在这篇文章中，我将介绍如何在Spark中使用Akka-http并结合Cassandra实现REST服务，在这个系统中Cassandra用于数据的存储。

　　我们已经见识到Spark的威力，如果和Cassandra正确地结合可以实现更强大的系统。我们先创建一个build.sbt文件，内容如下：

name := "cassandra-spark-akka-http-starter-kit"

version := "1.0"

scalaVersion := "2.11.8"

organization := "com.iteblog"

val akkaV = "2.4.5"
libraryDependencies ++= Seq(
  "org.apache.spark" % "spark-core_2.11" % "2.0.0",
  "org.apache.spark" % "spark-sql_2.11" % "2.0.0",
  "com.typesafe.akka" %% "akka-http-core" % akkaV,
  "com.typesafe.akka" %% "akka-http-experimental" % akkaV,
  "com.typesafe.akka" %% "akka-http-testkit" % akkaV % "test",
  "com.typesafe.akka" %% "akka-http-spray-json-experimental" % akkaV,
  "org.scalatest" %% "scalatest" % "2.2.6" % "test",
  "com.datastax.spark" % "spark-cassandra-connector_2.11" % "2.0.0-M3",
  "net.liftweb" % "lift-json_2.11" % "2.6.2"

)

assembleArtifact in assemblyPackageScala := false 

assemblyMergeStrategy in assembly := {
  case m if m.toLowerCase.endsWith("manifest.mf") => MergeStrategy.discard
  case m if m.toLowerCase.matches("meta-inf.*\\.sf$") => MergeStrategy.discard
  case "reference.conf" => MergeStrategy.concat
  case _ => MergeStrategy.first
}

ivyScala := ivyScala.value map {
  _.copy(overrideScalaVersion = true)
}
fork in run := true

上面我们把 assembleArtifact in assemblyPackageScala 设置为false，因为Spark已经包含了Scala library，所以我们不需要再包含了。

样本类User定义

User累仅仅包含id、名字以及Email等信息，定义如下：

package com.iteblog.domain

case class User(id: String, name: String, email: String)

数据访问层

下面代码片段是数据访问层的实现：

package com.iteblog.factories

import com.iteblog.domain.User
import com.typesafe.config.ConfigFactory
import org.apache.spark.SparkConf
import org.apache.spark.sql.SparkSession
import com.datastax.spark.connector._

import scala.util.Try


trait DatabaseAccess {

  import Context._

  def create(user: User): Boolean =
    Try(sc.parallelize(Seq(user)).saveToCassandra(keyspace, tableName)).toOption.isDefined

  def retrieve(id: String): Option[Array[User]] = Try(sc.cassandraTable[User](keyspace, tableName).where(s"id='$id'").collect()).toOption
}

object DatabaseAccess extends DatabaseAccess


object Context {
  val config = ConfigFactory.load()
  val url = config.getString("cassandra.url")
  val sparkConf: SparkConf = new SparkConf().setAppName("Saprk-cassandra-akka-rest-example").setMaster("local[4]")
    .set("spark.cassandra.connection.host", url)
  val spark = SparkSession.builder().config(sparkConf).getOrCreate()
  val sc = spark.sparkContext
  val keyspace = config.getString("cassandra.keyspace")
  val tableName = config.getString("cassandra.tableName")
}

服务层

下面是路由文件的实现代码：

package com.iteblog.routes

import java.util.UUID

import akka.actor.ActorSystem
import akka.event.Logging
import akka.http.scaladsl.model._
import akka.http.scaladsl.server.Directives._
import akka.http.scaladsl.server.{ExceptionHandler, Route}
import akka.stream.ActorMaterializer
import com.iteblog.domain.User
import com.iteblog.factories.DatabaseAccess
import net.liftweb.json._
import java.util.Date
import net.liftweb.json.Extraction._


trait SparkService extends DatabaseAccess {

  implicit val system:ActorSystem
  implicit val materializer:ActorMaterializer
  val logger = Logging(system, getClass)


  implicit def myExceptionHandler =
    ExceptionHandler {
      case e: ArithmeticException =>
        extractUri { uri =>
          complete(HttpResponse(StatusCodes.InternalServerError, entity = s"Data is not persisted and something went wrong"))
        }
    }

  implicit val formats: Formats = new DefaultFormats {
    outer =>
    override val typeHintFieldName = "type"
    override
    val typeHints = ShortTypeHints(List(classOf[String], classOf[Date]))
  }

  val sparkRoutes: Route = {
    get {
      path("create" / "name" / Segment / "email" / Segment) { (name: String, email: String) =>
        complete {
          val documentId = "user::" + UUID.randomUUID().toString
          try {
            val user = User(documentId,name,email)
            val isPersisted = create(user)
            if (isPersisted) {
              HttpResponse(StatusCodes.Created, entity = s"Data is successfully persisted with id $documentId")
            } else {
              HttpResponse(StatusCodes.InternalServerError, entity = s"Error found for id : $documentId")
            }
          } catch {
            case ex: Throwable =>
              logger.error(ex, ex.getMessage)
              HttpResponse(StatusCodes.InternalServerError, entity = s"Error found for id : $documentId")
          }
        }
      }
    } ~ path("retrieve" / "id" / Segment) { (listOfIds: String) =>
      get {
        complete {
          try {
            val idAsRDD: Option[Array[User]] = retrieve(listOfIds)
            idAsRDD match {
              case Some(data) => HttpResponse(StatusCodes.OK, entity = data.headOption.fold("")(x => compact(render(decompose(x)))))
              case None => HttpResponse(StatusCodes.InternalServerError, entity = s"Data is not fetched and something went wrong")
            }
          } catch {
            case ex: Throwable =>
              logger.error(ex, ex.getMessage)
              HttpResponse(StatusCodes.InternalServerError, entity = s"Error found for ids : $listOfIds")
          }
        }
      }
    }
  }
}

服务启动

现在我们需要编写一个用于启动服务的类，其主要目的是启动一个HTTP服务，这样可以供用户调用，如下：

package com.iteblog

import akka.actor.ActorSystem
import akka.http.scaladsl.Http
import akka.stream.ActorMaterializer
import com.iteblog.routes.SparkService
import com.iteblog.factories.Context


class StartSparkServer(implicit val system: ActorSystem,
                       implicit val materializer: ActorMaterializer) extends SparkService {
  def startServer(address: String, port: Int) = {
    Http().bindAndHandle(sparkRoutes, address, port)
  }
}

object StartApplication extends App {
  StartApp
}

object StartApp {
  implicit val system: ActorSystem = ActorSystem("Spark-Couchbase-Service")
  implicit val executor = system.dispatcher
  implicit val materializer = ActorMaterializer()
  val server = new StartSparkServer()
  val config = Context.config
  val serverUrl = config.getString("http.interface")
  val port = config.getInt("http.port")
  server.startServer(serverUrl, port)
}