SANY D2.3.4 Specification of the Sensor Service Architecture V3 Doc.V3.1
Copyright © 2007-2009 SANY Consortium Page 61 of 233
interaction model may be applied in the sensor domain of the SensorSA functional domains see Figure 6-1 and implemented by broadcast or multicast communication protocols in
sensor networks. However, these communication protocols are conceptually out of the scope of the SensorSA., because, as explained in section 2.1, the SensorSA specification is
independent of the specifics of a particular service platform.
6.3.3 Event-based Interaction Model
The event-based interaction model represents the basic form of interaction for cases in which timely delivery of observed actions is important but needs to be flexible. It is usually applied
in event-driven processing systems as defined in section 6.4.5. Flexibility and adaptability are among the key characteristics of event-driven processing systems, because event
generators don‟t call any specific type of event receivers. Indeed, they don‟t even need to know them.
Events will be more fully defined in section 6.4, but the event-based interaction model relies on two basic concepts:
-
an event that describes any happening of interest i.e. anything that happens, or is contemplated as happening, and
-
a notification, that transports the reified happening of interest.
In the event-based interaction model the initiator is the provider of the data, i.e. the
producer of notifications. The essential characteristic of this interaction model is that producers do not need to know any consumers. Thus the addressing scheme is indirect, which
means that the notifications are not addressed to any specific set of recipients but instead are mediated by a broker component which offers a notification service. A consumer may express
its interest in notifications by subscribing to the notification service.
The SensorSA provides the means and mechanisms to define, generate, distribute, receive, and process events. Three causes of events are observed most frequently:
1. Events based on singular observations made by a single sensor, 2. Events based on aggregated observations made by one or multiple sensors, and
3. Events related to the operation of the sensor network or the sensor services. The first type of event occurs if a sensor detects something that matches a previously
defined event condition. The occurrence may take place in the environment of the sensor or internally. Examples are a temperature value that exceeds a threshold, the detection of hotspot
pixels in a remote sensing image, or low battery power of a sensor.
The second type of event occurs if non-atomic conditions occur, e.g. both temperature and wind speed observation result values exceed thresholds. Events may be based on
conditions that remain for a well-defined number of time intervals, e.g. temperature exceeds a threshold for n time intervals continuously also known as time series analysis based events.
In a common example, one event is produced when e.g. the temperature exceeds a threshold the first time. A second but different event is produced if the temperature again falls below the
threshold. In this case the two different events follow a state change in the sensor.
SANY D2.3.4 Specification of the Sensor Service Architecture V3 Doc.V3.1
Copyright © 2007-2009 SANY Consortium Page 62 of 233
The third type of event occurs when some state has changed in the sensor or service network configuration e.g. addition of a new sensor or sensor service instance or some
unforeseen behaviour has been detected. The latter situation usually results in an exception on the software level. If deemed essential by the software engineer, such exceptions may be
escalated to other components in the form of events.
In the context of the SensorSA, all event types are handled equally. The event type is transparent to the receiver of an event notification. It is created by the event observer and
published or transferred to notification consumers. Still, the SensorSA addresses a very heterogeneous environment with sensors and
services provided by a number of institutions and organisations. The event-based architectural style of the SensorSA defined below takes these aspects into account.
6.4. Event-based Architectural Style
