## 2 Smart5 Grid NetApp Concept  
One of the Smart5 Grid's main aims is to provide a friendly environment that, abstracting the complexity of the underlying 5G network, facilitates the development of applications for the smart grid. Here is where the NetApp concept comes into play, conceived as a vertical application, composed of a chain of cloud-native virtual network functions (VNFs), able to leverage 5G and edge infrastructure by formally specifying its deployment and performance requirements in its so-called NetApp descriptor.  
The split of the functionality of the NetApp into decoupled VNFs is not a new concept. In fact, the European Telecommunications Standards Institute's (ETSI) network function virtualization (NFV) framework [27] describes the reference architecture, information models, and tools required to manage this kind of application. However, if advanced networking is needed when developing an end-to-end application to run on a 5G network, this framework requires telecommunications expertise from the developers, which usually exceeds their skillset. The Smart5 Grid NetApp concept intends to provide a solution to this problem by abstracting the complexities of network configuration from the developers. The Smart5 Grid NetApp contains the necessary components to offer the vertical service. It is a standalone application, but it may be also integrated with other external or legacy applications, as shown in Figure 1.  
Figure 1: Basic NetApp representation.  
The NetApp components can be deployed as container-based VNFs. By splitting these components whenever possible in the implementation phase, the NetApp brings the opportunity to take advantage of the cloud/edge infrastructure when deployed. Figure 2 shows an example of a NetApp comprised of two components. If there is a VNF that requires low latency, this could be placed at the edge of the computing infrastructure; the other, with less stringent requirements, in a cloud datacenter where resources are less constrained.  
Each NetApp is formally defined in a NetApp descriptor, which includes the necessary information regarding the vertical service that comprises it, its topology, as well as the performance requirements of each component so that the infrastructure over which it is instantiated can perform its intended functions, such as multiple-access edge computing (MEC) offloading, VNF scaling, or traffic policy enforcement via its management and orchestration (M&O) system. This information allows the M&O system to create end-to-end slices that fulfill these requirements, allowing developers to design applications with strict performance demands without having the expertise to implement the networks that support them. These concepts will be further explained in Sections 3 and 4.  
With the aim to leverage, as much as possible, the existing state of the art and standards, Smart5 Grid proposes to adopt ETSI-aligned descriptors and incorporate them into Smart5 Grid NetApps in such a way that existing VNFs can be reused. This, however, may be difficult to bring into reality, as multiple implementations in the orchestration layer are available in the different UC implementations within the project; even ETSI initiatives, such as open-source management and orchestration (OSM), include proprietary definitions of certain aspects, especially in the case of containerized workloads, which are still under specification. For this reason, we propose to use a hybrid solution where a placeholder for ETSI standard descriptors exists, while allowing existing specific implementations to work in such deployments. Figure 3 graphically illustrates the first version of the Smart5 Grid information model (IM) in relation to the existing ETSI NFV one.  
Figure 2: NetApp deployment over a 5G network.