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.spark.translation;
  
  import org.apache.spark.api.java.JavaSparkContext$;
  import org.apache.spark.rdd.RDD;
  import org.apache.spark.streaming.StreamingContext;
  import org.apache.spark.streaming.Time;
  import org.apache.spark.streaming.dstream.ConstantInputDStream;
  import org.apache.spark.streaming.dstream.QueueInputDStream;
  import scala.Option;
  
  /**
   * A specialized {@link ConstantInputDStream} that emits its RDD exactly once. Alternative to {@link
   * QueueInputDStream} when checkpointing is required.
   *
   * <p>Features:
   *
   * <ul>
   *   <li>Supports checkpointing
   *   <li>Guarantees single emission of data
   *   <li>Returns empty RDD after first emission
   * </ul>
   *
   * @param <T> The type of elements in the RDD
   */
  public class SingleEmitInputDStream<T> extends ConstantInputDStream<T> {
  
    private boolean emitted = false;
  
    public SingleEmitInputDStream(StreamingContext ssc, RDD<T> rdd) {
      super(ssc, rdd, JavaSparkContext$.MODULE$.fakeClassTag());
    }
  
    @Override
    public Option<RDD<T>> compute(Time validTime) {
      if (this.emitted) {
        return Option.apply(this.emptyRDD());
      } else {
        this.emitted = true;
        return super.compute(validTime);
      }
    }
  
    private RDD<T> emptyRDD() {
      return this.context().sparkContext().emptyRDD(JavaSparkContext$.MODULE$.fakeClassTag());
    }
  }
  