Combine.CombineFn (Apache Beam 2.12.0)

java.lang.Object
- org.apache.beam.sdk.transforms.Combine.CombineFn<InputT,AccumT,OutputT>

Type Parameters:

InputT - type of input values

AccumT - type of mutable accumulator values

OutputT - type of output values

All Implemented Interfaces:

java.io.Serializable, CombineFnBase.GlobalCombineFn<InputT,AccumT,OutputT>, HasDisplayData

Direct Known Subclasses:

ApproximateDistinct.ApproximateDistinctFn, ApproximateUnique.ApproximateUniqueCombineFn, Combine.AccumulatingCombineFn, Combine.BinaryCombineDoubleFn, Combine.BinaryCombineFn, Combine.BinaryCombineIntegerFn, Combine.BinaryCombineLongFn, Combine.IterableCombineFn, CombineFns.ComposedCombineFn, CovarianceFn, Sample.FixedSizedSampleFn, SketchFrequencies.CountMinSketchFn, TDigestQuantiles.TDigestQuantilesFn, VarianceFn

Enclosing class:

Combine
```
public abstract static class Combine.CombineFn<InputT,AccumT,OutputT>
extends java.lang.Object
```
A CombineFn<InputT, AccumT, OutputT> specifies how to combine a collection of input values of type InputT into a single output value of type OutputT. It does this via one or more intermediate mutable accumulator values of type AccumT.
The overall process to combine a collection of input InputT values into a single output OutputT value is as follows:
1. The input InputT 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 AccumT, initialized to represent the combination of zero values.
3. For each input InputT value in a batch, the addInput(AccumT, InputT) operation is invoked to add the value to that batch's accumulator AccumT value. The accumulator may just record the new value (e.g., if AccumT == List<InputT>, or may do work to represent the combination more compactly.
4. The mergeAccumulators(java.lang.Iterable<AccumT>) operation is invoked to combine a collection of accumulator AccumT values into a single combined output accumulator AccumT 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(AccumT) operation is invoked on the final accumulator AccumT value to get the output OutputT value.
For example:
```
 public class AverageFn extends CombineFn<Integer, AverageFn.Accum, Double> {
   public static class Accum implements Serializable {
     int sum = 0;
     int count = 0;

     @Override
     public boolean equals(Object other) {
       if (other == null) return false;
       if (other == this) return true;
       if (!(other instanceof Accum))return false;


       Accum o = (Accum)other;
       if (this.sum != o.sum || this.count != o.count) {
         return false;
       } else {
         return true;
       }
     }
   }

   public Accum createAccumulator() {
     return new Accum();
   }

   public Accum addInput(Accum accum, Integer input) {
       accum.sum += input;
       accum.count++;
       return accum;
   }

   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.
Note on Data Encoding

Some form of data encoding is required when using custom types in a CombineFn which do not have well-known coders. The sample code above uses a custom Accumulator which gets coder by implementing Serializable. By doing this, we are relying on the generic CoderProvider, which is able to provide a coder for any Serializable if applicable. In cases where Serializable is not efficient, or inapplicable, in general there are two alternatives for encoding:
- Default CoderRegistry. For example, implement a coder class explicitly and use the @DefaultCoder tag. See the CoderRegistry for the numerous ways in which to bind a type to a coder.
- CombineFn specific way. While extending CombineFn, overwrite both getAccumulatorCoder(org.apache.beam.sdk.coders.CoderRegistry, org.apache.beam.sdk.coders.Coder<InputT>) and getDefaultOutputCoder(org.apache.beam.sdk.coders.CoderRegistry, org.apache.beam.sdk.coders.Coder<InputT>).
See Also:

Serialized Form

Constructor Summary

Constructors
Constructor and Description

CombineFn()

Constructors
Constructor and Description
`CombineFn()`

Method Summary

All Methods Instance Methods Abstract Methods Concrete Methods
Modifier and Type	Method and Description
`abstract AccumT`	`addInput(AccumT accumulator, InputT input)` Adds the given input value to the given accumulator, returning the new accumulator value.
