Visitor API for DAG

[Tushar Gosavi]

Hi All,

How about adding visitor like API for DAG in Apex, and an api to
register visitor for the DAG.
Possible use cases are
-  Validator visitor which could validate the dag
-  Visitor to inject properties/attribute in the operator/streams from
some external sources.
-  Platform does not support validation of individual operators.
developer could write a validator visitor which would call validate
function of operator if it implements Validator interface.
- generate output schema based on operator config and input schema,
and set the schema on output stream.

Sample API :

dag.registerVisitor(DAGVisitor visitor);

Call order of visitorFunctions.
- preVisitDAG(Attributes) // dag attributes
  for all operators
  - visitOperator(OperatorMeta meta) // access to operator, name,
attributes, properties
 ports
  - visitStream(StreamMeta meta) // access to
stream/name/attributes/properties/ports
- postVisitDAG()

Regards,
-Tushar.