The Information Model and its renderings MUST treat strings, both entity and attribute names and their values, as being case-sensitive. Each GLUE entity MUST have an ID attribute (an exception is made for the Extension class) which is needed for identification or for access to the attributes of the related entity over time and across different information sources. As a general guideline, ID's SHOULD be persistent at least for a day when assigned to an entity. The ID MUST NOT be interpreted by the user or the system as having any meaning other than an identifier. In particular, there is no relationship between an ID and a network endpoint. Every ID MUST be a valid URI. The usage of URN (Uniform Resource Name, a subset of Uniform Resource Identifier or URI) is RECOMMENDED. The motivations for choosing URI’s reside in the fact that Grid services are evolving towards Web-based technologies, therefore it is meaningful to adopt the same identification system.
As regards units of measure, multiples of bytes MUST refer to the SI (Le Système International d'Unités) prefix ([http://en.wikipedia.org/wiki/SI_prefix](http://en.wikipedia.org/wiki/SI_prefix\)\), therefore GB is 10^9 Bytes and not 2^30 Bytes (the latter are GibiBytes).
In Appendix A, we provide guidelines for place-holder values that MUST be used when the attributes have no good default value or when the attribute cannot be measured for some reason.
As regards extensibility, two main approaches are introduced to extend the information associated to the existing classes: the OtherInfo attribute and the Extension class. The OtherInfo attribute is present in the Entity class, therefore it is inherited by all GLUE classes. Its type is string and its multiplicity is *. This SHOULD be used for associating a flat list of tags to a certain class instance. The Extension class is associated to the Entity class (and therefore also to all the derived classes) and enables linking key/value pairs to any GLUE class instance. This SHOULD be used when there is the need for advertising more structured information, for instance an attribute not present in the model with a related value.
Both solutions are proposed because they have a different impact in the implementations: the OtherInfo approach is easier to query, nevertheless it MAY require parsing in case of concatenation of different chunks of information (e.g., attribute name and attribute value). The Extension class offers a two-dimensional construct, but nevertheless it is more complex to query.
The extensibility regarding the addition of new classes and associations is not supported at the conceptual level. We RECOMMEND to create specializations of the conceptual model and to implement them by extending the concrete data models. Such extensions MUST NOT be considered part of the GLUE specification, but nevertheless we RECOMMEND submitting them to the GLUE WG for consideration in future revisions of the specification.