4.2.4.1 DAIS::Item Overview

   An item is a property of a node. While a node generally represents a real-world object, an item represents some characteristic of that object (for example, measurement value, control variable, or parameter related to the measurement/control process). The concept of an item in DAIS corresponds to the item in OPC. More precisely, it corresponds to the “leafs? in the OPC IOPCBrowseServerAddressSpace interface. In the Resource Description Framework (RDF) data model, the item corresponds to a combination of a subject and a predicate; that is, a resource and a property.

   A node may have many items, each representing a different characteristic of the same real world object. An item might represent a measured variable, a calculated variable, a control variable, or a configuration parameter. An item will typically have many values where each value corresponds to a time stamped sample (a single item value corresponds to a single statement in the RDF model). Each sample will also have its own quality. An item value is then qualified by a time stamp and a quality.

   An item value and its qualifications are represented by six fixed attributes:

• value - the value.

• time_stamp - the time when the value was last updated.

• quality - the quality of the value.

• canonical_data_type - the data type of the value. This actually belongs to the property (see Property) but it is mirrored at the item.

• access_rights - tells if the value is read only, write only, or both read/write. The access right is common for all item values and belongs to the item.

• scan_rate - the fastest rate with which the value can be expected to be updated. The scan rate is common for all item values and belongs to the item.

    For an information model describing this refer to Section 4.1.1, “Nodes, Items, Types, and Properties,? on page 4-2 and Section 4.1.3, “Item Values,? on page 4-4.

   Each item has a universal identity given by its ItemID. The ItemID is made up of the ResourceID of a node and the PropertyID of a property. The ItemID of an item is the same in all views provided by a DAIS server. Clients may construct ItemIDs given the identities of nodes and properties.

   DAIS servers may be coordinated with DAF servers so that valid ItemIDs can be constructed from DAF ResoureIDs.

   Within each view provided by the server, an item has a label that is unique among all items belonging to the same node. Within each view, an item has a unique pathname. The pathname is a string that contains the item’s label and the pathname of its node.

   The pathname must be a valid URI, but apart from that the syntax of pathnames is implementation dependent.

   Note – In OPC the pathname is the primary way to identify items and is called the ItemID. In DAIS the ResourceID, PropertyID pair is the primary identification and so it is called the ItemID. This is a potential point of confusion. The two ItemIDs play approximately the same role in OPC and DAIS respectively, but they are not the same type.

   The item interfaces permit the stock of items to be browsed. Once an item is located, the group interfaces are used to deliver its values, selected by source, at successive times. In addition, the item interfaces provide the most current value for the default source. In either case the item value and qualifications are represented by the six fundamental attributes listed above.

   The Item IDL defines a main interface Home for browsing among hierarchically structured items (leaf nodes). The information model describing the hierarchical structure is found in Section 4.1.1, “Nodes, Items, Types, and Properties,? on page 4-2 .

   Figure 4-9 DAIS data access item IDL in UML