## 1 Introduction
### Definitions
#### 1.2.3 Cloud Computing  
Cloud Computing (CC) has a disruptive business approach that enables the customers to pay for resources that they actually use and need instead of embarking on a sizable initial investment in hardware and specialized human resources to install it. Technology giants such as Google, Amazon, or Microsoft build and manage their own cloud infrastructure and provide resources to third-party companies with a usage-based pricing model. In this kind of ecosystem, there can be intermediary service providers that provide their services, renting resources to the cloud infrastructure providers. With this in consideration, CC creates new opportunities for companies, especially for small and medium-sized enterprises by offering higher performance at a lower cost, high scalability, availability, and accessibility, reducing business risks and big upfront investments and maintenance expenses [13].  
As a base foundation for the implementation of a CC system, an architecture is necessary as a guide. Usually, it is based on a four-layer structure, namely: a hardware/data center layer, infrastructure layer, platform layer, and application layer [14].  
According to Figure 2, there are three standard service models: Infrastructure as a Service (IaaS), Platform as a Service (PaaS), and Software as a Service (SaaS). According to [PERSON] [13], these services are suitable for IT professionals (IaaS), developers (PassS), and business end-users (SaaS). Access to these services is provided through standard interfaces namely WebServices, Service-Oriented Architecture (SOA), or Representational State Transfer (REST) services.  
Different service levels require different knowledge and effort from different stakeholders. Figure 3 depicts the several layers and typical interventions between the user and service provider.  
On the left side of Figure 3, the user manages all activities. The responsibility in managing systems decreases when using other service levels. In the IaaS paradigm, the user manages the five top layers. In PaaS, the user is only responsible for the two top layers, and when using Saas, the user has no intervention in any of these layers, as it is the responsibility of the service provider.  
CC can have several models, depending on the requirements of the end-users: they can be public, private, community, or hybrid [15]. The restrictions of access will be tightly aligned with the business process, type of information, and level of vision desired. A public cloud platform, as the name suggests, is used when manufacturing resources are shared with the general public. Companies and partners can publish or request data and services from the cloud platform. This kind of cloud is open to all parties but does not expose all the information. An example can be an email service such as Gmail from Alphabet, which is freely accessible, but each customer can only use or access their quote upon login. A private cloud is built to be accessible only from a single organization, to share resources among different subsidiaries of the enterprise. Although typically more expensive than the public clouds, they are more secure and manage the privacy of the organization more efficiently. The hybrid cloud is a combination of public and private clouds merging the best features from both approaches. The community cloud is a deployment model that is used for sharing resources between several organizations. As an example, cooperation among universities on a research topic can be supported by a community cloud [13; 16].  
Figure 3: Service levels of cloud services (adapted from [13]).  
Figure 2: CC architecture (based on [14]).