protected OperatorSpec<M, JM> getOperatorSpec() {
    return (OperatorSpec<M, JM>) joinOpSpec;
  }
Implementation of one side of a {@link JoinOperatorSpec} that buffers and joins its input messages of
type {@code M} with buffered input messages of type {@code OM} in the paired {@link PartialJoinOperatorImpl}.

@param <K> the type of join key
@param <M> the type of input messages on this side of the join
@param <OM> the type of input message on the other side of the join
@param <JM> the type of join result
samza-core/src/main/java/org/apache/samza/operators/impl/PartialJoinOperatorImpl.java