I have heard about variable registry, but I am not sure where to find it or access it. Please can someone help?
in Ambari -> NiFi -> configs -> Advanced nifi-properties, look for the 'nifi.variable.registry.properties' parameter.
nifi.variable.registry.properties can be set to the path of a simple text file (ensure the nifi user has rights to read it), which should be present in that same location on each of the NiFi nodes. After setting this value, each NiFi host needs a restart. The text file you're pointing to should a simple key/value entry for each line - and make sure there are no trailing whitespaces on these lines. For example;
[root@node2 ~]# cat /var/tmp/MyNiFiVariableRegistryFile somevariable=this-is-a-test anothervariable=this-is-another-test TESTPATH=/tmp/archive
You might be interested in the available documentation on using custom properties (https://nifi.apache.org/docs/nifi-docs/html/user-guide.html#Using_Custom_Properties) via:
Note that NiFi automatically picks up new or modified variables created in the UI (no restart required).