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communication channels. This is illustrated in
Figure 5.
In this example, the service manager is respon-
sible for parsing an OVF document and extracting
from it a set of appropriate scalability rules. All
virtual machines launched in the Cloud will
regularly output measurements of key perfor-
mance indicators via the use of probes intercon-
nected via an appropriate communication channel.
Such information is passed to a rule engine, which
will then trigger an appropriate management re-
sponse, such as the deployment of new instances.
In the example illustrated, the number of concur-
rent sessions handled by the load balancer is used
as a basis to deploy new instances of web servers.
With respect to the rule syntax itself, we adopt
a simple KPI based ratio system, illustrated in
Box 1 (based on the OVF example previously
shown in Figure 4).
In summary, we define a proportional relation-
ship between the average value of a collection of
KPI measurement obtained during a particular
time frame and the number of instances of a par-
ticular component that should be deployed. This
allows us for example to express that we should
deploy additional web servers should the average
number of sessions obtained during the last 10
minutes period (sampling with a frequency of 60
samples per period) is above 20 up to a maximum
of 5.
While more complex specifications based on
a rule-based syntax could be adopted, we found
that the majority of cases that were tackled could
be specified in this manner. As the KPI measure-
ments are forwarded to the infrastructure by the
application itself, the application level probes
can implement more complex operations on
performance indicators, such as aggregations of
multiple values before passing this information
to the infrastructure itself.
Service Level Objectives
While the potential of Cloud computing infrastruc-
tures is evidently great, there exists much risk in
service providers outsourcing hardware provision-
ing in this manner. Indeed, if the resources leased
fail, or do not meet the performance expectations
of the service provider, this may result in con-
siderable financial loss. In addition to the losses
incurred from the inability to provide the service
to the end-client, there may have also been costs
involved in porting the service or applications to
the leased infrastructure that cannot be recouped
Box 1.
<VirtualSystem ovf:id=”WebServer” xmlns:rsrvr=“http://schemas.telefonica.com/
claudia/ovf” rsrvr:min=”1” rsrvr:max=”5” rsrvr:initial=”1”>
…
<rsrvr:ElasticArraySection>
<Info>String</Info>
<rsrvr:Rule>
<rsrvr:KPIName>totalSessions</rsrvr:KPIName>
<rsrvr:Window unit=”minute”>10</rsrvr:Window>
<rsrvr:Frequency>60</rsrvr:Frequency>
<rsrvr:Quota>20</rsrvr:Quota>
</rsrvr:Rule>
</rsrvr:ElasticArraySection>
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