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management requires a comprehensive SLA
manager to provide a set of key functionalities
such as negotiation of QoS terms, acquisition
of usage data/QoS measurement, conformance
check, and billing. Therefore it is essential to have
a high level SLA reference architecture which can
provide guidelines for SLA application / service
development. Although various SLA managers
have been developed in many research projects
and industry applications, the generic SLA manger
reference architecture with identified key compo-
nents remains unclear.
Relevant SLA specification languages to be
reviewed include: (i) QoS Markup Language
(QML), (ii) Hierarchical QoS Markup Language
(HQML), (iii) Web Service Level Agreement
(WSLA), (iv) SLAng, (v) Web Service Manage-
ment Language (WSML), (vi) Web Service Of-
fering Language (WSOL), (vii) W3C WS-Policy
(WSP), (viii) WS-Agreement, (ix) WSDM, and
(x) WS-management.
(using xml DTD) mixes the QoS metrics and price
terms together.
Web Service Level Agreement (WSLA)
The Web Service Level Agreement (WSLA)
(Ludwig, 2003) is a specification language for
service level agreement. It was proposed by IBM
and version 1.0 was released in 2003. In WSLA,
the structure of SLA can include: (i) Parties, (ii)
Service definition, and (iii) Obligations (Ludwig,
2003).
•
“Parties” define parties involved in the
management of Web service such as cus-
tomer, service provider, third parties, etc.
•
“Service definition” describes: (i) defini-
tion of the service, (ii) SLA parameters, and
(iii) the way SLA parameters are measured
and computed. In service definition, a term
service object is used to describe what Web
service operations an SLA relates to.
QoS Markup Language
•
“Obligations” defines the service level
that is guaranteed with respect to the SLA
parameters, and promises to perform ac-
tions under particular conditions. It pro-
vides two kinds of guarantees: (i) Service
Level Objective (SLO), and (ii) Action
Guarantees. SLO expresses a commitment
to maintain a particular state of the service
in a given period, while Action Guarantees
expresses a commitment to perform partic-
ular activity if a given precondition is met.
QoS Markup Language (QML) (Svend, 1998)
was developed in 1998 by HP laboratory. It aims
to define multi-category QoS specification for
components in distributed object systems. QML
is now out-of-date. It is mainly used for QoS
terms specification and not appropriate for SLA
specification.
HQML
The Hierarchical QoS Markup Language (HQML)
(Gu et al., 2002) developed by University of Illinois
in 2002, is an XML based language to enhance
the distributed multimedia application over Web
with QoS capabilities. HQML schema is simple.
But it is more like a specification language for
QoS management rather than a specification
language for SLA. It is not closely tied up to the
use of Web service. The proposed XML schema
WSLA 1.0 is fully documented and publicly
available. It has been widely used. WSLA 1.0
specification clearly defines the structure of SLA;
especially it distinguishes the SLA parameter and
metrics. It provides a framework for specifying and
monitoring SLA for Web services. WSLA is also
extensible. All these make WSLA promising as a
QoS/SLA specification language. The identified
problem is that the v1.0 was released in 2003, and
it has not been officially supported since then.
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