64 Copyright © 2011 Open Geospatial Consortium,
Inc. All Rights Reserved.
9.5.2 Interoperability between Event Services
Another issue which came up during the integration of both the IfGI and IDS Event Service into the client applications was the different way of handling subscription
resources. Such resources were required to enable management of subscriptions on the client-side e.g. pausing or removing a subscription.
Listing 6 – Subscribe response
Listing 6 depicts the way both Event Services designate the resource of the subscription. The WS-N standards family does explicitly define a way how to designate subscription
resources by using service-specific parameters and because the Event Services is based on these standards using a proprietary element to identify resources is valid. During
OWS-8 the management of subscription resources on the client-side led to some minor problems as both Event Services used different namespaces for the ResourceId element.
This was probably due to first-time application of WS-A specific policies in an OWS service architecture. As a workaround both Event Services used the same namespace.
However, such a restriction is not necessary and may complicate the integration of additional services into the Event Architecture as these would need to apply domain-
specific policies on a domain-unspecific environment such as WS-A.
Two possible approaches have been discussed and can be applied in future testbeds. One approach would be to stay with the current design and delegate the responsibility of
managing the different namespaces to the clients which could address the complexity issues with existing software libraries and components. The other solution would be
URL-based resource management e.g. one specific URL for managing each subscription. This would foster the interoperability among different Event Service
implementations, but would also imply a modification of the underlying communication binding at the Event Service.
9.5.3 Additional Observations
The Digital NOTAM Event Specification states that “Missing a Digital NOTAM may be safety critical”. Regarding the Event Architecture there can be cases that a client who
receives DNOTAMs using subscriptions at an Event Service misses a DNOTAM due to the filter of the subscription. For instance, an Event had been published to create an ad-
hoc restricted airspace and the included spatial extent does not match the spatial filter criteria of the subscription. Due to a mistake in the definition of the spatial extent of this
wsnt:SubscribeResponse xmlns:wsnt=http:docs.oasis-open.orgwsnb-2 xmlns:wsa=http:www.w3.org200508addressing
wsnt:SubscriptionReference wsa:Addresshttp:v-tml.uni-
muenster.de:8080EventServiceservicesSubscriptionManagerContextPathwsa:Address wsa:ReferenceParameters
es-wsa:ResourceId xmlns:es- wsa=http:www.opengeospatial.orgprojectsinitiativesows-8es
Resource-7 es-wsa:ResourceId
wsa:ReferenceParameters wsnt:SubscriptionReference
wsnt:CurrentTime2011-08-17T12:47:33+02:00wsnt:CurrentTime wsnt:TerminationTime2011-08-17T13:47:33+02:00wsnt:TerminationTime
wsnt:SubscribeResponse
Copyright © 2011 Open Geospatial Consortium,
Inc. All Rights Reserved.
65
Event an additional Event for correcting the geometry is being created. The corrected geometry now matches the filter criteria of the client’s subscription but the client cannot
resolve the reference to the former published ad-hoc airspace feature. Although this situation presumably is a rare case the issue should be discussed and a feasible solution
needs to be developed within the Event Architecture as the issue can affect security.