The golang implementation is available in go-lumber. Feel free to follow/comment on this ticket. A Java (netty based) implementation is hidden in the logstash-input-beats repository: https://github.com/logstash-plugins/logstash-input-beats/tree/master/src/main/java/org/logstash/beats .
There is no publish protocol definition for v2. It's basically lumberjack with 'J' data-frame for JSON encoded events + some minor behaviour changes.
Some related post on NiFi implementation, giving some more details about ACK behaviour: Testers wanted: *beat integration with Apache NiFi