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Figure 34: Spatial filtering of events with dynamically computed filter geometry
As we can see in Figure 34, the aircraft will get events from the destination airport as well as alternative airports that are used in case that some event prevents the aircraft from
landing at the planned destination. In addition, events from some airports en route are received
– these airports are used for emergency landings. As the aircraft progresses on its route, the filter geometry buffer around the flight path is updated and its size
reduced. Once the aircraft has made enough progress, the updated filter ensures that events from airports that are no longer relevant e.g. the airport en route are not reported
anymore.
Obviously, using filters with dynamically updated filter properties can lead to significant savings in both computing and transmission resources. Dynamic filter properties are
supported through Event Processing. In the use cases described above, the event stream that contains the ve
hicle’s position updates would provide the information to update the filter geometry. Timer information can govern when the geometry the buffer in Figure
34 is updated. Event streams from other sources can then be filtered with the automatically updating geometry.
In OWS-7, the use case described above was applied in the Aviation thread. A static filter geometry was used, not a dynamically updating one. Other domains, for example the
Copyright © 2010 Open Geospatial Consortium, Inc.
103 Sensor Web domain, may also benefit from filter statements with dynamically updated
properties. The integration of Event Processing functionality in OGC services should therefore be pursued in the future. Some work in this direction has already been
performed – see section 9.2.3. It can serve as the basis for further developments.
9.4 Discovery of filter functionality
Various aspects are of interest when it comes to discovering which filter functionality is offered by a service. An Event Service needs to:
indicate which filter languages it supports indicate which functionality of a filter language is supported
indicate which additional capabilities are supported for a given filter language e.g. extensions to the filter functionality
– like automated unit conversion A service registry should harvest or be populated with the metadata of a service that
contains this information. Some initial work was performed on the discovery of Event Service’s filter functionality. The results are part of what is described in chapter 7.
Basically, a registry was populated with a model for the OGC Filter Encoding Specification’s
Filter_Capabilities
element. Sample data was used to search for event services that support certain event channels as well as certain filter functionality see for
example Listing 3.
10 Publishsubscribe specification guidelines
This chapter is about providing guidance and best practice on how to realize required parts of the Event Architecture in OGC documents and standards. The aim is to ensure
that such documents consider all relevant information and document it in a common and non-redundant fashion.
The following sections provide initial guidelines on specifying events and event channels. Work in this area was started in OWS-7
– feedback from the community will help improve the specification guidelines.
Future work on publish subscribe specification guidelines also needs to describe which information has to be provided by a specification to enable a chosen realization
technology.
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10.1 Specifying events
For specifying new events, the relevant information is: required:
o a unique identifier for the event o an exact definition of the event
optional: o a list of names known to be used by certain domains to label the event
o a list of suitable encodings
Note: different aspects of an event may be picked up by different encodings various application schema may document the same event differently.
The following tables can be integrated in specifications to define events.
Table 39: Template for event definition table
Event Identifier
a
Definition
identifier-value
Recommendation: use URL – like
http:www.opengis.netowseventsX.0EventX
Other option: combination of code space URL and name value that can easily be concatenated
to a single URL – like
http:www.opengis.netowseventsX.0
code space and
EventX
name value
Precise textual definition of the happening.
a Although some values listed in the column appear to contain spaces, they shall not contain spaces.
