@Override
  public Set<String> getOutputStreamIds() {
    return Collections.unmodifiableSet(new HashSet<>(outputStreams.keySet()));
  }
The 0-based position of the next operator in the graph.
Part of the unique ID for each OperatorSpec in the graph.
Should only accessed and incremented via {@link #getNextOpId(OpCode, String)}.
samza-core/src/main/java/org/apache/samza/application/descriptors/StreamApplicationDescriptorImpl.java