content: /*
   * 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.runners.core;
  
  import java.io.Serializable;
  import org.apache.beam.sdk.state.ReadableState;
  import org.apache.beam.sdk.state.Timers;
  import org.apache.beam.sdk.transforms.windowing.BoundedWindow;
  import org.apache.beam.sdk.transforms.windowing.PaneInfo;
  import org.apache.beam.sdk.values.WindowingStrategy;
  import org.joda.time.Instant;
  
  /**
   * Specification for processing to happen after elements have been grouped by key.
   *
   * @param <K> The type of key being processed.
   * @param <InputT> The type of input values associated with the key.
   * @param <OutputT> The output type that will be produced for each key.
   * @param <W> The type of windows this operates on.
   */
  public abstract class ReduceFn<K, InputT, OutputT, W extends BoundedWindow>
      implements Serializable {
  
    /** Information accessible to all the processing methods in this {@code ReduceFn}. */
    public abstract class Context {
      /** Return the key that is being processed. */
      public abstract K key();
  
      /** The window that is being processed. */
      public abstract W window();
  
      /** Access the current {@link WindowingStrategy}. */
      public abstract WindowingStrategy<?, W> windowingStrategy();
  
      /** Return the interface for accessing state. */
      public abstract StateAccessor<K> state();
  
      /** Return the interface for accessing timers. */
      public abstract Timers timers();
    }
  
    /** Information accessible within {@link #processValue}. */
    public abstract class ProcessValueContext extends Context {
      /** Return the actual value being processed. */
      public abstract InputT value();
  
      /** Return the timestamp associated with the value. */
      public abstract Instant timestamp();
    }
  
    /** Information accessible within {@link #onMerge}. */
    public abstract class OnMergeContext extends Context {
      /** Return the interface for accessing state. */
      @Override
      public abstract MergingStateAccessor<K, W> state();
    }
  
    /** Information accessible within {@link #onTrigger}. */
    public abstract class OnTriggerContext extends Context {
      /** Returns the {@link PaneInfo} for the trigger firing being processed. */
      public abstract PaneInfo paneInfo();
  
      /** Output the given value in the current window. */
      public abstract void output(OutputT value);
    }
  
    //////////////////////////////////////////////////////////////////////////////////////////////////
  
    /** Called for each value of type {@code InputT} associated with the current key. */
    public abstract void processValue(ProcessValueContext c) throws Exception;
  
    /** Called when windows are merged. */
    public abstract void onMerge(OnMergeContext context) throws Exception;
  
    /**
     * Called when triggers fire.
     *
     * <p>Implementations of {@link ReduceFn} should call {@link OnTriggerContext#output} to emit any
     * results that should be included in the pane produced by this trigger firing.
     */
    public abstract void onTrigger(OnTriggerContext context) throws Exception;
  
    /**
     * Called before {@link #onMerge} is invoked to provide an opportunity to prefetch any needed
     * state.
     *
     * @param c Context to use prefetch from.
     */
    public void prefetchOnMerge(MergingStateAccessor<K, W> c) throws Exception {}
  
    /**
     * Called before {@link #onTrigger} is invoked to provide an opportunity to prefetch any needed
     * state.
     *
     * @param context Context to use prefetch from.
     */
    public void prefetchOnTrigger(StateAccessor<K> context) {}
  
    /**
     * Called to clear any persisted state that the {@link ReduceFn} may be holding. This will be
     * called when the windowing is closing and will receive no future interactions.
     */
    public abstract void clearState(Context context) throws Exception;
  
    /** Returns true if there is no buffered state. */
    public abstract ReadableState<Boolean> isEmpty(StateAccessor<K> context);
  }
  