[jira] [Created] (MESOS-1160) Support flattening from {Try,Result} into Future.

["Benjamin Mahler (JIRA)" <ji...@apache.org>]

Benjamin Mahler created MESOS-1160:
--------------------------------------

             Summary: Support flattening from {Try,Result} into Future.
                 Key: MESOS-1160
                 URL: https://issues.apache.org/jira/browse/MESOS-1160
             Project: Mesos
          Issue Type: Improvement
          Components: libprocess
            Reporter: Benjamin Mahler


We should consider adding support for constructing a Future<T> from a Try<T>, the flattening is rather straightforward:
* Try is SOME, then Future is READY.
* Try is ERROR, then Future is FAILED.

Ideally we should consider consolidating terminology (Error vs. Failed / Failure) as well, but the flattening in itself is beneficial.

Consider the following code:

{code}
void Slave::checkDiskUsage()
{
  // TODO(vinod): We are making usage a Future, so that we can plug in
  // fs::usage() into async.
  Future<Try<double> >(fs::usage(flags.work_dir))
    .onAny(defer(self(), &Slave::_checkDiskUsage, lambda::_1));
}


void Slave::_checkDiskUsage(const Future<Try<double> >& usage)
{
  if (!usage.isReady()) {
    LOG(ERROR) << "Failed to get disk usage: "
               << (usage.isFailed() ? usage.failure() : "future discarded");
  } else {
    Try<double> result = usage.get();

    if (result.isSome()) {
      double use = result.get();

      LOG(INFO) << "Current usage " << std::setiosflags(std::ios::fixed)
                << std::setprecision(2) << 100 * use << "%."
                << " Max allowed age: " << age(use);

      // We prune all directories whose deletion time is within
      // the next 'gc_delay - age'. Since a directory is always
      // scheduled for deletion 'gc_delay' into the future, only directories
      // that are at least 'age' old are deleted.
      gc.prune(flags.gc_delay - age(use));
    } else {
      LOG(WARNING) << "Unable to get disk usage: " << result.error();
    }
  }
  delay(flags.disk_watch_interval, self(), &Slave::checkDiskUsage);
}
{code}

With flattening this code becomes:
{code}
void Slave::checkDiskUsage()
{
  // TODO(vinod): We are making usage a Future, so that we can plug in
  // fs::usage() into async.
  Future<double>(fs::usage(flags.work_dir))
    .onAny(defer(self(), &Slave::_checkDiskUsage, lambda::_1));
}


void Slave::_checkDiskUsage(const Future<double>& usage)
{
  if (!usage.isReady()) {
    LOG(WARNING) << "Failed to get disk usage: "
               << (usage.isFailed() ? usage.failure() : "future discarded");
  } else {
    LOG(INFO) << "Current usage " << std::setiosflags(std::ios::fixed)
              << std::setprecision(2) << 100 * usage.get() << "%."
              << " Max allowed age: " << age(usage.get());

    // We prune all directories whose deletion time is within
    // the next 'gc_delay - age'. Since a directory is always
    // scheduled for deletion 'gc_delay' into the future, only directories
    // that are at least 'age' old are deleted.
    gc.prune(flags.gc_delay - age(usage.get()));
  }
  delay(flags.disk_watch_interval, self(), &Slave::checkDiskUsage);
}
{code}



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