Seite 51 - Cloud Migration Version 2012 english

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"
Public Cloud", this categorisation has been extended to other deployment 
models besides these original forms, such as “Hybrid Cloud” or even more 
specialised forms like “Community Cloud”. The typical distinguishing 
features of these types are: 
Public Cloud 
"
The Cloud" is often used as a synonym of "Public Cloud". According to the 
NIST definition:  
The Cloud infrastructure is provisioned for open use by the general public. 
It may be owned, managed, and operated by a business, academic, or 
government organisation, or some combination of them. It exists on the 
premises of the Cloud provider. “
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This definition implies that Public Clouds are disparate resources by nature, 
and give rise to questions of security, law and availability, which we will be 
looking at in detail later on in this book. It is important to pay attention to 
this, because all the disadvantages or benefits of this deployment model are 
often implicitly assumed, therefore preventing an objective discussion of the 
proposed solution. 
In a Public Cloud an entity’s data is virtually segmented from another 
entity’s data while sharing a common IT infrastructure.  
Public Cloud can provide a number of services (business processes, 
workflows, IT applications and/or IT infrastructures) on a variable 
pay‐per‐use basis that are basically open to anybody at the same time 
(
multi‐tenant capable) over public networks. Cloud users share the 
underlying infrastructure, which is abstracted from the application layer 
(
and are either unaware of this fact, or of the scope). In an ideal Cloud‐only 
model none of the physical IT resources and infrastructures that support a 
specific service are tangible to the user, and all of the relevant applications 
reside outside of the enterprise firewall. One of the major distinguishing 
features of a Public Cloud is that the user cannot influence who uses the 
services offered by the provider (from a technology, or from a contractual 
point of view). The options for customising services to meet specific user 
requirements are typically very limited, as they conflict with the provider's 
scaling and efficiency interests. There is typically no localisation of 
resources, or no localisation option, or it is not part of the service 
agreement.  The owner and operator of a Public Cloud service is typically 
called an IT service provider.  
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