Initialize an RDD to empty

Question

I have an RDD called

JavaPairRDD<String, List<String>> existingRDD; 

Now I need to initialize this existingRDD to empty so that when I get the actual rdd's I can do a union with this existingRDD. How do I initialize existingRDD to an empty RDD except initializing it to null? Here is my code:

JavaPairRDD<String, List<String>> existingRDD;
if(ai.get()%10==0)
{
    existingRDD.saveAsNewAPIHadoopFile("s3://manthan-impala-test/kinesis-dump/" + startTime + "/" + k + "/" + System.currentTimeMillis() + "/",
    NullWritable.class, Text.class, TextOutputFormat.class); //on worker failure this will get overwritten                                  
}
else
{
    existingRDD.union(rdd);
}

Solution 1

To create an empty RDD in Java, you'll just to do the following:

// Get an RDD that has no partitions or elements.
JavaSparkContext jsc;
...
JavaRDD<T> emptyRDD = jsc.emptyRDD();

I trust you know how to use generics, otherwise, for your case, you'll need:

JavaRDD<Tuple2<String,List<String>>> emptyRDD = jsc.emptyRDD();
JavaPairRDD<String,List<String>> emptyPairRDD = JavaPairRDD.fromJavaRDD(
  existingRDD
);

You can also use the mapToPair method to convert your JavaRDD to a JavaPairRDD.

In Scala :

val sc: SparkContext = ???
... 
val emptyRDD = sc.emptyRDD
// emptyRDD: org.apache.spark.rdd.EmptyRDD[Nothing] = EmptyRDD[1] at ...

Solution 2

val emptyRdd=sc.emptyRDD[String]

Above statement will create empty RDD with String Type

From SparkContext class:

Get an RDD that has no partitions or elements

def emptyRDD[T: ClassTag]: EmptyRDD[T] = new EmptyRDD[T] (this)

Comments

[Dirk Groeneveld]

I'm pretty sure that creates an RDD[String] with one entry, the empty string.

[mjbsgll]

I like this way. I was doing val emptyRdd:RDD[String]=null.