Goals

JIRA: CDAP-4075: Error handling for Workflows.

Use Cases

1. When Workflow run finishes, user may want to send an email about its success or failure.
2. In case of hydrator pipeline, once the run is finish, user may wish to delete the data from the external source such as Oracle/Teradata etc.
3. If the Workflow fails for some reason, user may want to cleanup the files/data written by the nodes in the Workflow.
4. On failure of the Workflow, user may wish to keep certain local datasets for further debugging.
5. In a Workflow, user can have a custom action at the start of the workflow that writes to a dataset (which acts as a lock). Next node in the Workflow is a MapReduce program that fails for that run of the Workflow. User would like to be able to clean up the state that custom action wrote to dataset 

User Stories

1. As a developer of Workflow action, I want an ability to clean up the data that was written by Workflow action in case of Workflow action failure.
2. As a developer of Workflow action, I want an ability to clean up the data that was written by Workflow action in case Workflow fails.
   
3. As a developer of the Workflow, I want an ability to send an email once the Workflow run finishes. In case of failure, I should be able to access the nodes that failed and the failure cause.
4. As a developer of the Workflow, I want an ability to instruct the Workflow system, not to delete the certain local datasets for triage purpose.

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