A good first section for a simple producer is the JAX-RS endpoint that receives the ACK from the device that the delivered push-notification was used to launch, or bring up, the mobile app, on the physical device:

https://github.com/aerogear/aerogear-unifiedpush-server/blob/GSOC_2017_kafka/jaxrs/src/main/java/org/jboss/aerogear/unifiedpush/rest/registry/installations/InstallationRegistrationEndpoint.java#L230

Idea here is: in addition: write the data to a kafka topic