Database Reference
In-Depth Information
service. The P2P application contacts the directory service, informing the directory
service of its IP address and the names of objects in its local disk that it is making
available for sharing. When an active peer obtains a new object or removes one, it
informs the directory server, which then updates its database. In a distributed hash
table, peers are indexed through hashing keys and are found through complex queries
within a distributed system. This approach is good in performing load balancing and
offloading loads to less-loaded peers. The flooded request model is simple but scales
poorly. When a node wants to
nd a resource on the network, which may be on a node
it does not know about, it could simply broadcast its search query to its immediate
neighbours. If the neighbours do not have the resource, it then asks its neighbours to
forward the query. This is repeated until the resource is found or all the nodes have
been contacted, or perhaps a network-imposed hop limit is reached.
2.2 Cloud Computing
Cloud computing [
35
,
36
] assembles large networks of virtualised ICT services such as
hardware resources (such as CPU, storage, and network), software resources (such as
databases, application servers, and web servers) and applications. The advent of vir-
tualization has led to the transformation of traditional data centres into flexible cloud
infrastructure [
44
,
45
]. With the benet of virtualization, data centres progressively
provide flexible online application service hosting [
17
] such as: web hosting, search,
e-mails, and gaming. Largely, virtualization provides the opportunity to achieve high
availability of applications in data centres at reduced costs. In industry, these services
are referred to as Infrastructure as a Service (IaaS), Platform as a Service (PaaS), and
Software as a Service (SaaS) [
46
,
47
]. Cloud computing services are hosted in large
data centres, often referred to as data farms, operated by companies such as Amazon
[
10
], Apple, Google, and Microsoft [
8
]. Cloud computing gives developers the ability
to marshal virtually in
nite computing and storage based on amount of data to be
processed and stored; and number of people to be noti
ed in real time. Cloud-based
ICT resources can be acquired under pay-per-use models and as needed, instead of
requiring upfront investments in resources that may never be used optimally. As
de
ned by National Institute of Standards and Technology [
16
], the
ve essential
characterises of cloud computing are:
On-demand self-service.
Broad network access.
Resource pooling.
Rapid elasticity.
Measured service.
cant
momentum is Quality of Service (QoS) driven service delivery. For reliable and ef
Another important characteristic of cloud computing that is gaining signi
-
cient management of application performance hosted on the *aaS layers, system
administrators have to be fully aware of the compute, storage, networking resources,
application performance and their respective quality of service (QoS). QoS parameters
(e.g., latency, renting cost, throughput, etc.) play an important role in maintaining the
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