@Override
    public void accept(OperatorSpec opSpec) {
      if (opSpec instanceof JoinOperatorSpec || opSpec instanceof StreamTableJoinOperatorSpec) {
        joinOpSpecs.add(opSpec);
      }
    }
An {@link OperatorSpec} visitor that records all {@link JoinOperatorSpec}s and {@link StreamTableJoinOperatorSpec}s
encountered in the graph.
samza-core/src/main/java/org/apache/samza/execution/OperatorSpecGraphAnalyzer.java