WFS Support for Dynamic Features

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.