120
Copyright © 2010 Open Geospatial Consortium, Inc.
These lists of values are hard-coded into the schema significantly reducing the extensibility of the model. Also, the particular vocabulary used is focused on emergency
management which is fine if that is the intended context within which the notification messages are being used. Otherwise, the enumeration value
Other
tends to be used a lot significantly diminishing the usefulness of a parameter like
category
. This was certainly the case when CAP was used in the GSS experiment.
A better approach would have been to use code lists similar to the approach used in GML with the
gml:CodeType
type. This allows lists of values to be dynamically referenced allowing the vocabulary to be adjusted to fit the particular context. Also, from the point
of view of implementing a generic CAP client, code lists allow such clients to read these lists and generate lists at run-time thus adjusting the client to the context.
12.2.9.5 Resource references
The CAP message format defines the
cap:resource
element which may be used to embed or reference resources related to the notification. This was intended to allow references to
images, audio andor video content but was used effectively during the GSS experiment to reference, for example, modified features. The element could also contain a more
detailed representation of the event that the CAP alert summarizes information of.
12.2.9.6 Other features
The following list describes features of CAP that are anticipated to be useful in a general eventing environment but which were not exercised by the GSS during the OWS7 test
bed:
Multilingual support - alert messages can include the same information in multiple languages
Multi-audience support - alert messages can be targeted to multiple audiences Message management - the protocol defines procedures for updating and
canceling previously issued alerts Time - alert messages include onset and expiry times which allow for phased of
delayed alerts to be signaled
12.2.9.7 Interoperability
Interoperability with respect to different ways of encoding notifications can be achieved by using a gateway that provides uniform access to various notification technologies - the
Web Notification Service OGC best practice could become such a gateway right now it is limited, unfortunately. The gateway can then tie into emergency alert middleware that
uses CAP to route alert messages to recipients.
Copyright © 2010 Open Geospatial Consortium, Inc.
121
12.2.9.8 Geosynchronization
One of the services in the OWS-7 test bed that implemented notification was the Geosynchronization service. The GSS uses ATOM as the preferred event encoding
format but the specification does not specifically mandate which format shall be used to encode notification messages sent to subscribers.
During the OWS 7 test bed, both ATOM and CAP were used for encoding notification messages sent to subscribers. The choice of which format a subscriber would receive was
made using an outputFormat parameter on the subscribe request.
The following listing illustrates a notification message, encoded using CAP, generated by a GSS and sent to a subscriber when a new event i.e. entry appears in one of the
services event channels i.e. feeds:
