Provost is a Scala library inspired by GitHub's scientist and aims to allow easy side by side testing of code paths using Futures.

Features

• Asynchronous by way of Futures
• Returns when the control Future completes so that the other codepath won't slow things down

Notes

This is a pretty new library that was just refactored, so drop me a line if you've got ideas for improving it. It might change!

Using It

// Add the Dep
libraryDependencies += "provost" %% "provost" % "1.0.0"

// And a the resolver
resolvers += "gphat" at "https://raw.github.com/gphat/mvn-repo/master/releases/"

Usage

Provost uses Futures. An experiment and a control Future must be provided. The perform method returns a Future that will be completed when the control completes and it will return the result of the control Future.

import github.gphat.Experiment

// Some futures!
def fastOK = {
  Future {
    Thread.sleep(250)
    "OK"
  }
}

def slowOK = {
  Future {
    Thread.sleep(1000)
    "OK"
  }
}

// Make an experiment! Note that the experiment is parameterized to the type
// we expect from our experiments. We provide a future for the control and
// for the candidate! Names are optional.
val ex = new Experiment[String](name = Some("better_string"), control = slowOK, candidate = fastOK)
val result = ex.perform

// The returned Future is tied to the control and will return even if
// the candidate hasn't finished yet.
ex.map({ r =>
  // Do something!
  println(r) // This is the output of the control!
})

// You can also get a Future that is tied to *both* Futures completing.
// It returns a Result and you can block/await/callback it's completion.
val wholeExperiment = ex.getTotalFuture
wholeExperiment.map({ result =>
  // Now you can look at the two futures and compare them or whatever
  val control = ex.control
  val candidate = ex.candidate

  // Both Futures are complete
  control.isCompleted // True!
  candidate.isCompleted // True!

  // You can test if both underlying Trys were successful.
  result.succeeded

  // You can compare the execution times of each. Note that these times will be
  // measured from the start of the experiment to each Future's completion! This
  // means we might be missing some time since you created these Futures by
  // yourself! Note that these are Scala Duration objects.
  result.candidateDuration
  result.controlDuration

  // You can inspect the two results to determine equality, which Provost leaves
  // to you to do since equality is hard. These will be Try[A]. You can use the
  // aforementioned `succeeded` to determine how to unwind the Try
  result.candidateResult
  result.candidateResult

  // So, as an example, maybe you'd resolve the whole thing like this:
  val control = oldFunction // A Future[String] that we know works
  val candidate = newFunction // A Future[String] that we're testing

  val ex = Experiment[String](name = Some("better_string"), control = control, candidate = candidate)

  // Assuming that your type can be compared with `equals`, you can use this!
  if(ex.equalled) {
    println("Yay, equality!")
  } else {
    println("Boo, we didn't equal up :(")
  }

  // See below
  ex.getTotalFuture.onComplete(experimentLogger)

  ex.perform.map({ res =>
    // Do whatever you were gonna do with the control's result, since it's
    // now in `res`!
  })

  // A more thorough walk through!
  def experimentLogger(result: Result[String]) = {
    if(fullResult.isSuccess) {
      if(ex.equalled) {
        println("Yay, equality!")
      } else {
        println("Boo, we didn't equal up :(")
      }
    } else {
      if(fullResult.controlResult.isFailure && fullResult.candidateResult.isFailure) {
        println("Both the candidate and the control failed, maybe that's ok?")
      if(fullResult.control.isFailure) {
        println("The control failed and the candidate succeeded. Maybe it's better!")
      } else {
        println("The candidate failed! Back to the drawing board!")
      }
    }
    // Now we can report on duration
    println("Control took ${fullResult.controlDuration.toMillis} ms")
    println("Candidate took ${fullResult.candidateDuration.toMillis} ms")

    // You could even emit these as metrics using the Experiment's name!
    statsd.time("experiment.${ex.name.get}.control.duration_ms", fullResult.controlDuration.toMillis)
    statsd.time("experiment.${ex.name.get}.candidate.duration_ms", fullResult.candidateDuration.toMillis)
  }

})

Execution Contexts

You can also supply your own execution context. Provost uses the default Scala global execution context otherwise.

val ec = ExecutionContext.fromExecutor(Executors.newFixedThreadPool(1))
val ex = new Experiment[String](control = slowOK, candidate = fastOK)(ec)

Internals

The Future returned by perform uses completeWith to tie itself to the provided control Future. An onComplete is added to both the control and the experiment and an AtomicInteger in a this.synchronized block is used to track the completion of the two Futures. When both are complete the "whole experiment" Future is completed with success(Result).