`OutputT`	`apply(java.lang.Iterable<? extends InputT> inputs)` Applies this `CombineFn` to a collection of input values to produce a combined output value.
`AccumT`	`compact(AccumT accumulator)` Returns an accumulator that represents the same logical value as the input accumulator, but may have a more compact representation.
`abstract AccumT`	`createAccumulator()` Returns a new, mutable accumulator value, representing the accumulation of zero input values.
`OutputT`	`defaultValue()` Returns the default value when there are no values added to the accumulator.
`abstract OutputT`	`extractOutput(AccumT accumulator)` Returns the output value that is the result of combining all the input values represented by the given accumulator.
`java.lang.reflect.TypeVariable<?>`	`getAccumTVariable()` Returns the `TypeVariable` of `AccumT`.
`Coder<AccumT>`	`getAccumulatorCoder(CoderRegistry registry, Coder<InputT> inputCoder)` Returns the `Coder` to use for accumulator `AccumT` values, or null if it is not able to be inferred.
`Coder<OutputT>`	`getDefaultOutputCoder(CoderRegistry registry, Coder<InputT> inputCoder)` Returns the `Coder` to use by default for output `OutputT` values, or null if it is not able to be inferred.
`java.lang.String`	`getIncompatibleGlobalWindowErrorMessage()` Returns the error message for not supported default values in Combine.globally().
`java.lang.reflect.TypeVariable<?>`	`getInputTVariable()` Returns the `TypeVariable` of `InputT`.
`TypeDescriptor<InputT>`	`getInputType()` Returns a `TypeDescriptor` capturing what is known statically about the input type of this `CombineFn` instance's most-derived class.
`java.lang.reflect.TypeVariable<?>`	`getOutputTVariable()` Returns the `TypeVariable` of `OutputT`.
`TypeDescriptor<OutputT>`	`getOutputType()` Returns a `TypeDescriptor` capturing what is known statically about the output type of this `CombineFn` instance's most-derived class.
`abstract AccumT`	`mergeAccumulators(java.lang.Iterable<AccumT> accumulators)` Returns an accumulator representing the accumulation of all the input values accumulated in the merging accumulators.
`void`	`populateDisplayData(DisplayData.Builder builder)` Register display data for the given transform or component.

Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

- Constructor Detail
  - CombineFn
```
public CombineFn()
```
- Method Detail
  - createAccumulator
```
public abstract AccumT createAccumulator()
```
    Returns a new, mutable accumulator value, representing the accumulation of zero input values.
  - addInput
```
public abstract AccumT addInput(AccumT accumulator,
                                InputT 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 AccumT mergeAccumulators(java.lang.Iterable<AccumT> 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 OutputT extractOutput(AccumT accumulator)
```
    Returns the output value that is the result of combining all the input values represented by the given accumulator.
  - compact
```
public AccumT compact(AccumT accumulator)
```
    Returns an accumulator that represents the same logical value as the input accumulator, but may have a more compact representation.
    For most CombineFns this would be a no-op, but should be overridden by CombineFns that (for example) buffer up elements and combine them in batches.
    For efficiency, the input accumulator may be modified and returned.
    By default returns the original accumulator.
  - apply
```
public OutputT apply(java.lang.Iterable<? extends InputT> inputs)
```
    Applies this CombineFn to a collection of input values to produce a combined output value.
    Useful when using a CombineFn separately from a Combine transform. Does not invoke the mergeAccumulators(java.lang.Iterable<AccumT>) operation.
  - defaultValue
```
public OutputT defaultValue()
```
    Returns the default value when there are no values added to the accumulator.
    By default returns the extract output of an empty accumulator.
  - getOutputType
```
public TypeDescriptor<OutputT> getOutputType()
```
    Returns a TypeDescriptor capturing what is known statically about the output type of this CombineFn instance's most-derived class.
