NiFi has a bunch of Rest API’s that you can use. They are located here.
They are very comprehensive. The only thing that I would say is missing is getting the root process group of NiFi. It is not documented what the api call would be. All api calls must be authenticated as well.
The api call to get the root process group called “NiFi Flow” which is the main process group is.
https://lcoalhost/nifi-api/process-groups/root