I. Introduction
According to NIST [1], Cloud computing is a model for enabling convenient, on-demand network access to a shared pool of configurable computing resources as services. The cloud architecture is usually composed of several XaaS layers - including Software as a Service (SaaS, upper layer), Platform as a Service (PaaS, middle layer) and Infrastructure as a Service (IaaS, lower layer) - organized in a dependent way, i.e. the upper layer depends on the lower layer resources. Actors of the Cloud include end users that are using applications, software developers that are creating applications (using the SaaS or PaaS layers) and infrastructure administrators managing physical and virtual machines.