raft4s

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raft4s

An implementation of the Raft distributed consensus algorithm written in Scala with Cats library.

Implemented features

• Leader Election
• Log replication
• Cluster configuration
• Log Compaction

Usage

Complete example implementation can be found on https://github.com/alirezameskin/raft4s-kvstore-example

SBT settings

resolvers += "raft4s".at("https://maven.pkg.github.com/alirezameskin/raft4s")

libraryDependencies ++= Seq(
  "com.github.alirezameskin" %% "raft4s-effect" % "0.0.3"
  "com.github.alirezameskin" %% "raft4s-grpc"   % "0.0.3"
)

Defining a StateMachine and Commands

package demo

import cats.effect.IO
import cats.effect.concurrent.Ref
import raft4s.{ReadCommand, StateMachine, WriteCommand}

import java.io.{ByteArrayInputStream, ByteArrayOutputStream, ObjectInputStream, ObjectOutputStream}
import java.nio.ByteBuffer

case class SetCommand(key: String, value: String) extends WriteCommand[String]
case class DeleteCommand(key: String) extends WriteCommand[Unit]
case class GetCommand(key: String) extends ReadCommand[String]

class KvStateMachine(lastIndex: Ref[IO, Long], map: Ref[IO, Map[String, String]]) extends StateMachine[IO] {

  override def applyWrite: PartialFunction[(Long, WriteCommand[_]), IO[Any]] = {
    case (index, SetCommand(key, value)) =>
      for {
        _ <- map.update(_ + (key -> value))
        _ <- lastIndex.set(index)
      } yield value

    case (index, DeleteCommand(key)) =>
      for {
        _ <- map.update(_.removed(key))
        _ <- lastIndex.set(index)
      } yield ()
  }

  override def applyRead: PartialFunction[ReadCommand[_], IO[Any]] = {
    case GetCommand(key) =>
      for {
        items <- map.get
        _ = println(items)
      } yield items(key)
  }

  override def appliedIndex: IO[Long] = lastIndex.get

  override def takeSnapshot(): IO[(Long, ByteBuffer)] =
    for {
      items <- map.get
      index <- lastIndex.get
      bytes = serialize(items)
    } yield (index, bytes)


  override def restoreSnapshot(index: Long, bytes: ByteBuffer): IO[Unit] =
    for {
      _ <- map.set(deserialize(bytes))
      _ <- lastIndex.set(index)
    } yield ()

  private def serialize(items: Map[String, String]): ByteBuffer = {
    val stream: ByteArrayOutputStream = new ByteArrayOutputStream()
    val oos = new ObjectOutputStream(stream)
    oos.writeObject(items)
    oos.close

    ByteBuffer.wrap(stream.toByteArray)
  }

  private def deserialize(bytes: ByteBuffer): Map[String, String] = {

    val ois = new ObjectInputStream(new ByteArrayInputStream(bytes.array()))
    val response = ois.readObject().asInstanceOf[Map[String, String]]
    ois.close()

    response
  }

}

object KvStateMachine {
  def empty: IO[KvStateMachine] =
    for {
      index <- Ref.of[IO, Long](0L)
      map <- Ref.of[IO, Map[String, String]](Map.empty)
    } yield new KvStateMachine(index, map)
}

Creating the cluster

package demo

import cats.effect.{ExitCode, IO, IOApp, Resource}
import io.odin.consoleLogger
import raft4s.effect.rpc.grpc.io.implicits._
import raft4s.effect.storage.memory.MemoryStorage
import raft4s.effect.{odinLogger, RaftCluster}
import raft4s.{Cluster, Configuration, Node}

object SampleKVApp extends IOApp {

  val config = Configuration(Node("localhost", 8090), List.empty)

  implicit val logger = odinLogger[IO](consoleLogger())

  override def run(args: List[String]): IO[ExitCode] =
    makeCluster(config).use { cluster =>
      for {
        leader <- cluster.start
        _      <- IO(println(s"Election is completed. Leader is ${leader}"))
        _      <- cluster.execute(SetCommand("key", "value"))
        _      <- IO(println("Set command is executed"))
        result <- cluster.execute(GetCommand("key"))
        _      <- IO(println(s"Result of the get command is : ${result}"))
      } yield ExitCode.Success
    }

  private def makeCluster(config: Configuration): Resource[IO, Cluster[IO]] =
    for {
      stateMachine <- Resource.liftF(KvStateMachine.empty)
      storage      <- Resource.liftF(MemoryStorage.empty[IO])
      cluster      <- RaftCluster.resource(config, storage, stateMachine)
    } yield cluster
}