/*
 * 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 javax.annotation.Nullable;
import org.apache.beam.sdk.transforms.windowing.BoundedWindow;
import org.apache.beam.sdk.values.PCollectionView;

/**
 * The interface to objects that provide side inputs. Particular implementations may read a side
 * input directly or use appropriate sorts of caching, etc.
 */
public interface SideInputReader {
  /**
   * Returns the value of the given {@link PCollectionView} for the given {@link BoundedWindow}.
   *
   * <p>It is valid for a side input to be {@code null}. It is <i>not</i> valid for this to return
   * {@code null} for any other reason.
   */
  @Nullable
  <T> T get(PCollectionView<T> view, BoundedWindow window);

  /** Returns true if the given {@link PCollectionView} is valid for this reader. */
  <T> boolean contains(PCollectionView<T> view);

  /** Returns true if there are no side inputs in this reader. */
  boolean isEmpty();
}
