Enhance all Feature events with the feature definition

[thjaeckle]

We could simply add the Feature "definition" to all Feature events. That way, the definition would not have to be "enriched" (e.g. as also requested in issue #710) but would always be available (similar to the Feature's ID).

We could also add a Placeholder {{ feature:definition }} in order to e.g. add the definition in a header mapping (e.g. putting the definition as HTTP header for HTTP push connections).

[thjaeckle]

Implementation suggestions:

• add new interface WithFeatureDefinition which all FeatureEvents implement
• add definition field to all FeatureEvents (also add it to the serialized JSON)
• for DittoProtocol JSON, the definition would have to find its place e.g. in the headers, e.g.:

  {
  "topic": "com.acme/xdk_53/things/twin/events/modified",
  "headers": {
  "feature-definition": [
    "org.eclipse.vorto:Foobar:1.0.0"
  ]
  },
  "path": "/features/accelerometer/properties/x",
  "value": 42,
  "revision": 1
  }

• another option would be to add it "toplevel" to the DittoProtocol JSON, like:

  {
  "topic": "com.acme/xdk_53/things/twin/events/modified",
  "path": "/features/accelerometer/properties/x",
  "value": 42,
  "definition": [
  "org.eclipse.vorto:Foobar:1.0.0"
  ],
  "revision": 1
  }

[jokraehe]

I think the top-level definition in ditto-protocol would make sense, because we also could support the definition of a thing with this approach. We could name the interface WithDefinition then we can reuse it for thing events.

[thjaeckle]

Ok, but then the definition depends on the changed "path" which could be confusing. We could also add toplevel "definition" and optionally "feature-definition" (when a feature was affected).

[jokraehe]

Yes, you are right. The top-level definition is not unambiguous for feature related events. Maybe we could add something like "definitions": { "thing": ["org.eclipse.vorto:Foobar:1.0.0"], "feature": ["org.eclipse.vorto:Baz:1.0.0"] }

[thjaeckle]

that leads me to: what if an event contained updates for several features - then the JSON should probably look like:

...
"definitions": { 
  "thing": "org.eclipse.vorto:Foobar:1.0.0", 
  "features": {
    "featureA": ["org.eclipse.vorto:Baz:1.0.0"],
    "featureB": ["org.eclipse.vorto:BazBaz:1.0.0"] 
  }
}
...

[thjaeckle]

Closing in favor of #710