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Top1. Introduction
According to (Mell & Grance, 2011) cloud computing is a model for enabling ubiquitous, convenient, on-demand network access to a shared pool of configurable computing resources (e.g., networks, servers, storage, applications, and services) that can be rapidly provisioned and released with minimal management effort or service provider interaction.
This cloud model is composed of five essential characteristics, three service models, and four deployment models. At this time of its expansion, everybody should be familiar with the benefits and risks cloud computing bears. Before this paper analyses the two Greek adoption case studies, a short introduction to the cloud is included, for the sake of completeness. Briefly, according to (Hassan, 2011) these attributes characterize cloud computing:
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On-demand Computing Model: Organizations are able to escape from complex and expensive in-house infrastructure and choose the amount of resources they require for their operation;
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Autonomous: Clients are separated from the technical details of the cloud services they use;
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Predefined Quality of Service: Cloud providers state QoS terms in their service level agreements to inform clients about expected level of service;
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Internet-based: All cloud services are hosted beyond organizations and delivered over the Internet;
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Easy-to-use: Cloud providers offer easy-to-use interfaces that enable clients to make use of their services;
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Scalable: Clients are not limited with fixed amounts of resources. They can scale up and down at free will;
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Inexpensive: Cloud computing offers small-and-medium-sized enterprises (SMEs) a significantly lower-cost option than building an in-house infrastructure;
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Subscription-based Model: Clients subscribe to services they are interested in, and they are charged accordingly.
The architecture of cloud computing is pyramid shaped, starting with IaaS as a foundation and on top, SaaS (Varia, 2010). The main logic behind the pyramid shape, is that on the road to the top, the user is not required to know in detail how things work in the cloud:
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Infrastructure-as-a-Service (IaaS): IaaS provides hardware such as CPUs, memory, storage, networks, and load-balancers. The next architectures are based on IaaS in order to work;
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Platform-as-a-Service (PaaS): Supplies users with development and administration platforms that provide on-demand access to available hardware resources. Many PaaS platforms are available to enable access to IaaS resources;
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Data-as-a-Service (DaaS): Frees organizations from buying high-cost database engines and mass storage. This service offers database capabilities for storing client information;
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Software-as-a-Service (SaaS): The ultimate form of cloud resources that delivers software applications to clients in terms of accessible services. With SaaS, clients subscribe to applications offered by providers rather than building or buying them.
If the first pillar of this paper is the cloud computing, the second is the enterprises. The cloud services offer great amount of options, so every organization can enjoy the aspects of cloud it needs.