How to compare Scala Twitter Futures?

Question

I'm working on a backend migration on some scala code. The code current returns com.twitter.util.Future[A] and I am taking the response from my new backend and transforming it into Future[A] as well

To monitor the migration, I want to do a comparison so I can tell how often the response from the 2 backends differ, so I have something like

val b1Resp: Future[A] = getResponseFromBackend1()
val b2Resp: Future[A] = getResponseFromBackend2() map { resp => convert(resp)}

return b1Resp

I want to verify that the results of the futures are either the same or that they both fail. On discrepancies, I will be publishing a metric.

Moreover, I don't want to delay the return. I don't really care when the metric is published (within reason of course), so it's okay to allow execution to continue

How do I do the comparison?

Answer 1

The broad outline of what you want to do is

{
  val b1Resp: Future[A] = getResponseFromBackend1()
  val b2Resp: Future[A] = getResponseFromBackend2() map { resp => convert(resp)}

  Future.join(b1Resp, b2Resp)  // combine the two futures into a future of a pair of their values
    .foreach {                 // when both are complete...
      case (b1, b2) =>
        // compare and tickle a metric or whatever
        ()  // explicit Unit value, in case the compiler warns about discarding a value
    }

  b1Resp  // return should generally be avoided in Scala
}

Note that Future.join.foreach just arranges for the comparison etc. to happen in the future; it should only add a few microseconds to the block.

For standard library futures, the equivalent to Future.join.foreach is (assuming an implicit ExecutionContext is in scope):

b1Resp.zip(b2Resp)
  .foreach {
    // as above
  }