double) and getFractionConsumed(), which can be called concurrently from a different thread. There will not be multiple concurrent calls to splitAtFraction(double) but there can be for getFractionConsumed() if splitAtFraction(double) is implemented.

If the source does not implement splitAtFraction(double), you do not need to worry about thread safety. If implemented, it must be safe to call splitAtFraction(double) and getFractionConsumed() concurrently with other methods.

Implementing splitAtFraction(double)

In the course of dynamic work rebalancing, the method splitAtFraction(double) may be called concurrently with Source.Reader.advance() or Source.Reader.start(). It is critical that their interaction is implemented in a thread-safe way, otherwise data loss is possible.

Sources which support dynamic work rebalancing should use RangeTracker to manage the (source-specific) range of positions that is being split. If your source supports dynamic work rebalancing, please use that class to implement it if possible; if not possible, please contact the team at [email protected].

Constructor Summary

Constructors

Constructor and Description
BoundedReader()

Method Summary

Modifier and Type	Method and Description
abstract BoundedSource<T>	getCurrentSource() Returns a Source describing the same input that this Reader reads (including items already read).
org.joda.time.Instant	getCurrentTimestamp() By default, returns the minimum possible timestamp.
Double	getFractionConsumed() Returns a value in [0, 1] representing approximately what fraction of the source (getCurrentSource()) this reader has read so far.
BoundedSource<T>	splitAtFraction(double fraction) Tells the reader to narrow the range of the input it's going to read and give up the remainder, so that the new range would contain approximately the given fraction of the amount of data in the current range.

Methods inherited from class com.google.cloud.dataflow.sdk.io.Source.Reader

advance, close, getCurrent, start

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

BoundedReader

public BoundedReader()

Method Detail

getFractionConsumed

public Double getFractionConsumed()

Returns a value in [0, 1] representing approximately what fraction of the source (getCurrentSource()) this reader has read so far.

It is recommended that this method should satisfy the following properties:

• Should return 0 before the Source.Reader.start() call.
• Should return 1 after a Source.Reader.start() or Source.Reader.advance() call that returns false.
• The returned values should be non-decreasing (though they don't have to be unique).

By default, returns null to indicate that this cannot be estimated.

Thread safety

If splitAtFraction(double) is implemented, this method can be called concurrently to other methods (including itself), and it is therefore critical for it to be implemented in a thread-safe way.

Returns:

A value in [0, 1] representing the fraction of this reader's current input read so far, or null if such an estimate is not available.

getCurrentSource

public abstract BoundedSource<T> getCurrentSource()

Description copied from class: Source.Reader

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.

Specified by:

getCurrentSource in class Source.Reader<T>

splitAtFraction

public BoundedSource<T> splitAtFraction(double fraction)

Tells the reader to narrow the range of the input it's going to read and give up the remainder, so that the new range would contain approximately the given fraction of the amount of data in the current range.

Returns a BoundedSource representing the remainder.

Detailed description

Assuming the following sequence of calls:

   BoundedSource<T> initial = reader.getCurrentSource();
   BoundedSource<T> residual = reader.splitAtFraction(fraction);
   BoundedSource<T> primary = reader.getCurrentSource();
 

• The "primary" and "residual" sources, when read, should together cover the same set of records as "initial".
• The current reader should continue to be in a valid state, and continuing to read from it should, together with the records it already read, yield the same records as would have been read by "primary".
• The amount of data read by "primary" should ideally represent approximately the given fraction of the amount of data read by "initial".

For example, a reader that reads a range of offsets [A, B) in a file might implement this method by truncating the current range to [A, A + fraction*(B-A)) and returning a Source representing the range [A + fraction*(B-A), B).

This method should return null if the split cannot be performed for this fraction while satisfying the semantics above. E.g., a reader that reads a range of offsets in a file should return null if it is already past the position in its range corresponding to the given fraction. In this case, the method MUST have no effect (the reader must behave as if the method hadn't been called at all).

Statefulness

Since this method (if successful) affects the reader's source, in subsequent invocations "fraction" should be interpreted relative to the new current source.

Thread safety and blocking

This method will be called concurrently to other methods (however there will not be multiple concurrent invocations of this method itself), and it is critical for it to be implemented in a thread-safe way (otherwise data loss is possible).

It is also very important that this method always completes quickly, in particular, it should not perform or wait on any blocking operations such as I/O, RPCs etc. Violating this requirement may stall completion of the work item or even cause it to fail.

E.g. it is incorrect to make both this method and Source.Reader.start()/Source.Reader.advance() synchronized, because those methods can perform blocking operations, and then this method would have to wait for those calls to complete.

RangeTracker makes it easy to implement this method safely and correctly.

By default, returns null to indicate that splitting is not possible.

getCurrentTimestamp

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

By default, returns the minimum possible timestamp.

Specified by:

getCurrentTimestamp in class Source.Reader<T>

Throws:

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