gistfile5.scala

    case r @ Req("webservices" :: "product" :: id :: _, _, PutRequest) => {
      def updateResource(id: String, r: Req): LiftResponse = {
        val reqETag = r.header("If-Match") openOr ""
        val product = Product.findByKey(id.toLong)
        product match {
          case Full(x) => {
            compareETags(reqETag, x.updatedAt.toLong.toString) match {
              case true => {
                save(r.xml.open_!, x)
                NoContentResponse(x.updatedAt.toLong.toString)
              }
              case false => PreConditionFailedResponse(x.updatedAt.toLong.toString,
                "Update failed. Resource state changed.")
            }
          }
          case _ => NotFoundResponse("Resource not found")
        }
      }

      r.xml_? match {
        case true => updateResource(id, r)
        case false => NotFoundResponse("Resource not found")
      }
    }

gistfile6.scala

case class NoContentResponse(eTag: String) extends LiftResponse with HeaderDefaults {
  def toResponse = InMemoryResponse(Array(), "ETag" -> ("\"" + eTag + "\"") :: headers, cookies, 204)
}

case class PreConditionFailedResponse(eTag: String, msg: String) extends LiftResponse with HeaderDefaults {
  def toResponse = InMemoryResponse(msg.getBytes("UTF-8"), "ETag" -> ("\"" + eTag + "\"") :: headers, cookies, 412)
}

NotModifiedResponse.scala

case class NotModifiedResponse(eTag: String) extends LiftResponse with HeaderDefaults {
  def toResponse = InMemoryResponse(Array(), "ETag" -> ("\"" + eTag + "\"") :: headers, cookies, 304)
}

Product.scala

package code.model

import net.liftweb.mapper._

class Product extends LongKeyedMapper[Product] with CreatedUpdated {
  def getSingleton = Product
  def primaryKeyField = itemNumber

  object itemNumber extends MappedLongIndex(this)
  object name extends MappedString(this, 20)
  object price extends MappedDouble(this)
  object quantity extends MappedInt(this)
}

object Product extends Product with LongKeyedMetaMapper[Product]

WebService1.scala

object WebService extends RestHelper {
  serve {
    case r @ Req("webservices" :: "product" :: id :: _, _, GetRequest) => {
      val reqETag = r.header("If-None-Match") openOr ""
      val product = Product.findByKey(id.toLong)
      product match {
        case Full(x) => {
          val resourceETag = x.updatedAt.toLong.toString
          compareETags(reqETag, resourceETag) match {
            case true => NotModifiedResponse(resourceETag)
            case false => XmlResponse(toXml(x), resourceETag)
          }
        }
        case _ => NotFoundResponse("Resource not found")
      }
    }
  }

  def compareETags(reqETags: String, respETag: String) = reqETags split(",") map (_.trim) contains("\"" + respETag + "\"")
}

XmlResponse.scala

class XmlResponse(val xml: Node, val code: Int, val mime: String, val _headers: List[(String, String)],
                  val cookies: List[HTTPCookie]) extends NodeResponse {
  def docType = Empty
  def out: Node = xml
  def headers = _headers
}

object XmlResponse {
  def apply(xml: Node, eTag: String) =
    new XmlResponse(xml, 200, "application/xml", ("Content-Type" -> "application/xml") :: "ETag" -> ("\"" + eTag + "\"") :: Nil, Nil)
}