Kotlin for Apache® Spark™

Your next API to work with Apache Spark.

This project adds a missing layer of compatibility between Kotlin and Apache Spark. It allows Kotlin developers to use familiar language features such as data classes, and lambda expressions as simple expressions in curly braces or method references.

We have opened a Spark Project Improvement Proposal: Kotlin support for Apache Spark to work with the community towards getting Kotlin support as a first-class citizen in Apache Spark. We encourage you to voice your opinions and participate in the discussion.

Table of Contents

• Supported versions of Apache Spark
• Releases
• How to configure Kotlin for Apache Spark in your project
• Kotlin for Apache Spark features
  • Creating a SparkSession in Kotlin
  • Creating a Dataset in Kotlin
  • Null safety
  • withSpark function
  • withCached function
  • toList and toArray
  • Column infix/operator functions
  • Overload Resolution Ambiguity
  • Tuples
  • Streaming
  • User Defined Functions
• Examples
• Reporting issues/Support
• Code of Conduct
• License

Supported versions of Apache Spark

Deprecated versions

Releases

The list of Kotlin for Apache Spark releases is available here. The Kotlin for Spark artifacts adhere to the following convention: [name]_[Apache Spark version]_[Scala core version]:[Kotlin for Apache Spark API version]

The only exception to this is scala-tuples-in-kotlin_[Scala core version]:[Kotlin for Apache Spark API version], which is independent of Spark.

How to configure Kotlin for Apache Spark in your project

You can add Kotlin for Apache Spark as a dependency to your project: Maven, Gradle, SBT, and leinengen are supported.

Here's an example pom.xml:

<dependency>
  <groupId>org.jetbrains.kotlinx.spark</groupId>
  <artifactId>kotlin-spark-api_3.3.1_2.13</artifactId>
  <version>${kotlin-spark-api.version}</version>
</dependency>
<dependency>
    <groupId>org.apache.spark</groupId>
    <artifactId>spark-sql_2.13</artifactId>
    <version>${spark.version}</version>
</dependency>

Note that you must match the version of the Kotlin for Apache Spark API to the Spark- and Scala version of your project. You can find a complete example with pom.xml and build.gradle in the Quick Start Guide.

If you want to try a development version. You can use the versions published to GH Packages. They typically have the same version as the release version, but with a -SNAPSHOT suffix. See the GitHub Docs for more information.

Once you have configured the dependency, you only need to add the following import to your Kotlin file:

import org.jetbrains.kotlinx.spark.api.*

Jupyter

The Kotlin Spark API also supports Kotlin Jupyter notebooks. To it, simply add

%use spark

to the top of your notebook. This will get the latest version of the API, together with the latest version of Spark. To define a certain version of Spark or the API itself, simply add it like this:

%use spark(spark=3.3.1, scala=2.13, v=1.2.2)

Inside the notebook a Spark session will be initiated automatically. This can be accessed via the spark value. sc: JavaSparkContext can also be accessed directly. The API operates pretty similarly.

There is also support for HTML rendering of Datasets and simple (Java)RDDs. Check out the example as well.

To use Spark Streaming abilities, instead use

%use spark-streaming

This does not start a Spark session right away, meaning you can call withSparkStreaming(batchDuration) {} in whichever cell you want. Check out the example.

NOTE: You need kotlin-jupyter-kernel to be at least version 0.11.0.83 for the Kotlin Spark API to work. Also, if the %use spark magic does not output "Spark session has been started...", and %use spark-streaming doesn't work at all, add %useLatestDescriptors above it.

For more information, check the wiki.

Kotlin for Apache Spark features

Creating a SparkSession in Kotlin

val spark = SparkSession
        .builder()
        .master("local[2]")
        .appName("Simple Application").orCreate

This is not needed when running the Kotlin Spark API from a Jupyter notebook.

Creating a Dataset in Kotlin

spark.dsOf("a" to 1, "b" to 2)

The example above produces Dataset<Pair<String, Int>>. While Kotlin Pairs and Triples are supported, Scala Tuples are recommended for better support.

Null safety

There are several aliases in API, like leftJoin, rightJoin etc. These are null-safe by design. For example, leftJoin is aware of nullability and returns Dataset<Pair<LEFT, RIGHT?>>. Note that we are forcing RIGHT to be nullable for you as a developer to be able to handle this situation. NullPointerExceptions are hard to debug in Spark, and we're doing our best to make them as rare as possible.

In Spark, you might also come across Scala-native Option<*> or Java-compatible Optional<*> classes. We provide getOrNull() and getOrElse() functions for these to use Kotlin's null safety for good.

Similarly, you can also create Option<*>s and Optional<*>s like T?.toOptional() if a Spark function requires it.

withSpark function

We provide you with useful function withSpark, which accepts everything that may be needed to run Spark — properties, name, master location and so on. It also accepts a block of code to execute inside Spark context.

After work block ends, spark.stop() is called automatically.

Do not use this when running the Kotlin Spark API from a Jupyter notebook.

withSpark {
    dsOf(1, 2)
        .map { it X it } // creates Tuple2<Int, Int>
        .show()
}

dsOf is just one more way to create Dataset (Dataset<Int>) from varargs.

withCached function

It can easily happen that we need to fork our computation to several paths. To compute things only once we should call cache method. However, it becomes difficult to control when we're using cached Dataset and when not. It is also easy to forget to unpersist cached data, which can break things unexpectedly or take up more memory than intended.

