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2.2.1 Cloud Essential Characteristics
According to definition [5] on-demand characteristic gives the freedom to the cloud user
to self-provisioning the IT cloud resources without a human intervention. Provisioning is
mostly made by self-service portals where the user can chose computing power, storage
capacity, network connection, eventual software etc. This
characteristic enables the
main concept of “service-based” and “usage-driven content” of a cloud environment.
Broad access defines that cloud services must be largely accessible by the users using
heterogeneous access devices [6]. Users in fact need to be able to connect to the service
using different types of terminal (PC, tablet, mobile phones), transport protocols and se-
curity technologies. Due to the broad access, the service might be tailored fit to suit the
requirements, additional Application Programming Interface (API) will be required.
Resource pooling, and multinenancy, refers to the dynamical allocation of IT resources
in order to meet the customer demand. The resource allocation should be totally trans-
parent to the end user and not related to the location where the cloud service is hosted.
Resource pooling mostly uses virtualization technologies and allows the cloud provider
to serve multiple could customers using the same infrastructure,
this is called mul-
titenancy. Different tenants are not
aware of each other’s presence, due to resource iso-
lation, and might dynamically reserve and release IT resources.
Rapid elasticity is the ability of the cloud service to automatically, and transparently, al-
locate IT resources in order to satisfy the cloud users need. Users have the illusion of
infinite resources. Scaling is usually done using probes and scaling agents that can de-
tect the needs and immediately allocate more IT resources such as network, memory,
storage, processing power, VM. This characteristic is a core reason of the cloud service
existence itself.
As per the NIST definition measurability is a key cloud element defining the characteris-
tics that all the cloud services need to have measurable features for billing, monitoring
and reporting purpose. This is a fundamental requirement for both not-charged usage
and for more common pay-per-use cloud services.
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Resilience is a characteristic that originally was not included in the NIST cloud definition,
however over the years this aspect gained a significant importance in the cloud solutions
justifying the usage of the cloud itself against the on-premises systems.
In cloud computing the resiliency refers to the capability of the cloud service to failover
and distributes the service over redundant pool of IT resources across physical locations
or within the same cloud. Usually the failover mechanism is fully automatic and relies on
probes that detect the failures and react according to a pre-defined set of instructions
[7].
