FinagleHttpClient.scala

package com.dwolla.http.finagle

import com.twitter.finagle.Http.Client
import com.twitter.finagle.http.{Method, Request, Response}
import com.twitter.finagle.loadbalancer.LoadBalancerFactory
import com.twitter.finagle.{Http, Service}

object FinagleHttpClient {

  private def newService(dest: String): Service[Request, Response] = {
    val modifiedStack = Http.Client.stack.insertAfter(LoadBalancerFactory.role, new HttpHostHeaderAppendingModule[Request, Response])
    Client(stack = modifiedStack).newService(dest)
  }

  def apply(dest: String): FinagleHttpClient = new FinagleHttpClient(newService(dest))

}

HttpHostHeaderAppendingModule.scala

package com.dwolla.http.finagle

import java.net.SocketAddress

import com.dwolla.http.finagle.HttpHostHeaderAppendingModule.toHostHeaderValue
import com.twitter.finagle.Stack.Role
import com.twitter.finagle.client.Transporter.EndpointAddr
import com.twitter.finagle.http.Request
import com.twitter.finagle.{Filter, ServiceFactory, Stack}

class HttpHostHeaderAppendingModule[Req <: Request, Rep] extends Stack.Module1[EndpointAddr, ServiceFactory[Req, Rep]] {
  val role = Role("sets-host-header")
  val description = "adds the Host header to the request"

  def addHostHeaderWith(dest: EndpointAddr) = Filter.mk[Req, Rep, Req, Rep] { (req, svc) ⇒
    req.headerMap.add("Host", toHostHeaderValue(dest.addr))
    svc(req)
  }

  override def make(dest: EndpointAddr, next: ServiceFactory[Req, Rep]) = {
    addHostHeaderWith(dest) andThen next
  }
}

object HttpHostHeaderAppendingModule {
  private val addrRegex = """/?(.+)""".r

  def toHostHeaderValue(addr: SocketAddress): String = {
    addr.toString match {
      case addrRegex(inet) ⇒ inet
      case _ ⇒ throw new RuntimeException(s"could not parse $addr")
    }
  }
}