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OPC is a service API providing access to data managed by the server. The data (for example, nodes, items, areas, sources,
conditions, reasons) are not instantiated as objects at the client. This means that OPC does not define any particular APIs
for the data instances that a client can deal with directly. OPC has a few coarse objects like OPCGroup and OPCEventSubscription
supporting the data access. DAIS has adopted this principle and is identical to OPC in this respect.
In OPC each client has its own OPC server object. In DAIS there is only one DAIS server object shared by all clients. To support
individual client sessions a new session interface is defined. There is one interface for data access session objects and
one for alarms & events session objects. The session objects correspond to the OPC server object.
The OPC server object has interfaces for browsing server data (for example, IOPCBrowseServerAddressSpace and IOPCEventAreaBrowser)
and information model (for example, IOPCItemProperties). In DAIS each type of data has its own object for browsing, these
objects are called “home? objects. The OPC browse interface methods are hence divided among the different home objects.