ReadinessProbe.scala

import akka.http.scaladsl.server.Directives._
import akka.cluster.Member
import akka.actor.Address
import akka.http.scaladsl.model.StatusCodes
import de.heikoseeberger.akkahttpcirce.FailFastCirceSupport._
import io.circe.generic.auto._

val cluster = Cluster(system)

final case class ClusterView(status: MemberStatus,
                             address: Address,
                             leader: Option[Address],
                             members: Set[Member])
get {
  import cluster.selfUniqueAddress
  val clusterState = cluster.state
  val selfStatus = clusterState.members
    .find(_.uniqueAddress == selfUniqueAddress)
    .map(_.status)
    .getOrElse(MemberStatus.Removed)
  val clusterView =
    ClusterView(
      selfStatus,
      selfUniqueAddress.address,
      clusterState.leader,
      clusterState.members
    )
  clusterView.status match {
    case MemberStatus.Up =>
      complete {
        StatusCodes.OK -> clusterView
      }
    case _ =>
      complete {
        StatusCodes.ServiceUnavailable -> clusterView
      }
  }
}

SimplifiedProbe.scala

import akka.http.scaladsl.server.Directives._
import akka.cluster.MemberStatus
import akka.actor.Address
import akka.http.scaladsl.model.StatusCodes
import de.heikoseeberger.akkahttpcirce.FailFastCirceSupport._
import io.circe.syntax._

val cluster = Cluster(system)

get {
  val selfStatus = cluster.state.members
    .find(_.uniqueAddress == cluster.selfUniqueAddress)
    .map(_.status)
    .getOrElse(MemberStatus.Removed)
 
  selfStatus match {
    case MemberStatus.Up =>
      complete {
        StatusCodes.OK -> Json.obj("status" -> "Up".asJson)
      }
    case other =>
      complete {
        StatusCodes.ServiceUnavailable -> Json.obj("status" -> other.toString.asJson)
      }
  }
}