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KDnuggets Home » News » 2016 » Feb » Tutorials, Overviews » Apache Spark: RDD, DataFrame or Dataset? ( 16:n05 )

Apache Spark: RDD, DataFrame or Dataset?


 
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There are now 3 Apache Spark APIs. Here’s how to choose the right one.



Dataset API

The Dataset API, released as an API preview in Spark 1.6, aims to provide the best of both worlds; the familiar object-oriented programming style and compile-time type-safety of the RDD API but with the performance benefits of the Catalyst query optimizer. Datasets also use the same efficient off-heap storage mechanism as the DataFrame API.

When it comes to serializing data, the Dataset API has the concept of encoders which translate between JVM representations (objects) and Spark’s internal binary format. Spark has built-in encoders which are very advanced in that they generate byte code to interact with off-heap data and provide on-demand access to individual attributes without having to de-serialize an entire object. Spark does not yet provide an API for implementing custom encoders, but that is planned for a future release.

Additionally, the Dataset API is designed to work equally well with both Java and Scala. When working with Java objects, it is important that they are fully bean-compliant. In writing the examples to accompany this article, we ran into errors when trying to create a Dataset in Java from a list of Java objects that were not fully bean-compliant.

Example: Creating Dataset from a list of objects

Scala

val sc = new SparkContext(conf)
val sqlContext = new SQLContext(sc)
import sqlContext.implicits._
val sampleData: Seq[ScalaPerson] = ScalaData.sampleData()
val dataset = sqlContext.createDataset(sampleData)

Java:

JavaSparkContext sc = new JavaSparkContext(sparkConf);
SQLContext sqlContext = new SQLContext(sc);
List data = JavaData.sampleData();
Dataset dataset = sqlContext.createDataset(data, Encoders.bean(JavaPerson.class));

Transformations with the Dataset API look very much like the RDD API and deal with the Person class rather than an abstraction of a row.

Example: Filter by attribute with Dataset

Scala

dataset.filter(_.age < 21);

Java

dataset.filter(person -> person.getAge() < 21);

Despite the similarity with RDD code, this code is building a query plan, rather than dealing with individual objects, and if age is the only attribute accessed, then the rest of the the object’s data will not be read from off-heap storage.

Conclusions

If you are developing primarily in Java then it is worth considering a move to Scala before adopting the DataFrame or Dataset APIs. Although there is an effort to support Java, Spark is written in Scala and the code often makes assumptions that make it hard (but not impossible) to deal with Java objects.

If you are developing in Scala and need your code to go into production with Spark 1.6.0 then the DataFrame API is clearly the most stable option available and currently offers the best performance.

However, the Dataset API preview looks very promising and provides a more natural way to code. Given the rapid evolution of Spark it is likely that this API will mature very quickly through 2016 and become the de-facto API for developing new applications.

Author Bio: Andy Grove is Chief Architect at AgilData. He is responsible for product architecture. He is also actively involved in R&D and product implementation.

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