hierarchies. This is especially useful, if information in the IoT system is to be
modelled using ontologies.
Service descriptions
Services provide access to functions for retrieving information or executing
actuation tasks on IoT Devices. As a basis for finding and interacting with
services, services need to be appropriately described, which is done in the form
of Service Descriptions. Service Descriptions contain information about the
interface of the service, both on a syntactic as well as a semantic level, e.g. the
required inputs, the provided outputs or the necessary pre-conditions as well as
post-conditions. Furthermore, the Service Description may include information
regarding the functionality of the resources, e.g. the type of resource, the
processing method or algorithm etc., or information regarding the device on
which the resource is running, e.g. it‘s hardware or its geographical location.
Different specification languages for describing services are available, so again,
there are different design choices.
Associations between Virtual Entities and Services
Services can provide information or enable actuation, but the services
themselves may not be aware of e.g., which Virtual Entities can provide what
information or can enable what kind of actuation. This information is captured by
associations that relate to the Virtual Entity and the Service. The association
includes the attribute of the Virtual Entity for which the Service provides the
information or enables the actuation as a result of a change in its value.
4.2.3.2 Information handling
Information in the system is handled by IoT Services. IoT Services may provide
access to On-Device Resources, e.g. sensor resources, which make real-time
information about the physical world accessible to the system. Other IoT
Services may further process and aggregate the information provided by IoT
Services/Resources, deriving additional higher-level information. Furthermore,
information that has been gathered by the mentioned IoT Services or has been
added directly by a user of the IoT system can be stored by a special class of
IoT Service, the history storage. A history storage may exist on the level of data
values directly gathered from sensor resources as a resource history storage or
as a history storage providing information about a Virtual Entity as a Virtual
Entity history storage.
IoT Services are registered to the IoT system using Service Descriptions.
Service Descriptions can be provided by the services themselves, by users or
by special management components that want to make the service visible and
discoverable within the IoT system. The IoT Service Resolution is responsible
for managing Service Descriptions and providing access to Service
Descriptions. In detail, the IoT Service Resolution provides an interface for
discovering Service Descriptions based on service specifications given by the
requestor, for looking up a Service Description based on the identifier of a
service and for resolving a service identifier to a service locator. The latter can