Service level objective
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A service level objective (SLO) is a key element of a service level agreement (SLA) between a service provider and a customer. SLOs are agreed as a means of measuring the performance of the Service Provider and are outlined as a way of avoiding disputes between the two parties based on misunderstanding.
There is often confusion in the use of SLA and SLO. The SLA is the entire agreement that specifies what service is to be provided, how it is supported, times, locations, costs, performance, and responsibilities of the parties involved. SLOs are specific measurable characteristics of the SLA such as availability, throughput, frequency, response time, or quality.
The SLO may be composed of one or more quality-of-service measurements (service level indicators, SLIs) that are combined to produce the SLO achievement value. As an example, an availability SLO may depend on multiple components, each of which may have a QOS availability measurement. The combination of Quality of Service (QOS) measures into an SLO achievement value will depend on the nature and architecture of the service.
In  the authors argue that SLOs must be:
- Mutually acceptable
In  the authors define a SLO as a "commitment to maintain a particular state of the service in a given period" with respect to the state of the SLA parameters.
In  the authors define the SLO as "the quality of service aspect of the agreement. Syntactically, it is an assertion over the terms of the agreement as well as such qualities as date and time".
SLOs should generally be specified in terms of an achievement value or service level, a target measurement, a measurement period, and where and how measured. As an example, "90% of calls to the helpdesk should be answered in less than 20 seconds measured over a one-month period as reported by the ACD system". Results can be reported by the percent of time that the target answer time was achieved compared to the desired service level (90%).
The use of the term SLO is deprecated in ITIL V3 to Service Level Target, not to be confused with Service Level Requirement defined in the service design. However the SLO term is found in various scientific papers, for instance in the reference architecture of the SLA@SOI project, and it is used in the Open Grid Forum document on WS-Agreement.
- Rick Sturm, Wayne Morris "Foundations of Service Level Management", April 2000, Pearson.
- Alexander Keller, Heiko Ludwig "The WSLA Framework: Specifying and Monitoring Service Level Agreements for Web Services", Journal of Network and Systems Management, Vol 11, n. 1, March 2003.
- Alain Andrieux, Karl Czajkowski, Asit Dan, Kate Keahey, Heiko Ludwig, Toshiyuki Nakata, Jim Pruyne, John Rofrano, Steve Tuecke, Ming Xu "Web Services Agreement Specification (WS-Agreement)", GFD-R-P.107, March 2007, Open Grid Forum.
- Jens Happe, Wolfgang Theilmann, Andrew Edmonds, and Keven T. Kearney "A Reference Architecture for Multi-Level SLA Management" in "Service Level Agreements for Cloud Computing", eds. Wieder, Philipp and Butler, Joe M. and Theilmann, Wolfgang and Yahyapour, Ramin, Springer New York, 2011, DOI:10.1007/978-1-4614-1614-2_2