void validate() {
    validateInputStreams();
    validateOutputStreams();
    validateInternalStreams();
    validateReachability();
  }
Validate the graph has the correct topology, meaning the input streams are coming from external streams,
output streams are going to external streams, and the nodes are connected with intermediate streams.
Also validate all the nodes are reachable from the input streams.
samza-core/src/main/java/org/apache/samza/execution/JobGraph.java