Hi,
As @tmetzke says, I’m using Kafka as external queue to process all external tasks, or user tasks events.
I created a plugin for Camunda and when receive a new external task event, I puliblish into a Kafka topic. Other systems are subscribed to this topic an process the business logic for this external tasks.
Other way to interact with external tasks is use the External Task Client for Java (or Javascript) https://github.com/camunda/camunda-external-task-client-java.
In this case is a pooling system instead a reactive, but may fit for your solution.