Support receiving cloud events using HTTP endpoint

[ctron]

I think it makes sense for Ditto to support receiving Ditto Protocol JSON messages also via Cloud Events.

While cloud events provide bindings for Kafka, MQTT, … they also have an HTTP endpoint binding, which can easily be used in the combination with Knative. So it would be possible, to easily map incoming messages from any Knative eventing source, to Ditto, acting as a Knative eventing sink.

The payload of these events would be expected to be "Ditto Protocol payload (JSON)".

I would be volunteering to work on this.

[thjaeckle]

Cool 👍
As discussed in Gitter.IM we would appreciate such a contribution.
Implementation hints:

• HTTP handling in Ditto is done with Akka HTTP
• the Ditto service responsible for terminating HTTP requests is the gateway service, see also the architecture
• RootRoute.buildApiSubRoutes(RequestContext, DittoHeaders) would be the place to add a new top-level route below /api/2/
• the "ditto-protocol-adapter" module must be used in order to parse Ditto Protocol JSON messages to Signals which have to be forwarded in the cluster
  • https://search.maven.org/artifact/org.eclipse.ditto/ditto-protocol-adapter/1.4.0/bundle
  • Example use:

private Signal<?> jsonToDittoSignal(final String jsonString) {
   final JsonObject jsonObject = JsonObject.of(jsonString);
   final JsonifiableAdaptable jsonifiableAdaptable = ProtocolFactory.jsonifiableAdaptableFromJson(jsonObject);
   final DittoProtocolAdapter dittoProtocolAdapter = DittoProtocolAdapter.newInstance();
   return dittoProtocolAdapter.fromAdaptable(jsonifiableAdaptable);
}

• The created Signal has to be passed into a handlePerRequest variant of org.eclipse.ditto.services.gateway.endpoints.routes.AbstractRoute

Regarding naming and verb of the new endpoint I would suggest:

• using POST as verb
• regarding the "noun" I am not yet sure ..
  • /events for example would not fit IMO as Ditto Protocol JSON messages are not always events, but also commands
  • /protocolMessages could be an option, however I would like to prevent using camel case or a dash in the endpoint
  • maybe /signals as all Ditto Protocol JSON messages represent Signals: https://www.eclipse.org/ditto/basic-signals.html

[thjaeckle]

I would close this as the basic functionality is available via #895