fixed in version 1.01.06

Should ArrayDimensions be an Array or Property?

Is there a reason to make it a property?

How much code do you have to change?

For telco 2007-10-09 an attribute seems suitable.

From an information model perspective there is no difference between Attributes and Properties: both define a Facet that describes a node. However, UA introduced the concept of Attributes because it was felt that application developers would benefit if some Facets had a more efficient access mechanism. The Attributes that exist today fall into three groups:

1) Facets which are intrinsic to the definition of the NodeClass (i.e. the NodeClass would not make sense if these Facets did not exist). NodeId, NodeClass, BrowseName, DisplayName and Value are examples of intrinsic Facets.

2) Facets which are accessed frequently by almost every application and would benefit from an optimized access mechanism. AccessLevel, DataType and EventNotifier are examples of frequently used Facets.

3) Facets which are absolutely required to describe Property Nodes since Properties cannot have Properties (a restriction which allows developers to better optimize their implementations of Properties). MinimumSamplingInterval and Historizing are examples of Property qualifying facets.

In other words, Attributes are a mechanism to facilitate optimization. For this reason, the set of Attributes must be small and not extensible. Developers are expected to take advantage of this fact when they write their applications. This also means that the overhead associated with supporting new attributes will likely be much larger because developers may hardcode support for the known Attributes into their applications instead of relying on dynamic lists which provide flexibility at the cost of performance.

For this reason, we must consider whether the Attribute meets the criteria above when we look at a proposed attribute such as ArrayDimensions.

In this case, the ArrayDimensions attribute is not intrinsic to the node nor is it used frequently by many applications because many servers will not be able to provide a value for it. The ArrayDimensions is also not required for Properties since it is optional and a server can always use a DataVariable instead of a Property. In fact we have already decided that servers must use DataVariables when exposing information such as EURange, EUInformation or EnumStrings even though we know those values will be used much more frequently than the ArrayDimensions value. For that reason, I feel that ArrayDimensions should be a Property instead of an Attribute.

That said, ArrayDimensions could be a legimate attribute if there are additional use cases that demonstrate why this value is important enough to justify the additional overhead that will need to be written into applications to support the attribute.

Agreed to leave ArrayDimensions as an attribute in today's telecon.