Option #3 (based on discussion with terence)

1) No new user level apps are deployed. Config store is used to store user drafts of hydrator apps.

2) REST endpoint 'configure', can accept partial config and return a config response with suggestions of values for fields in a plugin, exceptions if any during configuring the plugin. 

#3 

Story 1 - Schema and field value suggestions : 

Plugin annotation @Endpoint: 

REST API :

Schema Propagation and Validation through backend - DryRuns:

Story 2 - Drafts

The ConsoleSettingsHttpHandler currently makes use of ConfigStore. It's however not name-spaced and has few other issues, it can be fixed and can be improved to store configs.

Along with pipeline drafts ConsoleSettingsHttpHandler also stores the following information currently:

GET namespaces/{namespace-id}/plugin-templates/{plugin-template-id}/ 
// create a new plugin template
POST namespaces/{namespace-id}/plugin-templates/{plugin-template-id}/ -d '@plugin-template.json' 
// update existing plugin template
PUT namespaces/{namespace-id}/plugin-templates/{plugin-template-id}/ -d '@plugin-template.json'
// delete the plugin template
DELETE namespaces/{namespace-id}/plugin-templates/{plugin-template-id}/ 

 // create/update defaults this include user's plugin version preferences, etc.
 PUT : namespaces/{namespace-id}/defaults -d '@default.json' 
 GET : namespaces/{namespace-id}/defaults 

JAVA API - Config Store:

void create(String namespace, String type, Config config) throws ConfigExistsException;

void createOrUpdate(String namespace, String type, Config config);

void delete(String namespace, String type, String id) throws ConfigNotFoundException;

List<Config> list(String namespace, String type);

Config get(String namespace, String type, String id) throws ConfigNotFoundException; 

void update(String namespace, String type, Config config) throws ConfigNotFoundException;

// get a particular version of an entry. 
Config get(String namespace, String type, String id, int version) throws ConfigNotFoundException; 
// get all the versions of an entry.
Config getAllVersions(String namespace, String type, String id) throws ConfigNotFoundException; 
// delete all entries of specified type.
void delete(String namespace, String type) 

Open Questions :

1) ConfigStore stores the configs in "config.store.table", currently the table properties doesn't have versioning, drafts would need versioning, this would also need CDAP-upgrade to update properties for the existing dataset? 

2) rename ConsoleSettingsHttpHandler to ConfigurationsHttpHanlder ?

3) Dependent UI changes. 

User Stories (3.5.0)

1. For the hydrator use case, the backend app should be able to support hydrator related functionalities listed below:
2. query for plugins available for a certain artifacts and list them in UI
3. obtaining output schema of plugins provided the input configuration information
4. deploying pipeline and start/stop the pipeline
5. query the status of a pipeline run and current status of execution if there are multiple stages.
6. get the next schedule of run, ability to query metrics and logs for the pipeline runs.
7. creating and saving pipeline drafts
8. get the input/output streams/datasets of the pipeline run and list them in UI. 
9. explore the data of streams/datasets used in the pipeline if they are explorable. 
10. Add new metadata about a pipeline and retrieve metadata by pipeline run,etc.
11. delete hydrator pipeline
12. the backend app's functionalities should be limited to hydrator and it shouldn't be like a proxy for CDAP.  

Having this abilities will remove the logic in CDAP-UI to make appropriate CDAP REST calls, this encapsulation will simplify UI's interaction with the back-end and also help in debugging potential issues faster. In future, we could have more apps similar to hydrator app so our back-end app should define and implement generic cases that can be used across these apps and it should also allow extensibility to support adding new features. 

Generic Endpoints