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example, notes that Private Clouds lack “the freedom from capital investment and
the virtually unlimited flexibility of cloud computing.”
4.5.2 Hybrid Clouds and Federations of Clouds
Single Clouds can be combined resulting in multiple-Cloud environments.
Contingent on which types of Clouds (public or private) are combined, two types of
multiple-Cloud environments can be distinguished:
• Hybrid Clouds and
• Federation of Clouds.
Hybrid Clouds
combine Public and Private Clouds and allow an organization
to both run some applications on an internal Cloud infrastructure and others in
a Public Cloud (Sun 2009a). This way, companies can benefit from scalable IT
resources offered by external Cloud providers while keeping specific applications
or data inside the firewall. A mixed Cloud environment adds complexity regarding
the distribution of applications across different environments, monitoring of the
internal and external infrastructure involved, security and privacy, and may there-
fore not be suited for applications requiring complex databases or synchronization
(Sun 2009a).
The terms
Federated Clouds
or
Federation of Clouds
denote collaboration
among mainly Public Clouds even though Private Clouds may be involved. Cloud
infrastructure providers are supposed to provide massively scalable computing
resources. This allows users and Cloud SaaS providers not to worry about the
computational infrastructure required to run their services. The Cloud infrastructure
providers, however, may face a scalability problem themselves. A single hosting
company may not be able to provide seemingly infinite computing infrastructure,
which is required to serve increasing numbers of applications, each with massive
amounts of users and access at anytime from anywhere. Consequently, Cloud infra-
structure providers may eventually partner to be able to truly serve the needs of
Cloud service providers, i.e. providing seemingly infinite compute utility. Thus,
the
Cloud might become a federation of infrastructure providers or alternatively there
might be a federation of clouds (RESERVOIR 2008).
Federated Clouds are a collection of single Clouds that can interoperate, i.e.
exchange data and computing resources through defined interfaces. According
to basic federation principles, in a Federation of Clouds each single Cloud
remains independent, but can interoperate with other Clouds in the federation
through standardized interfaces. At present, a Federation of Clouds seems still to
be a theoretical concept as there is no common Cloud interoperability standard.
One new initiative that tries to develop a common standard is the Open Cloud
Computing Interface, which is developed by the Open Cloud Computing Interface
Working Group (http://www.occi-wg.org/) of the Open Grid Forum (OGF). The
goal is through a standardized API among Clouds to enable both interoperability
among Clouds from different vendors and new business models and platforms as
(according to OCCI 2009):
