Using Elastic APM with Scalatra

Elastic APM hooks into Scalatra pretty easily. Since APM’s Java agent supports the servlet API, particularly Jetty, you can instrument a Scalatra app by just slapping on the agent.

But you may notice the resulting transaction names have only the request’s method and servlet’s name. For example, all GETs in HealthServlet will fall under the transaction name HealthServlet#doGet:

HealthServlet transaction names

This is much better than no monitoring at all, but if you’ve got multiple routes in a single servlet, you may wish to disambiguate them.

Fortunately, getting more precise transaction names isn’t too hard. By extending ScalatraServlet, you can hook into your routes and set a unique transaction name for each. Observe:

import co.elastic.apm.api.ElasticApm
import org.scalatra.{PathPatternRouteMatcher, RailsRouteMatcher, RegexRouteMatcher, Route, RouteTransformer, ScalatraServlet, SinatraRouteMatcher}

trait ScalatraApmServlet extends ScalatraServlet {
  /**
   * Calculate the route portion of an APM transaction name.
   *
   * We ignore a couple matchers that don't have obvious text representations. This should be fine for most purposes.
   *
   * @param transformers The route's transformers.
   * @return Transaction name path.
   */
  private def getApmTransactionPath(transformers: Seq[RouteTransformer]): String = {
    val transformersWithPath = transformers.filter {
      case _: PathPatternRouteMatcher => true
      case _: RailsRouteMatcher => true
      case _: RegexRouteMatcher => true
      case _: SinatraRouteMatcher => true
      case _ => false
    }

    val concatenated = transformersWithPath.map(_.toString).mkString("; ")
    if (concatenated != "/") {
      concatenated
    } else {
      ""
    }
  }

  private def wrapWithApmTransactionName(transactionPath: String, action: => Any): Any = {
    ElasticApm.currentTransaction.setName(s"${request.getMethod} ${request.getServletPath}${transactionPath}")

    action
  }

  override def get(transformers: RouteTransformer*)(action: => Any): Route = {
    val transactionPath = getApmTransactionPath(transformers)
    super.get(transformers: _*) { wrapWithApmTransactionName(transactionPath, action) }
  }
  override def post(transformers: RouteTransformer*)(action: => Any): Route = {
    val transactionPath = getApmTransactionPath(transformers)
    super.post(transformers: _*) { wrapWithApmTransactionName(transactionPath, action) }
  }
  override def put(transformers: RouteTransformer*)(action: => Any): Route = {
    val transactionPath = getApmTransactionPath(transformers)
    super.put(transformers: _*) { wrapWithApmTransactionName(transactionPath, action) }
  }
  override def delete(transformers: RouteTransformer*)(action: => Any): Route = {
    val transactionPath = getApmTransactionPath(transformers)
    super.delete(transformers: _*) { wrapWithApmTransactionName(transactionPath, action) }
  }
  override def options(transformers: RouteTransformer*)(action: => Any): Route = {
    val transactionPath = getApmTransactionPath(transformers)
    super.options(transformers: _*) { wrapWithApmTransactionName(transactionPath, action) }
  }
  override def head(transformers: RouteTransformer*)(action: => Any): Route = {
    val transactionPath = getApmTransactionPath(transformers)
    super.head(transformers: _*) { wrapWithApmTransactionName(transactionPath, action) }
  }
  override def patch(transformers: RouteTransformer*)(action: => Any): Route = {
    val transactionPath = getApmTransactionPath(transformers)
    super.patch(transformers: _*) { wrapWithApmTransactionName(transactionPath, action) }
  }
}

If you make your servlets extend ScalatraApmServlet instead of ScalatraServlet, their routes will be distinguished in APM.

This works because when an action is evaluated, the APM agent will have already established a transaction for us. We’re just overriding its name.

I use this with Scalatra 2.7.0. I imagine it’s prone to breaking with future Scalatra releases, but am hopeful it won’t be too hard to adapt when that happens.

Ryan Conway