Add event listeners to the REST endpoint.
These should use HTTP Server-Sent-Events
https://html.spec.whatwg.org/multipage/server-sent-events.html

Cache listeners:

GET /v2/caches/cacheName?action=listen[&includeCurrentState=true|false][&filterFactoryName=...][&converterFactoryName=...][&includeMetadata=true|false]

This will result in a response with content-type text/event-stream followed by a stream of cache events.
Each event follows the following structure:

event: [created|modified|removed|expired]
data: [json representantion of event]

the data field will contain a JSON representation of the event:

{ 
  key: {},
  value: {},
  metadata: {}
}