public Set<OperatorSpec> getJoins() {
      return Collections.unmodifiableSet(joinOpSpecs);
    }
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