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indicators (SLIs). The SLA class inherits from the SLACommon class and
has the same data structures as the SLACommon class. The SLATemplate
class is derived from the SLACommon class. Most of the terminology
described in our proposed SLA model follows the WS-agreement specii -
cation draft version 1.1 [9].
As shown in
Figure 10.5
,
the SLACommon class consists of the follow-
ing classes: (1) ContextInfo, which includes the information of the partici-
pants, (e.g., who are agreeing to the SLA, and how long is the SLA valid);
(2) ServiceDescriptionTerm, which describes the service properties (e.g.,
service name, type); (3) GuaranteeTerm, which expresses the desired or
required service properties, behaviors, consequences, and so on; (4) the
SLI, which represents the name and details of the terms used, how they
are measured, and so on. GuaranteeTerm consists of a PricingModel
that
describes the service charges, PaymentProtocolModel that describes how
and when the payment will be made, and ServiceLevelObjectives (SLO)
that specify the measurable characteristics of the service such as response
time, reliability, availability, throughput, and so on. With the structured
information in the GuaranteeTerms, the proposed SLA model allows the
service provider and its clients to express the business values associated
with the service qualities in the SLA.
10.4.2
SLA Negotiation and the Implementations
Based on the SLA model, a service client and the service provider can
negotiate with each other via the SLA-based service. A basic SLA negotia-
tion process may involve three phases: (1) retrieving SLA templates, (2)
proposing SLAs, and (3) agreeing/disagreeing to an SLA.
10.4.2.1
Retrieving SLA Templates
As explained before, SLA templates hold the default values and creation
constraints for the service. A service customer uses the SLA template to put
together an SLA proposal. To subscribe a service, the client may i rst request
one or more SLA templates from the service provider. As a result, a set of
entity references of the SLATemplate instances will be sent to the client.
The client can then access the SLATemplate instances using these entity
references.
10.4.2.2
Proposing SLAs
Based on the SLATemplate obtained from the service provider, the client
can create an SLA proposal object by adding the user requirements. To
propose an SLA, the client invokes the proposeSLA() method on the ser-
vice provider interface. Once an SLA is proposed, the service provider
receives a copy of the SLA proposal. After receiving the SLA proposal, the
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