    In the normal case of a concrete CombineFn subclass with no generic type parameters of its own, this will be a complete non-generic type.
  - getInputType
```
public TypeDescriptor<InputT> getInputType()
```
    Returns a TypeDescriptor capturing what is known statically about the input type of this CombineFn instance's most-derived class.
    In the normal case of a concrete CombineFn subclass with no generic type parameters of its own, this will be a complete non-generic type.
  - getAccumulatorCoder
```
public Coder<AccumT> getAccumulatorCoder(CoderRegistry registry,
                                         Coder<InputT> inputCoder)
                                  throws CannotProvideCoderException
```
    Description copied from interface: CombineFnBase.GlobalCombineFn
    
    Returns the Coder to use for accumulator AccumT values, or null if it is not able to be inferred.
    By default, uses the knowledge of the Coder being used for InputT values and the enclosing Pipeline's CoderRegistry to try to infer the Coder for AccumT 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.
    
    Specified by:
    
    getAccumulatorCoder in interface CombineFnBase.GlobalCombineFn<InputT,AccumT,OutputT>
    
    Throws:
    
    CannotProvideCoderException
  - getDefaultOutputCoder
```
public Coder<OutputT> getDefaultOutputCoder(CoderRegistry registry,
                                            Coder<InputT> inputCoder)
                                     throws CannotProvideCoderException
```
    Description copied from interface: CombineFnBase.GlobalCombineFn
    
    Returns the Coder to use by default for output OutputT values, or null if it is not able to be inferred.
    By default, uses the knowledge of the Coder being used for input InputT values and the enclosing Pipeline's CoderRegistry to try to infer the Coder for OutputT values.
    
    Specified by:
    
    getDefaultOutputCoder in interface CombineFnBase.GlobalCombineFn<InputT,AccumT,OutputT>
    
    Throws:
    
    CannotProvideCoderException
  - getIncompatibleGlobalWindowErrorMessage
```
public java.lang.String getIncompatibleGlobalWindowErrorMessage()
```
    Description copied from interface: CombineFnBase.GlobalCombineFn
    
    Returns the error message for not supported default values in Combine.globally().
    
    Specified by:
    
    getIncompatibleGlobalWindowErrorMessage in interface CombineFnBase.GlobalCombineFn<InputT,AccumT,OutputT>
  - getInputTVariable
```
public java.lang.reflect.TypeVariable<?> getInputTVariable()
```
    Returns the TypeVariable of InputT.
  - getAccumTVariable
```
public java.lang.reflect.TypeVariable<?> getAccumTVariable()
```
    Returns the TypeVariable of AccumT.
  - getOutputTVariable
```
public java.lang.reflect.TypeVariable<?> getOutputTVariable()
```
    Returns the TypeVariable of OutputT.
  - populateDisplayData
```
public void populateDisplayData(DisplayData.Builder builder)
```
    Register display data for the given transform or component.
    populateDisplayData(DisplayData.Builder) is invoked by Pipeline runners to collect display data via DisplayData.from(HasDisplayData). Implementations may call super.populateDisplayData(builder) in order to register display data in the current namespace, but should otherwise use subcomponent.populateDisplayData(builder) to use the namespace of the subcomponent.
    By default, does not register any display data. Implementors may override this method to provide their own display data.
    
    Specified by:
    
    populateDisplayData in interface HasDisplayData
    
    Parameters:
    
    builder - The builder to populate with display data.
    
    See Also:
    
    HasDisplayData

Class Combine.CombineFn<InputT,AccumT,OutputT>

Note on Data Encoding

Constructor Summary

Method Summary

Methods inherited from class java.lang.Object

Constructor Detail

CombineFn

Method Detail

createAccumulator

addInput

mergeAccumulators

extractOutput

compact

apply

defaultValue

getOutputType

getInputType

getAccumulatorCoder

getDefaultOutputCoder

getIncompatibleGlobalWindowErrorMessage

getInputTVariable

getAccumTVariable

getOutputTVariable

populateDisplayData