Describes a set of non
functional requirements of
Example : RTO time –
Return to Operation Time
if case of failure
SLO – Service Level
Objective. That is, the
objective to be achieved.
KPI – Key Performance
Service Level Objective:
Objective of service quality
that has to be achieved.
Set of measurable KPIs
with thresholds to decide if
the objective is fulfilled or
o The fulfillment of an SLOs describes a state of service when all of the
SLOs key performance indicators are within a specified thresholds.
o KPIs usually consist from one or more raw monitored values
including min, avg and max specifying the scale
o They can also represent some aggregated measurement ( e.g.
average output ) within a sliding window that is combined from one or
more monitoring outputs.
o The Cloud Computing infrastructures are usually large scale,
therefore SLAs need to be formally described to enable their
automated handling and protection.
Automated SLA protection
• Automated SLA protection is based on a set of policy rules.
• Each policy rule is formed by one or more conditions (KPI's value
matching pattern) and one or more actions.
• KPIs are periodically evaluated according to defined policies.
• If one or more conditions are met, then appropriate actions are
• Service Level Agreement (SLA) describes agreement on non-functional
requirements between provider and customer.
• SLA consists of service level objectives (SLOs) that are evaluated
according to measurable Key Performance Indicators (KPIs).
• Automatic SLA protection enables further increase of the system
utilization and system profit.
• In currently available systems only some basic SLAs like "uptime over
a time period guarantee" are available.
Main benefits of cloud computing:
• Clients will be able to add more capacity at peak demand, reduce costs,
experiment with new services, and remove unneeded capacity.
• Service providers will increase utilization via multiplexing, and allow for
larger investments in software and hardware.
More about cloud computing
• Service-oriented software
• Grid-computing technologies
• Management of large facilities
• Power efficiency
Services in consumer perspective:
• infrastructure-as-a-service (IaaS)
• platform-as-a-service (PaaS)
• or software-as-a-service (SaaS) .
Cloud is divided into three basic service models, each addressing a
specific business need.
• IAAS (Infrastructure as a Service). This is the most basic of the
cloud service models. The end customer is purchasing raw
compute, storage, and network transfer. Offerings of this type are
delivered as an operating system on a server with some amount of
storage and network transfer. These offerings can be delivered as a
single server or as part of a collection of servers integrated into a
VPDC (virtual private data center).
• PAAS (Platform as a Service). This is the next layer up, where the
end customer is purchasing an application environment on top of
the bare-bones infrastructure. Examples of this would be
application stacks: Ruby on Rails, Java, or LAMP. The advantage of
PaaS is that the developer can buy a fully functional development
and/or production environment.
• SAAS (Software as a Service). This currently is the highest layer in
the cloud stack. The end customer is purchasing the use of a
working application. Examples of this are NetSuite and
SalesForce.com. (This service is not the focus of this article.)
THE ESSENTIAL CHARACTERISTICS OF CLOUD COMPUTING
• On-demand access. Cloud computing offers rapid fulfillment of demand
for computing and has the continuing ability to fulfill that demand as
• Elasticity. Computing is provided in the amount required and disposed of
when no longer needed.
• Pay per use. Much like a utility, cloud resource charges are based on the
• Connectivity. All of the servers are connected to a high-speed network that
allows data to flow to the Internet, as well as between computing and
• Resource pooling. The cloud provider’s infrastructure is shared across
some number of end customers, providing economies of scale at the
computing and services layers.
• Abstracted infrastructure. The cloud end customers do not know the exact
locations or the type(s) of computer(s) their applications are running on.
Instead, the cloud provider provides performance
Why is Service Level Agreement important in Cloud Computing?
Many cloud providers are vague on the specifics of the
underlying hardware and software stack they use to deliver a
virtual server to the end customer, which allows for over
commitment. Techniques for overcommitting hardware include
(but are not limited to):
• Specify memory allocation and leave CPU allocation
unspecified, allowing total hardware memory to dictate the
number of customers the hardware can support
• Quote shared resource maximums instead of private
• Offer a range of performance for a particular instance, such as
a range of GHz
• Overallocate resources on a physical server, or thin
provisioning. Commercial virtualization management software
such as VMware or Virtuozzo offer the ability to overallocate
resources on the underlying hardware, resulting in reduced
performance during peak loads.
VARIABLE PERFORMANCE. People can see
advertisements for cloud computing breaking through the
previous price floor for a virtual server instance. It makes one
wonder how cloud providers can do this and stay in business.
THEY OVERCOMMIT THEIR COMPUTING RESOURCES
and cut corners on infrastructure. The result is variable and
unpredictable performance of the virtual infrastructure.5
SLA ensures the customer gets the quality of service he expects.
SLA has to point the volume of service rendered for the cloud services.
• The volume of service
• The quality of service
• Peak and average loads of work
• The volume of demand at different times of day
• The penalty for the cloud provider in case the provider fails to meet
these service requirements.
Why Is Service Level Agreement important in cloud computing
• A signed agreement with each customer.
• Transactions by hour and jobs by day for each application.
• A method of reporting SLA results.
• Priority of services in case of insufficient availability.
• Agreed methods of penalty in case customer exceeds his limits.
• Agreed methods of penalty in case cloud provider fails to meet contract
• Schedule of virtual or actual meeting between the customer and the
cloud provider if necessary.