Information Technology Reference
In-Depth Information
5.2 QUALITY-OF-SERVICE MEASUREMENTS
5.2.1 QoS Attributes
Each functionality of a service can be evaluated by several QoS
properties and parameters. The QoS parameters are sorted into run
time, transaction support, configuration management and cost, and
security-related ones. Each parameter comprises several metrics and
submetrics. In this chapter, we focus on configuration-related QoS
attributes. Generally, there are two kinds of QoS attributes, that is, cost
and benefit. For cost attributes, the higher the value, the less optimal the
solution, and for benefit attributes, the higher the value, the better the
solution. These include availability and reliability. Some of the QoS
attributes can be measured quantitatively as shown in Table 5.1.
5.2.2 Aggregation
In the following discussion, a dummy web service is the one to be
decided during the service composition/configuration process and a
nondummy one is the one whose QoS values are available, to be
formally defined via a disassembly Petri net in the next section. A
place in the net corresponds to an atomic service (called a leaf place), a
service to be instantiated by an atomic one, or a composite one. The
latter two cases lead to non-leaf places. Suppose that we have
q
QoS
Table 5.1
QoS Attributes and Measures Used for Evaluating
Candidate Congurations
QoS attributes
Measures
Cost
The fee to be paid by a service requestor for invoking a Web
service each time
Availability
The attribute for evaluating an immediate availability of a Web
service. It can be computed as the ratio of the service
accessible time to the total time of observation
Reliability
The probability of receiving the result within the expected
duration time after a Web service is successfully invoked
Successful
execution rate
The probability that a request is correctly responded within the
anticipated time indicated in a Web service invocation
context, for example, an operation is successfully completed
