kafka-streams-scala
This is a thin Scala wrapper for the Kafka Streams API. It does not intend to provide a Scala-idiomatic API, but rather intends to make the original API simpler to use from Scala. In particular, it provides the following adjustements:
- Scala lambda expressions can be used directly
- when aggregating and counting, counts are converted from Java
Long
s to ScalaLong
s - when using a
flatMap
operation, this lets you use a ScalaIterable
Serde
s (Serializers/Deserializers) can be implicitly found in the scope
This API also contains a few Serde
s (Serializers/Deserializers):
- to convert Scala
Int
/Long
/Double
to/from their binary representation - to convert Scala
Int
/Long
/Double
to/from string representation - to convert case classes to/from JSON
Usage of the Kafka Streams API in Scala
The main objects are:
KStreamsBuilderS
as the entry point to build streams or tablesKStreamS
as a wrapper aroundKStream
KGroupedStreamS
as a wrapper aroundKGroupedStream
KTableS
as a wrapper aroundKTable
KGroupedTable
as a wrapper aroundKGroupedTable
Using the builder
With the original Java API, you would create an instance of KStreamBuilder
, then use it to create streams or tables. Here, KStreamsBuilderS
is an object
thqt can be used directly:
val stream: KStreamS[String, String] = KStreamBuilderS.stream[String, String]("my-stream")
val table: KTableS[String, String] = KStreamBuilderS.table[String, String]("my-table")
When starting the application, you just need to unwrap the KStreamBuilder
by calling KStreamBuilderS.inner
:
val streams = new KafkaStreams(KStreamBuilderS.inner, config)
Serdes (declare them as implicit)
It is a common mistake to forget to specify Serde
s when using the Java API, then resulting in class cast errors when objects are serialized or deserialized.
To work around this issue, this API requires Serde
s to be used. Most of the times, it is enough to declare your Serde
s as implicit
values, and they will be picked up automatically:
implicit val stringSerde: Serde[String] = Serdes.String()
implicit val userSerde: Serde[User] = new MyUserSerde
val usersByIdStream = KStreamBuilderS.stream[String, User]("users-by-id")
Resolution is based on the type of the object to serialize/deserialize, so make sure you have a Serve
of the appropriate type. If not, you should see an error such as:
Error:(87, 80) could not find implicit value for parameter keySerde: org.apache.kafka.common.serialization.Serde[String]
If, on the other hand, you have multiple Serde
s for the same type, you might see the following error:
Error:(88, 80) ambiguous implicit values:
both value stringSerde2 of type org.apache.kafka.common.serialization.Serde[String]
and value stringSerde1 of type org.apache.kafka.common.serialization.Serde[String]
match expected type org.apache.kafka.common.serialization.Serde[String]
In this case, just pass the Serde
explicitly:
val usersByIdStream = KStreamBuilderS.stream[String, User]("users-by-id")(stringSerde, userSerde)
Usage of the Serdes in Scala
To convert Scala Int
/Long
/Double
to/from their binary representation:
import com.github.aseigneurin.kafka.serialization.scala._
implicit val intSerde = IntAsStringSerde
implicit val longSerde = LongAsStringSerde
implicit val doubleSerde = DoubleAsStringSerde
To convert Scala Int
/Long
/Double
to/from string representation:
import com.github.aseigneurin.kafka.serialization.scala._
implicit val intSerde = IntSerde
implicit val longSerde = LongSerde
implicit val doubleSerde = DoubleSerde
To convert case classes to/from JSON:
- define a
case class
- create an instance of
JsonSerde
with the case class as the generic type
Example:
import com.github.aseigneurin.kafka.serialization.scala._
case class User(name: String)
implicit val stringSerde = Serdes.String
implicit val userSerde = new JsonSerde[User]
// read JSON -> case class
KStreamBuilderS.stream[String, User]("users")
.mapValues { user => user.name }
.to("names")
// write case class -> JSON
KStreamBuilderS.stream[String, String]("names")
.mapValues { name => User(name) }
.to("users")
Example
This repository contains a Scala version of the Java Word Count Demo.
Here is the code to implement a word count:
val props = new Properties()
// ...
implicit val stringSerde = Serdes.String
implicit val longSerde = LongAsStringSerde
val source = KStreamBuilderS.stream[String, String]("streams-file-input")
val counts: KTableS[String, Long] = source
.flatMapValues { value => value.toLowerCase(Locale.getDefault).split(" ") }
.map { (_, value) => (value, value) }
.groupByKey
.count("Counts")
counts.to("streams-wordcount-output")
val streams = new KafkaStreams(KStreamBuilderS.inner, props)
streams.start()