public boolean isKeyed() {
    return isKeyed;
  }
An intermediate stream is both an input and an output stream (e.g. a repartitioned stream).
<p>
This implementation accepts a pair of {@link InputOperatorSpec} and {@link OutputStreamImpl} associated
with the same logical {@code streamId}. It provides access to its {@link OutputStreamImpl} for
the partitionBy operator to send messages out to. It's also a {@link MessageStreamImpl} with
{@link InputOperatorSpec} as its operator spec, so that further operations can be chained on the
{@link InputOperatorSpec}.

@param <M> the type of message in the output {@link MessageStreamImpl}
samza-core/src/main/java/org/apache/samza/operators/stream/IntermediateMessageStreamImpl.java