Figure 1: Event Model (Class Diagram)
How to publish event streams as Linked Data.
Increasingly, real-time processing is becoming important and sites provide direct access to data streams, which provide data as soon as the data appears. For example, Twitter provides API access to a stream of Twitter messages in real-time. In addition, with the proliferation of sensors, more streaming data sources will become available. Simultaneously, there is an increasing amount of public data on the Web which can be linked to and reused in RDF. This means that a lot of static data is available to be used as context of events.
N-ary predicates. [N-ARY-REL]
Need a class event, time stamp (atomic or interval).
Data sources:
Triples/Quads occur once in NiceWeatherStream and once in historical data source for events.
An event is defined as:
(optional) (either lat/lon or URIs to locations which contain descriptions of geometry)
The NeoGeo vocabulary [NEOGEO] and [BASIC-GEO] could be a starting point:
e:e1 {
<http://events.event-processing.org/ids/e1#event>
:location [ geo:lat "53.27194"^^xsd:double; geo:long "-9.04889"^^xsd:double ] .
}
Figure 1: Event Model (Class Diagram)
Figure 1 shows the event model in a class diagram. The class "Event" is the superclass for any event to conform to our model. This class makes use of related work by inheriting from the class "DUL:Event" from Dolce Ultralight [DUL]. That class provides a notion of time and helps distinguish events (things that happen) from facts (which are always valid).
An instance of class Event MUST have (i) a type, (ii) at least one timestamp and (iii) a relevant stream. We describe the event properties in detail as follows.
The type of an event must be specified using rdf:type. The type must be the class Event or any subclass.
The event model supports interval-based events as well as point-based events by either using just the property :endTime for a point or both :startTime and :endTime for an interval. The property :endTime thus has a cardinality of [1..1] whereas :startTime has a cardinality of [0..1]. Both temporal properties are subproperties of DUL:hasEventDate from the super class. We improve the semantics by distinguishing start from end whereas the superclass has an alternative, more difficult way of formulating intervals using subobjects reifying the interval.
The property :stream associates an event with a stream. Streams are used in our system as a unit of organisation for events governing publish/subscribe and access control. Streams themselves are modelled using title, description and a topic needed for topic-based publish/subscribe.
The first optional property is :location For for geo-referencing of events (where necessary) we re-use the basic geo vocabulary from the W3C [BASIC-GEO]. The property may be used to locate events in physical locations on a globe. The property is subproperty of DUL:hasLocation and geo:location to inherit the semantics from those schemas.
Inter-event relationships may be supported by linking a complex event to the simple events which caused it. Thus, RDF Lists may be used in :members to maintain an ordered and complete account of member events. The linked events are identified by their URI. These linked events could have further member events themselves. This allows modelling of composite events [EP-GLOSS]. The :members property is a subproperty of DUL:hasConstituent from the superclass.
The property :eventPattern may be used to link a complex event to the pattern which caused the event to be detected. Using such links can help in recording provenance of derived events.
The source of an event may be specified using the :source property. This is an optional property to record the provenance of a event where needed. The property is a subproperty of DUL:involvesAgent. Agents may be human or non-human.
A human readable synopsis of an event may be added using the :message property. This proves useful in scenarios where events are received by human end users. The :message property is a subproperty of dc:title, a popular way of describing things using natural language.
@prefix : <http://events.event-processing.org/types/> .
@prefix dbpedia: <http://dbpedia.org/resource/#> .
@prefix e: <http://events.event-processing.org/ids/> .
@prefix geo: <http://www.w3.org/2003/01/geo/wgs84_pos#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix s: <http://streams.event-processing.org/ids/> .
@prefix src: <http://sources.event-processing.org/ids/> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
e:e1 {
<http://events.event-processing.org/ids/e1#event>
a :AvgTempEvent ;
:endTime "2011-08-24T14:42:01.011"^^xsd:dateTime ;
:members (<http://events.event-processing.org/ids/e2#event> <http://events.event-processing.org/ids/e3#event>) ;
:source <http://sources.event-processing.org/ids/NiceWeatherAggregator#source> ;
:startTime "2011-08-24T14:40:59.837"^^xsd:dateTime ;
:stream <http://streams.event-processing.org/ids/NiceWeatherStream#stream> ;
dbpedia:Nice :avgTemp [ rdf:value "25" ; :event <http://events.event-processing.org/ids/e1#event> ] .
}e1 as part of its URI. There is a distinction made between the event e1:event and the Web resource e1 to separate the URI for the thing event from the information resource describing the event. The fragment identifier #event is used to identify the event.
:members to maintain ordered and complete account of related events.
dbpedia:Nice to maintain knowledge about a temperature measurement and at the same time its belonging to a specific event.
:AvgTempEvent is inherited. This hierarchy can be extended by anyone by referencing the RDF type Event as a super class.
e2:event, e3:event in different streams which were used as input to create the :AvgTempEvent. These events are depicted below. They could have further input events themselves.
dbpedia:Nice from Linked Data where further context for the event can be retrieved.
e:e1#evente1:event which avoids defining a new namespace e1, e2, e3, ..., eN for each unique event .
event-processing.org was chosen as a generic home for this schema.
e:e2 {
<http://events.event-processing.org/ids/e2#event>
a :TempEvent ;
:endTime "2011-08-24T14:40:59.837"^^xsd:dateTime ;
:source <http://sources.event-processing.org/ids/NiceWeatherStation01#source> ;
:stream <http://streams.event-processing.org/ids/NiceTempStream#stream> ;
dbpedia:Nice :curTemp [ rdf:value "23" ; :event <http://events.event-processing.org/ids/e2#event> ] .
}
e:e3 {
<http://events.event-processing.org/ids/e3#event>
a :TempEvent ;
:endTime "2011-08-24T14:42:01.011"^^xsd:dateTime ;
:source <http://sources.event-processing.org/ids/NiceWeatherStation02#source> ;
:stream <http://streams.event-processing.org/ids/NiceTempStream#stream> ;
dbpedia:Nice :curTemp [ rdf:value "27" ; :event <http://events.event-processing.org/ids/e3#event> ] .
}Streams make use of namespace prefixes to reduce the number of characters to be transmitted.
Further bandwidth savings can be achieved by gzipping the HTTP stream.
Another option could be a binary RDF serialization like [HDT-ISWC2010].
Changes since 2012-11-27:
<dependency>
<groupId>eu.play-project</groupId>
<artifactId>play-commons-eventtypes</artifactId>
<version>1.1</version>
</dependency>
<repository>
<id>ow2.release</id>
<name>OW2 Maven Releases Repository</name>
<url>http://repository.ow2.org/nexus/service/local/staging/deploy/maven2</url>
</repository>