Push Data to external Source after a ResourceUpdateMessage

[ghost]

As previousely discussed with @juliapampus. First draft below.

Problem: Connector A requested a resource + artifact from Connector B. Connector B provides an updated dataset and sends a ResourceUpdateMessage to Connector A. Connector A automatically updates the resource internally. However, Connector A can not inform other external systems which are processing the data that a new dataset for the resource is available. Currently, external systems would have to request the data from Connector A manually (e.g. schedule, on-use).

Proposed Solution: Provide the possibility for external services to "subscribe" to updates of specific resources, so that the connector can itself inform these systems when new data is available. Possible integration:

• Expose a new REST-Endpoint for resource subscription (like PUT on /subscribe/{resourceID} with required parameter uri, maybe other CRUD operations for maintenance)
• Integration of a new service (e.g. ResourceForwardingService) inside the Connector
  • The service keeps track of the "subscriptions", e.g. in form List<resourceID -> List< URL >> with all "subscribed" URLs listed for each resource
  • The service is called in the ResourceUpdateMessageHandler message processing
  • On call, the service checks if the updated resource is present and sends a http POST to each specified endpoint with an empty body
  • Additional logic must be added in the subscribing endpoint: Data retrieving can be triggered here.

[ghost]

Further discussion led to the following updates:

• a new controller for subscribing to resources is needed
• persistence of the subscription in DB instead of in-memory
• naming: SubscriberNotificationService
• put ResourceID in POST RequestBody of the provided backend URL to allow processing
• Update is send after the resource has been updated internally, so that external services fetch the newest version of data