International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
The architecture of Figure |, presents the functions needed
for QoS-aware service chaining. The key componets are;
the service discoverv mechanism, the service chain gen-
erator and the service orchestration and execution man-
agement engine. Typically, the client specifies a request
for services with QoS constraints. The Qos requirements
are end-to-end, save for exceptional cases where business
policy dictates otherwise (like when services from certain
nodes are not allowed), hence they apply to the service
chain as a whole and not the service instances per se. Based
on the request specifications , the service chain genera-
tor determines the type of services necessary, the sequence
of execution, and QoS-related constraints for each. The
service discovery mechanism queries the registry to find
services that meet specified requirements, which are then
orchestrated and managed by the service orchestration and
execution management engine to ensure delivery of the ser-
vice as requested. Once defined, the service chain can be
re-used as a single service in a more elaborate business
process. Its notable that the services that are discovered
and choreographed are those that optimise the QoS of the
entire chain.
4 QOS MODEL FOR GI SERVICE
In defining a QoS model for GI services, important non-
functional quality characteristics are identified that play a
role in enhancing user experience or necessary for proper
operation of Internet GIS applications. For each character-
istic the definition includes its semantics and a method of
estimating its value.
l. Performance: Performance is about the timing aspects
of a service. User perceived service performance refers
to the time duration in seconds between the instant a
request for an operation is submitted and the duration
a response is made available. It comprises two com-
ponents:
e Transmission delay 75,;,4,, the time it takes to
relay the request from client to the geo-service
and to relay the generated response back to the
client and is a function of the volume of data
being relayed, the bandwidth, propagation dis-
tance or number of network hops. Estimates of
Taelay Can be obtained from time series of ping
measurements to the service over different times
ofthe day and a profile can then be generated in-
dicating the latency over time.
e Processing delay, /4,,,,, which is the duration
between the instant a request is received for ser-
vice and the instant when a response is gener-
ated.
Therefore
Per formance (Rf = Topi + Pais
and in the geo-service node, processing delay is the
average response time for executing the specified op-
eration and includes anytime spent waiting for resources
214
N
Wine and the time spent on servicing the request
Sinne therefore Fy, Wisrme + Stime À service pub-
lishes its performance or provides a means of acquir-
ing the information.
Cost: Cost refers to the price charged for executing an
operation at a particular site. Several charging schemes
may be used include a flat rate cost price, charging de-
pending on time spent in providing the service, amount
of information or data processed, etc. Providers pub-
lish cost information or provide a means for acquiring
it.
Reliability: This is the probability that the service will
execute correctly and adhere to specified service level
agreements for the duration its required. Reliability
can be provider specified. However better estimates
can be obtained from observations on the service as
N
1
Reliability [r] = N SS p.
1
Availability: This is the fraction of time the GI service
is accessible to provide service over some fixed period
time i.e.
t
Availability |A| — —
yA = 7.
where # is the time the service is accessible and 7
is the fixed time period. A service may publish its
availability or provide a means of obtaining it. It can
also be estimated from observations on the service.
Security: Security refers to the requirement that the
service supports certain security protocols and mech-
anisms e.g. registration, authentication, secure trans-
actions etc.
Reputation/Trustworthiness: This is a measure of the
reliability of the service as perceived by the user. It
depends on the users experience with the use of the
service and is independent of published QoS of the
service. Reputation is an estimate of the quality of ex-
perience (QoE), and a rank can be assigned, say [1,10]
or some other convenient interval.
Interactivity support: This quality measure is valid for
portrayal services that generate graphical displays of
spatial data or geographically referenced information.
Interactivity refers to the extent the user can interact
with the graphical output through querying features,
pan, zoom in, zoom out, etc.
Location: This is not a quality measure of the service
perse but refers to the geographic region to which the
data accessed by the service refers. This characteris-
tic is therefore only valid to data access services, and
will aid distinction of GI services depending on geo-
graphical region its data covers. A bounding box or
some other convenient means of describing the spatial
extent may be applied but this need be agreed upon by
the negotiating entities.
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