Streams that produce elements with an associated ordering.
Say you have a bunch of events that all have a timestamp, sequence number, or other ordering
attribute. If you get these events from multiple
Streams, then you
should be able to produce a "composite" stream by joining each of the individual streams, so
long as each originating stream is ordered.
However, if you actually implement this, you discover that you need to buffer at least one element from each stream in order to avoid ordering inversions if the sources are independent (including just running in different tasks). This presents a problem if one of the sources rarely produces events: that slow source can stall all other streams in order to handle the case where the slowness is due to an earlier element instead of just having no elements.
OrderedStream] trait provides a way to solve this problem: if you can ask a stream if it
will ever have any events that should be delivered before a given event, then you can often
avoid blocking the composite stream when data is ready.