Combine.KeyedCombineFn (Apache Beam SDK 0.2.0-incubating API)

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

Type Parameters:

K - type of keys

InputT - type of input values

AccumT - type of mutable accumulator values

OutputT - type of output values

All Implemented Interfaces:

Serializable, CombineFnBase.PerKeyCombineFn<K,InputT,AccumT,OutputT>, HasDisplayData

Direct Known Subclasses:

CombineFns.ComposedKeyedCombineFn

Enclosing class:

Combine
```
public abstract static class Combine.KeyedCombineFn<K,InputT,AccumT,OutputT>
extends Object
```
A KeyedCombineFn<K, InputT, AccumT, OutputT> specifies how to combine a collection of input values of type InputT, associated with a key of type K, 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 associated with an input K key 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(K) 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(K, 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(K, 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(K, AccumT) operation is invoked on the final accumulator AccumT value to get the output OutputT value.
All of these operations are passed the K key that the values being combined are associated with.
For example:
```
 
 public class ConcatFn
     extends KeyedCombineFn<String, Integer, ConcatFn.Accum, String> {
   public static class Accum {
     String s = "";
   }
   public Accum createAccumulator(String key) {
     return new Accum();
   }
   public Accum addInput(String key, Accum accum, Integer input) {
       accum.s += "+" + input;
       return accum;
   }
   public Accum mergeAccumulators(String key, Iterable<Accum> accums) {
     Accum merged = new Accum();
     for (Accum accum : accums) {
       merged.s += accum.s;
     }
     return merged;
   }
   public String extractOutput(String key, Accum accum) {
     return key + accum.s;
   }
 }
 PCollection<KV<String, Integer>> pc = ...;
 PCollection<KV<String, String>> pc2 = pc.apply(
     Combine.perKey(new ConcatFn()));
  
```
Keyed combining functions used by 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

KeyedCombineFn()

Constructors
Constructor and Description
`KeyedCombineFn()`

Method Summary

All Methods Instance Methods Abstract Methods Concrete Methods
Modifier and Type	Method and Description
`abstract AccumT`	`addInput(K key, AccumT accumulator, InputT value)` Adds the given input value to the given accumulator, returning the new accumulator value.
`OutputT`	`apply(K key, Iterable<? extends InputT> inputs)` Applies this `KeyedCombineFn` to a key and a collection of input values to produce a combined output value.
`AccumT`	`compact(K key, 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(K key)` Returns a new, mutable accumulator value representing the accumulation of zero input values.
`abstract OutputT`	`extractOutput(K key, AccumT accumulator)` Returns the output value that is the result of combining all the input values represented by the given accumulator.
`Combine.CombineFn<InputT,AccumT,OutputT>`	`forKey(K key, Coder<K> keyCoder)` Returns the a regular `CombineFnBase.GlobalCombineFn` that operates on a specific key.
`TypeVariable<?>`	`getAccumTVariable()` Returns the `TypeVariable` of `AccumT`.
`Coder<AccumT>`	`getAccumulatorCoder(CoderRegistry registry, Coder<K> keyCoder, 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<K> keyCoder, Coder<InputT> inputCoder)` Returns the `Coder` to use by default for output `OutputT` values, or null if it is not able to be inferred.
`TypeVariable<?>`	`getInputTVariable()` Returns the `TypeVariable` of `InputT`.
`TypeVariable<?>`	`getKTypeVariable()` Returns the `TypeVariable` of `K`.
`TypeVariable<?>`	`getOutputTVariable()` Returns the `TypeVariable` of `OutputT`.
`abstract AccumT`	`mergeAccumulators(K key, 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
  - KeyedCombineFn
```
public KeyedCombineFn()
```
- Method Detail
  - createAccumulator
```
public abstract AccumT createAccumulator(K key)
```
    Returns a new, mutable accumulator value representing the accumulation of zero input values.
    
    Parameters:
    
    key - the key that all the accumulated values using the accumulator are associated with
  - addInput
```
public abstract AccumT addInput(K key,
                                AccumT accumulator,
                                InputT value)
```
    Adds the given input value to the given accumulator, returning the new accumulator value.
    For efficiency, the input accumulator may be modified and returned.
    
    Parameters:
    
    key - the key that all the accumulated values using the accumulator are associated with
  - mergeAccumulators
```
public abstract AccumT mergeAccumulators(K key,
                                         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.
    
    Parameters:
    
    key - the key that all the accumulators are associated with
  - extractOutput
```
public abstract OutputT extractOutput(K key,
                                      AccumT accumulator)
```
    Returns the output value that is the result of combining all the input values represented by the given accumulator.
    
    Parameters:
    
    key - the key that all the accumulated values using the accumulator are associated with
  - compact
```
public AccumT compact(K key,
                      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.
  - forKey
```
public Combine.CombineFn<InputT,AccumT,OutputT> forKey(K key,
                                                       Coder<K> keyCoder)
```
    Description copied from interface: CombineFnBase.PerKeyCombineFn
    
    Returns the a regular CombineFnBase.GlobalCombineFn that operates on a specific key.
  - apply
```
public OutputT apply(K key,
                     Iterable<? extends InputT> inputs)
```
    Applies this KeyedCombineFn to a key and a collection of input values to produce a combined output value.
    Useful when testing the behavior of a KeyedCombineFn separately from a Combine transform.
  - getAccumulatorCoder
```
public Coder<AccumT> getAccumulatorCoder(CoderRegistry registry,
                                         Coder<K> keyCoder,
                                         Coder<InputT> inputCoder)
                                  throws CannotProvideCoderException
```
    Description copied from interface: CombineFnBase.PerKeyCombineFn
    
    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 K keys and input 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.PerKeyCombineFn<K,InputT,AccumT,OutputT>
    
    Throws:
    
    CannotProvideCoderException
  - getDefaultOutputCoder
```
public Coder<OutputT> getDefaultOutputCoder(CoderRegistry registry,
                                            Coder<K> keyCoder,
                                            Coder<InputT> inputCoder)
                                     throws CannotProvideCoderException
```
    Description copied from interface: CombineFnBase.PerKeyCombineFn
    
    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 K keys and input InputT values and the enclosing Pipeline's CoderRegistry to try to infer the Coder for OutputT values.
    
    Specified by:
    
    getDefaultOutputCoder in interface CombineFnBase.PerKeyCombineFn<K,InputT,AccumT,OutputT>
    
    Throws:
    
    CannotProvideCoderException
  - getKTypeVariable
```
public TypeVariable<?> getKTypeVariable()
```
    Returns the TypeVariable of K.
  - getInputTVariable
```
public TypeVariable<?> getInputTVariable()
```
    Returns the TypeVariable of InputT.
  - getAccumTVariable
```
public TypeVariable<?> getAccumTVariable()
```
    Returns the TypeVariable of AccumT.
  - getOutputTVariable
```
public 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.KeyedCombineFn<K,InputT,AccumT,OutputT>

Constructor Summary

Method Summary

Methods inherited from class java.lang.Object

Constructor Detail

KeyedCombineFn

Method Detail

createAccumulator

addInput

mergeAccumulators

extractOutput

compact

forKey

apply

getAccumulatorCoder

getDefaultOutputCoder

getKTypeVariable

getInputTVariable

getAccumTVariable

getOutputTVariable

populateDisplayData