Service level agreement in cloud computing
Describes a set of non functional requirements of the service.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 IndicatorsService LevelObjective:Objective of service quality that has to be achieved.Set of measurable KPIs with thresholds to decide if the objective is fulfilled or not.
The fulfillment of an SLOs describes a state of service when all of the SLOs key performance indicators are within a specified thresholds.KPIs usually consist from one or more raw monitored values including min,avgand max specifying the scaleThey can also represent some aggregated measurement ( e.g. average output ) within a sliding window that is combined from one or more monitoring outputs.
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 triggered.
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.
Key forces behind cloud computing are :The ubiquity of broadband wireless and wired networking.Falling storage costs.Progressive improvements in Internet computing software.
Main benefits of cloud computing:Clientswill be able to add more capacity at peak demand, reduce costs,experiment with new services, and remove unneededcapacity.Serviceproviders will increase utilization via multiplexing, and allow for larger investments in software and hardware.
More about cloud computing
Technical underpinnings:VirtualizationService-oriented softwareGrid-computing technologiesManagement of large facilitiesPower efficiencyServices in consumer perspective:infrastructure-as-a-service(IaaS)platform-as-a-service (PaaS)or software-as-a-service (SaaS).
SERVICE MODELSCloud 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 ofPaaSis 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 CLOUDCOMPUTING•On-demand access. Cloud computing offers rapid fulfillment ofdemandfor computing and has the continuing ability to fulfill that demand as required.•Elasticity.Computing is provided in the amount required and disposed ofwhenno longer needed.•Pay per use.Much like a utility, cloud resource charges are based on thequantityused.
•Connectivity.All of the servers are connected to a high-speed network thatallows data to flow to the Internet, as well as between computing andstorage elements.•Resource pooling.The cloud provider’s infrastructure is shared acrosssome number of end customers, providing economies of scale at thecomputing and services layers.•Abstracted infrastructure.The cloud end customers do not know the exactlocations 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 allocations• Offer a range of performance for a particular instance, such as a range of GHz•Overallocateresources on a physical server, orthin provisioning. Commercial virtualization management software such as VMware orVirtuozzooffer the ability tooverallocateresources 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 resourcesand cut corners on infrastructure. The result is variable and unpredictable performance of the virtual infrastructure.5SLA ensures the customer gets the quality of service he expects.
SLA has to point the volume of service rendered for the cloud services.Contract KPIs:Thevolume ofserviceThe quality of servicePeak and average loads of workThe volume of demand at different times of dayThepenalty 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.Transactionsby 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 specifications.Schedule of virtual or actual meeting between the customer and the cloud provider if necessary.
Durkee, D. (2010). Why the cloud computing will never be free. ACM New York.
Dikaiakos, M.,Katsaros, D., &Mehra, P. V. (2009). Cloud Computing - Distributed Internet Computing for IT and Scientific Research.Internet Computing, IEEE, 10-13.