Clouds are a type of parallel and distributed system harnessing physical and virtual computers presented as a unified computing resource. Clouds build the infrastructure on top of which services are implemented and delivered to customers. Such infrastructures can be of different types and provide useful information about the nature
and the services offered by the cloud.
A more useful classification is given according to the administrative domain of a cloud: It identifies the boundaries within which cloud computing services are implemented, provides hints on the underlying infrastructure adopted to support such services, and qualifies them.
It is then possible to differentiate four different types of clouds:
Public Cloud
Public clouds are a realization of the canonical view of cloud computing in which the services offered are made available to anyone, from anywhere, and at any time through the Internet.
From a structural point of view, they are a distributed system, most likely composed of one or more data centers connected together, on top of which the specific services offered by the cloud are implemented.
Any customer can easily sign in with the cloud provider, enter her credential and billing details, and use the services offered.
A fundamental characteristic of public clouds is multitenancy. A public cloud is meant to serve
a multitude of users, not a single customer. Any customer requires a virtual computing environment
that is separated, and most likely isolated, from other users.
Public clouds are appealing and provide a viable option to cut IT costs and reduce capital expenses, but they are not applicable in all scenarios.
For example, a very common critique of the use of cloud computing in its canonical implementation is the loss of control.
In particular, institutions such as government and military agencies will not consider public clouds as an option for processing or storing their sensitive data. The risk of a breach in the security infrastructure of the provider could expose such information to others; this could be considered unacceptable.
Private Cloud
Private clouds are virtual distributed systems that rely on private infrastructure and provide internal users with dynamic provisioning of computing resources.
Instead of a pay-as-you-go model as in public clouds, there could be other schemes in place, taking into account the usage of the cloud and proportionally billing the different departments or sections of an enterprise.
Private clouds have the advantage of keeping the core business operations in-house by relying on the existing IT infrastructure and reducing the burden of maintaining it once the cloud has been set up.
In this scenario, security concerns are less critical, since sensitive information does not flow out of the private infrastructure.
Moreover, existing IT resources can be better utilized because the private cloud can provide services to a different range of users.
Another interesting opportunity that comes with private clouds is the possibility of testing applications and systems at a comparatively lower price than public clouds before deploying them on the public virtual infrastructure.
From an architectural point of view, private clouds can be implemented on more heterogeneous hardware:
- They generally rely on the existing IT infrastructure already deployed on the private premises. This could be a data center, a cluster, an enterprise desktop grid, or a combination of them.
- The physical layer is complemented with infrastructure management software or a PaaS solution, according to the service delivered to the users of the cloud.
Community Cloud
Community clouds are distributed systems created by integrating the services of different clouds to address the specific needs of an industry, a community, or a business sector.
The National Institute of Standards and Technologies (NIST) characterizes community clouds as follows:
The users of a specific community cloud fall into a well-identified community, sharing the same concerns or needs; they can be government bodies, industries, or even simple users, but all of them focus on the same issues for their interaction with the cloud.
This is a different scenario than public clouds, which serve a multitude of users with different needs. Community clouds are also different from private clouds, where the services are generally delivered within the institution that owns the cloud.
From an architectural point of view, a community cloud is most likely implemented over multiple administrative domains. This means that different organizations such as government bodies, private enterprises, research organizations, and even public virtual infrastructure providers contribute with their resources to build the cloud infrastructure.
Hybrid Cloud
Public clouds are large software and hardware infrastructures that have a capability that is huge enough to serve the needs of multiple users, but they suffer from security threats and administrative pitfalls.
Although the option of completely relying on a public virtual infrastructure is appealing for companies that did not incur IT capital costs and have just started considering their IT needs (i.e., start-ups), in most cases, the private cloud option prevails because of the existing IT infrastructure.
Private clouds are the perfect solution when it is necessary to keep the processing of information within an enterprise’s premises or it is necessary to use the existing hardware and software infrastructure.
One of the major drawbacks of private deployments is the inability to scale on demand and to efficiently address peak loads. In this case, it is important to leverage the capabilities of public clouds as needed. Hence, a hybrid solution could be an interesting opportunity for taking advantage of the best of the private and public worlds. This led to the development and diffusion of hybrid clouds.
Hybrid clouds allow enterprises to exploit existing IT infrastructures, maintain sensitive information within the premises, and naturally grow and shrink by provisioning external resources and releasing them when they’re no longer needed. Security concerns are then only limited to the public portion of the cloud that can be used to perform operations with less stringent constraints but that are still part of the system workload.
It is a heterogeneous distributed system resulting from a private cloud that integrates additional services or resources from one or more public clouds. For this reason, they are also called heterogeneous clouds. Dynamic provisioning is a fundamental component in this scenario.
Hybrid clouds address scalability issues by leveraging external resources for exceeding capacity demand. These resources or services are temporarily leased for the time required and then released. This practice is also known as cloud bursting. Whereas the concept of hybrid cloud is general, it mostly applies to IT infrastructure rather than software services.
Service-oriented computing already introduces the concept of integration of paid software services with existing applications deployed in private premises.
In an IaaS scenario, dynamic provisioning refers to the ability to acquire on-demand virtual machines in order to increase the capability of the resulting distributed system and then release them.
Infrastructure management software and PaaS solutions are the building blocks for deploying and managing hybrid clouds.
In particular, with respect to private clouds, dynamic provisioning introduces a more complex scheduling algorithm and policies, the goal of which is also to optimize the budget spent to rent public resources.
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