Controller Services are shared services that can be used by reporting tasks, processors, and other services to utilize for configuration or task execution.
Note: Controller Services defined on the controller level are limited to reporting tasks and other services defined there. Controller Services for use by processors in your dataflow must be defined in the configuration of the root process group or sub-process group(s) where they will be used.
Note: If your FlexFiles instance is secured, your ability to view and add Controller Services is dependent on the privileges assigned to you. If you do not have access to one or more Controller Services, you are not able to see or access it in the UI. Access privileges can be assigned on a global or Controller Service-specific basis (see Accessing the UI with Multi-Tenant Authorization for more information).
The following documentation from NiFi covers specific use cases for Controller Services: