/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.beam.sdk.state;
import org.apache.beam.sdk.annotations.Experimental;
import org.apache.beam.sdk.annotations.Experimental.Kind;
import org.apache.beam.sdk.transforms.Combine.CombineFn;
/**
* A {@link ReadableState} cell defined by a {@link CombineFn}, accepting multiple input values,
* combining them as specified into accumulators, and producing a single output value.
*
* <p>Implementations of this form of state are expected to implement {@link #add} efficiently, not
* via a sequence of read-modify-write.
*
* @param <InputT> the type of values added to the state
* @param <AccumT> the type of accumulator
* @param <OutputT> the type of value extracted from the state
*/
@Experimental(Kind.STATE)
public interface CombiningState<InputT, AccumT, OutputT> extends GroupingState<InputT, OutputT> {
/**
* Read the merged accumulator for this state cell. It is implied that reading the state involves
* reading the accumulator, so {@link #readLater} is sufficient to prefetch for this.
*/
AccumT getAccum();
/**
* Add an accumulator to this state cell. Depending on implementation this may immediately merge
* it with the previous accumulator, or may buffer this accumulator for a future merge.
*/
void addAccum(AccumT accum);
/** Merge the given accumulators according to the underlying {@link CombineFn}. */
AccumT mergeAccumulators(Iterable<AccumT> accumulators);
@Override
CombiningState<InputT, AccumT, OutputT> readLater();
}