Information Technology Reference
In-Depth Information
1.2.2 Platform as a Service
The platform-as-a-service (PaaS) model presents more flexibility than the
SaaS model. Using this model, it is possible to develop a new, fully custom-
ized application and then execute it in the provider's cloud environment.
It is also possible to modify an existing application to be compatible with
the provider's model of execution; in the majority of cases, this is a realistic
scenario for scientific applications [4]. The majority of the services provided
in this model consist of an environment to execute web-based applications.
This kind of application processes a large number of simultaneous requests
from different users. The regular architecture of these applications is com-
posed of a web page, which interacts with the user; a processing layer,
which implements the business model; and a database, used for data per-
sistence. Each user request is treated uniquely in the system and has no
relationship with other requests. Due to this, it is impossible to create a
system to perform distributed computing. However, the processing layer
of this model can be used if the service does not have a huge demand for
processing power.
In the PaaS model, the provider defines the programming languages and
the operating system that can be used; this is a limitation for general-purpose
scientific application development.
1.2.3 Infrastructure as a Service
The infrastructure-as-a-service (IaaS) model is the most flexible service
model of cloud computing. The model delivers raw computational resources
to the user, normally in the form of virtual machines (VMs). It is possible
to choose the size of the VM, defining the number of cores and the amount
of memory. The user can even choose the operating system and install any
desired software in the VM. The user can allocate any desired quantity of
VMs and build a complete parallel system. With this flexibility, it is possible
to use IaaS for applications that need a large amount of resources by the con-
figuration of a cluster in the cloud.
1.3 Cloud Implementation Models
The service models, presented in the previous section, can be delivered
using four different implementation models: private cloud, community
cloud, public cloud, and hybrid cloud. Each one has strong and weak points.
The four models can be used to build an e-science service, and they are
analyzed to present their main characteristics to help the user decide which
one to choose.
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