eu.fbk.rdfpro

Interface Reducer

Functional Interface:

This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.

@FunctionalInterface
public interface Reducer

Reduce function in a MapReduce job.

A Reducer object is used in a MapReduce job (see RDFProcessors.mapReduce(Mapper, Reducer, boolean)) to process a partition of statements associated to a certain Value key produced by a Mapper in a previous map phase (e.g., all the statements having a specific subject).

Implementations of this interface should be thread-safe, as multiple reduce jobs can be fired in parallel with method reduce() being invoked concurrently by different threads on different statement partitions.

Field Summary

Fields

Modifier and Type	Field and Description
static Reducer	IDENTITY The identity reducer that emits all the quads of a partition unchanged.

Method Summary

Modifier and Type	Method and Description
static Reducer	concat(Reducer... reducers) Returns a Reducer that emits the output of multiple reductions on the same quad partition.
static Reducer	filter(Reducer reducer, Predicate<org.openrdf.model.Statement> existsPredicate, Predicate<org.openrdf.model.Statement> forallPredicate) Returns a filtered version of the input reducer that operates only on partitions satisfying the existential and forall predicates supplied.
static Mapper	parse(String expression) Parses a Reducer out of the supplied expression string.
void	reduce(org.openrdf.model.Value key, org.openrdf.model.Statement[] statements, org.openrdf.rio.RDFHandler handler) Processes the statement partition associated to a certain key, emitting output statements to the supplied RDFHandler.

Field Detail

IDENTITY

static final Reducer IDENTITY

The identity reducer that emits all the quads of a partition unchanged.

Method Detail

filter

static Reducer filter(Reducer reducer,
                      @Nullable
                      Predicate<org.openrdf.model.Statement> existsPredicate,
                      @Nullable
                      Predicate<org.openrdf.model.Statement> forallPredicate)

Returns a filtered version of the input reducer that operates only on partitions satisfying the existential and forall predicates supplied.

Parameters:

reducer - the reducer to filter

existsPredicate - the exists predicate, that must be satisfied by at least a partition quad in order for the partition to be processed; if null no existential filtering is done

forallPredicate - the forall predicate, that must be satisfied by all the quads of a partition in order for it to be processed; if null, no forall filtering is applied

Returns:

the resulting filtered reducer

concat

static Reducer concat(Reducer... reducers)

Returns a Reducer that emits the output of multiple reductions on the same quad partition.

Parameters:

reducers - the reducers whose output has to be concatenated

Returns:

the created Reducer

parse

@Nullable
static Mapper parse(@Nullable
                              String expression)

Parses a Reducer out of the supplied expression string. The expression must be a language: expression script.

Parameters:

expression - the expression to parse

Returns:

the parsed reducer, or null if a null expression was supplied

reduce

void reduce(@Nullable
            org.openrdf.model.Value key,
            org.openrdf.model.Statement[] statements,
            org.openrdf.rio.RDFHandler handler)
     throws org.openrdf.rio.RDFHandlerException

Processes the statement partition associated to a certain key, emitting output statements to the supplied RDFHandler.

Parameters:

key - the partition key, possibly null

statements - a modifiable array with the statements belonging to the partition, not null

handler - the RDFHandler where to emit output statements, not null

Throws:

org.openrdf.rio.RDFHandlerException - on error