...
A single registry instance could be an attribute of a process group (for processors) and controller (for reporting tasks, controller services). Being an entity at these levels could help support communication of variable registry configuration within a cluster (via flow.xml) but and also allow the registry to leverage multi-tenancy functionality for access (see Authorization). Each process group would contain a registry which can override a parent process group’s registry; therefore a variable referred by a component can be resolved by searching within the registry of that component’s process group. If needed the search could continue up the hierarchy to the root process group’s variable registry to discover referenced variables.
The variable registry at the root level process group could be preloaded with system variables such that they can maintain their global availability while custom variables can be configured within the root group or a child/descendant process group.
...