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.sdk.schemas;
  
  import java.util.Map;
  import org.apache.beam.sdk.values.TupleTag;
  
  /**
   * A factory for operations that execute a projection on a {@link Schema}-aware {@link
   * org.apache.beam.sdk.values.PCollection}.
   *
   * <p>Typically this interface will be implemented by a reader {@link
   * org.apache.beam.sdk.transforms.PTransform} or some component thereof that is capable of pushing
   * down a projection to an external source.
   */
  public interface ProjectionProducer<T> {
    /** Whether {@code this} supports projection pushdown. */
    boolean supportsProjectionPushdown();
  
    /**
     * Actuate a projection pushdown.
     *
     * @param outputFields Map keyed by the {@link TupleTag} for each output on which pushdown is
     *     requested. The value is the {@link FieldAccessDescriptor} containing the list of fields
     *     needed for that output; fields not present in the descriptor should be dropped.
     * @return {@code T} that implements the projection pushdown. The return value is assumed to be a
     *     drop-in replacement for {@code this}; it must have all the same functionality. For this
     *     reason, {@code T} is usually the same class as {@code this}.
     */
    T actuateProjectionPushdown(Map<TupleTag<?>, FieldAccessDescriptor> outputFields);
  }
  