vice placement and identification of existing controllers that meet the service request parameters and have the required capacity. If existing controllers (infrastructure, network, or application) do not exist or do not have capacity, the MSO will obtain a recipe for instantiation of a new controller under which the requested service can be placed. SDC is the module of ONAP where orchestration process flows are defined. These process flows will start with a template that may include common functions such as homing determina- tion, selection of infrastructure, network and application controllers, consultation of policies, and interrogation of A&AI to obtain necessary information to guide the process flows. The MSO does not provide any process-based functionality without a recipe for the requested activity regardless of whether that request is a customer order or a service adjustment/configuration update to an existing service. MSO will interrogate A&AI to obtain information regarding existing network and application controllers to support a service request. A&AI will provide the addresses of candidate controllers that are able to support the service request. The MSO may then interrogate the controller to validate its continued available capacity. The MSO and the controllers report reference information back to A&AI upon completion of a service request to be used in subsequent operations. 7.3 SDC ENVIRONMENT 7.3.1 METADATA-DRIVEN DESIGN TIME AND RUNTIME EXECUTION Metadata is generally described as "data about data" and is a critical architecture concept deal- ing with both abstraction and methodology. Metadata expresses structural and operational aspects of the virtualized elements comprising products, services, and resources as expressed in terms of logical objects within a formally defined model space. The attributes of these objects, and the relationships among them, embody semantics that correspond to real-world aspects of the modeled elements. The modeling process abstracts common features and internal behavio