ecmascript 6 - How do you implement a "raceToSuccess" helper, given a list of promises?

Question

I'm puzzled by something in the ES6 Promise API. I can see a clear use case for submitting multiple async jobs concurrently, and "resolving" on the first success. This would, for example, serve a situation where multiple equivalent servers are available, but some are perhaps down, and others heavily loaded and slow, so my goal would be to get a response from the first one to succeed, and ignore the rest (yes, I know this is an obnoxious way for a client to behave from a server's perspective, but it's great for the end user ;)

However, as far as I can see, I have either "all" or "race" behaviors to play with. The "all" behavior seems to wait until all the requests have completed, which means that I have to wait for the slowest, even if a server has already completed (indeed, I might have to wait for a timeout, with would be a disaster for this scenario.) The "race" behavior, however, seems to give me the first to complete, which if that happens to be a failure, is also a disaster.

Is there something in the API that permits a "raceToSuccess" kind of behavior, or do I have to build it by hand. For that matter, how would I build it by hand?

As a side note, I found the same puzzle in the Java 8 CompletableFuture, which seems to be a closely parallel API. So, am I missing something at a philosophical level?

Answer 1

This is a classic example where inverting your logic makes it much clearer. Your "race" in this case is that you want your rejection behavior to in fact be success behavior.

function oneSuccess(promises){
  return Promise.all(promises.map(p => {
    // If a request fails, count that as a resolution so it will keep
    // waiting for other possible successes. If a request succeeds,
    // treat it as a rejection so Promise.all immediately bails out.
    return p.then(
      val => Promise.reject(val),
      err => Promise.resolve(err)
    );
  })).then(
    // If '.all' resolved, we've just got an array of errors.
    errors => Promise.reject(errors),
    // If '.all' rejected, we've got the result we wanted.
    val => Promise.resolve(val)
  );
}