com.google.cloud.dataflow.sdk.transforms

Class Combine.CombineFn<VI,VA,VO>

java.lang.Object

com.google.cloud.dataflow.sdk.transforms.Combine.CombineFn<VI,VA,VO>

Type Parameters:

VI - type of input values

VA - type of mutable accumulator values

VO - type of output values

All Implemented Interfaces:

java.io.Serializable

Direct Known Subclasses:

ApproximateUnique.ApproximateUniqueCombineFn, Combine.AccumulatingCombineFn, Combine.BinaryCombineDoubleFn, Combine.BinaryCombineFn, Combine.BinaryCombineIntegerFn, Combine.BinaryCombineLongFn, Combine.SimpleCombineFn, Sample.FixedSizedSampleFn

Enclosing class:

Combine

public abstract static class Combine.CombineFn<VI,VA,VO>
extends java.lang.Object
implements java.io.Serializable

A CombineFn<VI, VA, VO> specifies how to combine a collection of input values of type VI into a single output value of type VO. It does this via one or more intermediate mutable accumulator values of type VA.

The overall process to combine a collection of input VI values into a single output VO value is as follows:

1. The input VI values are partitioned into one or more batches.
2. For each batch, the createAccumulator() operation is invoked to create a fresh mutable accumulator value of type VA, initialized to represent the combination of zero values.
3. For each input VI value in a batch, the addInput(VA, VI) operation is invoked to add the value to that batch's accumulator VA value. The accumulator may just record the new value (e.g., if VA == List<VI>, or may do work to represent the combination more compactly.
4. The mergeAccumulators(java.lang.Iterable<VA>) operation is invoked to combine a collection of accumulator VA values into a single combined output accumulator VA value, once the merging accumulators have had all all the input values in their batches added to them. This operation is invoked repeatedly, until there is only one accumulator value left.
5. The extractOutput(VA) operation is invoked on the final accumulator VA value to get the output VO value.

For example:

 public class AverageFn extends CombineFn<Integer, AverageFn.Accum, Double> {
   public static class Accum {
     int sum = 0;
     int count = 0;
   }
   public Accum createAccumulator() { return new Accum(); }
   public void addInput(Accum accum, Integer input) {
       accum.sum += input;
       accum.count++;
   }
   public Accum mergeAccumulators(Iterable<Accum> accums) {
     Accum merged = createAccumulator();
     for (Accum accum : accums) {
       merged.sum += accum.sum;
       merged.count += accum.count;
     }
     return merged;
   }
   public Double extractOutput(Accum accum) {
     return ((double) accum.sum) / accum.count;
   }
 }
 PCollection<Integer> pc = ...;
 PCollection<Double> average = pc.apply(Combine.globally(new AverageFn()));
  

Combining functions used by Combine.Globally, Combine.PerKey, Combine.GroupedValues, and PTransforms derived from them should be associative and commutative. Associativity is required because input values are first broken up into subgroups before being combined, and their intermediate results further combined, in an arbitrary tree structure. Commutativity is required because any order of the input values is ignored when breaking up input values into groups.

See Also:

Serialized Form

Constructor Summary

Constructors

Constructor and Description
Combine.CombineFn()

Method Summary

Methods

Modifier and Type	Method and Description
abstract VA	addInput(VA accumulator, VI input) Adds the given input value to the given accumulator, returning the new accumulator value.
VO	apply(java.lang.Iterable<? extends VI> inputs) Applies this CombineFn to a collection of input values to produce a combined output value.
<K> Combine.KeyedCombineFn<K,VI,VA,VO>	asKeyedFn() Converts this CombineFn into an equivalent Combine.KeyedCombineFn, which ignores the keys passed to it and combines the values according to this CombineFn.
abstract VA	createAccumulator() Returns a new, mutable accumulator value, representing the accumulation of zero input values.
abstract VO	extractOutput(VA accumulator) Returns the output value that is the result of combining all the input values represented by the given accumulator.
Coder<VA>	getAccumulatorCoder(CoderRegistry registry, Coder<VI> inputCoder) Returns the Coder to use for accumulator VA values, or null if it is not able to be inferred.
Coder<VO>	getDefaultOutputCoder(CoderRegistry registry, Coder<VI> inputCoder) Returns the Coder to use by default for output VO values, or null if it is not able to be inferred.
abstract VA	mergeAccumulators(java.lang.Iterable<VA> accumulators) Returns an accumulator representing the accumulation of all the input values accumulated in the merging accumulators.

Methods inherited from class java.lang.Object

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

Constructor Detail

Combine.CombineFn

public Combine.CombineFn()

Method Detail

createAccumulator

public abstract VA createAccumulator()

Returns a new, mutable accumulator value, representing the accumulation of zero input values.

addInput

public abstract VA addInput(VA accumulator,
          VI input)

Adds the given input value to the given accumulator, returning the new accumulator value.

For efficiency, the input accumulator may be modified and returned.

mergeAccumulators

public abstract VA mergeAccumulators(java.lang.Iterable<VA> accumulators)

Returns an accumulator representing the accumulation of all the input values accumulated in the merging accumulators.

May modify any of the argument accumulators. May return a fresh accumulator, or may return one of the (modified) argument accumulators.

extractOutput

public abstract VO extractOutput(VA accumulator)

Returns the output value that is the result of combining all the input values represented by the given accumulator.

apply

public VO apply(java.lang.Iterable<? extends VI> inputs)

Applies this CombineFn to a collection of input values to produce a combined output value.

Useful when testing the behavior of a CombineFn separately from a Combine transform.

getAccumulatorCoder

public Coder<VA> getAccumulatorCoder(CoderRegistry registry,
                            Coder<VI> inputCoder)

Returns the Coder to use for accumulator VA values, or null if it is not able to be inferred.

By default, uses the knowledge of the Coder being used for VI values and the enclosing Pipeline's CoderRegistry to try to infer the Coder for VA values.

This is the Coder used to send data through a communication-intensive shuffle step, so a compact and efficient representation may have significant performance benefits.

getDefaultOutputCoder

public Coder<VO> getDefaultOutputCoder(CoderRegistry registry,
                              Coder<VI> inputCoder)

Returns the Coder to use by default for output VO values, or null if it is not able to be inferred.

By default, uses the knowledge of the Coder being used for input VI values and the enclosing Pipeline's CoderRegistry to try to infer the Coder for VO values.

asKeyedFn

public <K> Combine.KeyedCombineFn<K,VI,VA,VO> asKeyedFn()

Converts this CombineFn into an equivalent Combine.KeyedCombineFn, which ignores the keys passed to it and combines the values according to this CombineFn.

Type Parameters:

K - the type of the (ignored) keys