public Task StartFlowAsync()
{
var sourceCancellationUnit = new JobCancellationUnit();
// tap - start scrolling - out = es docs
var sourceStream = _tap.StartFlowingToEnd(sourceCancellationUnit);
// batcher/transformer - start batching - in = es docs, out = es doc batches
var batchStream = _transformer.StartTransforming(sourceCancellationUnit, sourceStream);
// TODO: throttler
// sink - start indexing - in = es doc batches
var destinationCancellationUnit = new JobCancellationUnit();
return(_sink.StartDrainingAsync(destinationCancellationUnit, batchStream));
}