Hi @Seongmin Park
The "NiFi Flow" itself is also a process group, see on the left hand side when you log in, note the Process group ID that belongs to my NiFi Flow. I will also set a var on the NiFi Flow and retrieve it with REST:
Setting a random var on it;
And getting the variable data of the NiFi Flow over REST:
[root@c3175-node4 ~]# curl -i -X GET http://172.25.35.14:9090/nifi-api/process-groups/dadadbcd-0164-1000-16c1-6c50ce94361e/variable-regis...
HTTP/1.1 200 OK
Date: Thu, 02 Aug 2018 09:34:52 GMT
Content-Type: application/json
Content-Length: 362
{"processGroupRevision":{"clientId":"f9e30047-0164-1000-e3ba-51b6ec77ae6f","version":3},"variableRegistry":{"variables":[{"variable":{"name":"FlowVarExample","value":"This is a flow variable example : - )","processGroupId":"dadadbcd-0164-1000-16c1-6c50ce94361e","affectedComponents":[]},"canWrite":true}],"processGroupId":"dadadbcd-0164-1000-16c1-6c50ce94361e"}}