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SLAng: A Language for Defining
Service Level Agreement
development of WSOL has made much refer-
ence to WSLA and WSML discussed previously.
One of the distinct features of WSOL is that it
has defined external ontologies of QoS metrics
and measurement units for the specification of
QoS constraints. In the current implementation
of WSOL, it is assumed that ontology of QoS
metrics is a collection of names with information
about appropriate data types and measurement
units. Similarly, ontology of measurement units
is a simple collection of names without any ad-
ditional information.
Authors of WSOL have identified the draw-
backs of WSDL and proposed such a solution
attempting to address them. However, WSOL is
not widely used and accepted. The W3C (http://
www.w3.org/) recommended WS-Policy now
becomes a standard to address these needs.
SLAng is SLA language developed by University
of College London (UCL) under the TAPAS proj-
ect (http://tapas.sourceforge.net/) (2002-2005).
SLAng defines six different types of SLA, which
are divided into Vertical SLAs and Horizontal
SLAs (Lamanna et al., 2003). SLAng does not
clearly describe the structure of SLA. The clas-
sification of vertical SLAs and horizontal SLAs
is easy to confuse people. The TAPAS project
finished in 2005, and the further development of
SLAng cannot be guaranteed.
Web Service Management Language
Web Service Management Language (WSML)
(Sahai et al., 2001) was developed in 2001 by HP
Laboratories. It can be regarded as an extension
of QoS Modeling Language (QML) (Svend et al.,
1998) by allowing the definition of SLO, validity
period and mathematical operation of measured
data, etc. which were not supported in QML.
However, WSML does not enable specification
of management third parties. Further, WSML
does not define the language for expressions to
be evaluated. It is assumed that expressions will
be written in some other mathematical languages,
such as MathML. This means that the infrastructure
for WSML constraints evaluation needs to support
these mathematical languages.
WS-Policy
WS-Policy is a W3C recommendation since Sep-
tember 2007. WS-Policy is a standard to describe
the properties that characterize a Web service
(http://www.w3.org/TR/ws-policy-primer/). By
means of this specification, the functional descrip-
tion of a service can be tied to a set of assertions
that describe how the Web service should work in
terms of aspects like security, transactionability,
and reliable messaging. WS-Policy document
is in charge of composing assertions to identify
how a Web service should work. These assertions
can be used to express both functional aspects
(e.g. constraints on exchanged data), and non-
functional aspects (e.g. security, transactionability,
and message reliability). WS-Policy language is
an extensible language by design. The Policy ,
ExactlyOne , All and wsp:PolicyReference ele-
ments are extensible.
Although WS-Policy is recommended by
W3C, it actually provides no advantage for QoS
specification, other than it is a standard way of
associating QoS-like descriptions with service
(Dobson, 2004; Chaari et al., 2008). Also WS-
Web Service Offering Language
Web Service Offering Language (WSOL) (Tosic
et al., 2003) claims to be a language for the formal
specification of various constraints, management
statements, and classes of service for Web services.
It was developed in 2003 by Carleton University,
Canada. The motivation of development of WSOL
is that the WSDL cannot support specification
of various constraints, management statements,
classes of service, SLAs and other contracts. The
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