To solve these problems we've added withCached function

withSpark {
    dsOf(1, 2, 3, 4, 5)
        .map { tupleOf(it, it + 2) }
        .withCached {
            showDS()
  
            filter { it._1 % 2 == 0 }.showDS()
        }
        .map { tupleOf(it._1, it._2, (it._1 + it._2) * 2) }
        .show()
}

Here we're showing cached Dataset for debugging purposes then filtering it. The filter method returns filtered Dataset and then the cached Dataset is being unpersisted, so we have more memory t o call the map method and collect the resulting Dataset.

toList and toArray methods

For more idiomatic Kotlin code we've added toList and toArray methods in this API. You can still use the collect method as in Scala API, however the result should be casted to Array. This is because collect returns a Scala array, which is not the same as Java/Kotlin one.

Column infix/operator functions

Similar to the Scala API for Columns, many of the operator functions could be ported over. For example:

dataset.select( col("colA") + 5 )
dataset.select( col("colA") / col("colB") )

dataset.where( col("colA") `===` 6 )
// or alternatively
dataset.where( col("colA") eq 6)

To read more, check the wiki.

Overload resolution ambiguity

We had to implement the functions reduceGroups and reduce for Kotlin separately as reduceGroupsK and reduceK respectively, because otherwise it caused resolution ambiguity between Kotlin, Scala and Java APIs, which was quite hard to solve.

We have a special example of work with this function in the Groups example.

Tuples

Inspired by ScalaTuplesInKotlin, the API introduces a lot of helper- extension functions to make working with Scala Tuples a breeze in your Kotlin Spark projects. While working with data classes is encouraged, for pair-like Datasets / RDDs / DStreams Scala Tuples are recommended, both for the useful helper functions, as well as Spark performance. To enable these features simply add

import org.jetbrains.kotlinx.spark.api.tuples.*

to the start of your file.

Tuple creation can be done in the following manners:

val a: Tuple2<Int, Long> = tupleOf(1, 2L)
val b: Tuple3<String, Double, Int> = t("test", 1.0, 2)
val c: Tuple3<Float, String, Int> = 5f X "aaa" X 1

To read more about tuples and all the added functions, refer to the wiki.

Streaming

A popular Spark extension is Spark Streaming. Of course the Kotlin Spark API also introduces a more Kotlin-esque approach to write your streaming programs. There are examples for use with a checkpoint, Kafka and SQL in the examples module.

We shall also provide a quick example below:

// Automatically provides ssc: JavaStreamingContext which starts and awaits termination or timeout
withSparkStreaming(batchDuration = Durations.seconds(1), timeout = 10_000) { // this: KSparkStreamingSession

    // create input stream for, for instance, Netcat: `$ nc -lk 9999`
    val lines: JavaReceiverInputDStream<String> = ssc.socketTextStream("localhost", 9999)
  
    // split input stream on space
    val words: JavaDStream<String> = lines.flatMap { it.split(" ").iterator() }

    // perform action on each formed RDD in the stream
    words.foreachRDD { rdd: JavaRDD<String>, _: Time ->
      
          // to convert the JavaRDD to a Dataset, we need a spark session using the RDD context
          withSpark(rdd) { // this: KSparkSession
            val dataframe: Dataset<TestRow> = rdd.map { TestRow(word = it) }.toDS()
            dataframe
                .groupByKey { it.word }
                .count()
                .show()
            // +-----+--------+
            // |  key|count(1)|
            // +-----+--------+
            // |hello|       1|
            // |   is|       1|
            // |    a|       1|
            // | this|       1|
            // | test|       3|
            // +-----+--------+
        }
    }
}

For more information, check the wiki.

User Defined Functions

Spark has a way to call functions from SQL using so-called UDFs. Using the Scala/Java API from Kotlin is not that obvious, so we decided to add special UDF support for Kotlin. This support grew into a typesafe, name-safe, and feature-rich solution for which we will give an example:

// example of creation/naming, and registering of a simple UDF
val plusOne by udf { x: Int -> x + 1 }
plusOne.register()
spark.sql("SELECT plusOne(5)").show()
// +----------+
// |plusOne(5)|
// +----------+
// |         6|
// +----------+

// directly registering
udf.register("plusTwo") { x: Double -> x + 2.0 }
spark.sql("SELECT plusTwo(2.0d)").show()
// +------------+
// |plusTwo(2.0)|
// +------------+
// |         4.0|
// +------------+

// dataset select
val result: Dataset<Int> = myDs.select(
  plusOne(col(MyType::age))
)

We support:

• a notation close to Spark's
• smart naming (with reflection)
• creation from function references
• typed column operations
• UDAF support and functional creation
• (Unique!) simple vararg UDF support

For more, check the extensive examples. Also, check out the wiki.

Examples

For more, check out examples module. To get up and running quickly, check out this tutorial.

Reporting issues / support

Please use GitHub issues for filing feature requests and bug reports. You are also welcome to join kotlin-spark channel in the Kotlin Slack.

Contribution guide

Contributions are more than welcome! Pull requests can be created for the main branch and will be considered as soon as possible. Be sure to add the necessary tests for any new feature you add. The main branch always aims to target the latest available Apache Spark version. Note that we use Java Comment Preprocessor to build the library for all different supported versions of Apache Spark and Scala. The current values of these versions can be edited in gradle.properties and should always be the latest versions for commits. For testing, all versions need a pass for the request to be accepted. We use GitHub Actions to test and deploy the library for all versions, but locally you can also use the gradlew_all_versions file.

Of the main branch, development versions of the library are published to GitHub Packages. This way, new features can be tested quickly without having to wait for a full release.

For full releases, the release branch is updated.

Code of Conduct

This project and the corresponding community is governed by the JetBrains Open Source and Community Code of Conduct. Please make sure you read it.

License

Kotlin for Apache Spark is licensed under the Apache 2.0 License.