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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Schema vs Schema-free (was: RE: XML Binary Characterization WG
 Elliote Rusty Harold wrote: > this does rule out a number of the proposals that have > been made, particularly those that try to represent > numbers and similar things as binary rather than text, > and thus lose the distinction between 1, 0001, and +1. Certainly, it is sometimes important to ensure that leading zeros, gratuitous "+" signs, etc. are passed through systems untouched. However, it is important to understand that the importance varies depending on whether or not you are working with a schema. In a schema based system, if a field is defined as having "integer" type and if the definition of integer states that "1," "00001", "+001", and "+1" are equivalent, then no one should feel compelled to preserve any syntactic sugar unless there is some other normative information that requires it to be preserved. Basically, the author of the schema, in defining the schema and using type definitions that establish an equivalence between representations, has implicitly "authorized" the substitutions. On the other hand, in a schema-free system, there is no way for a processor to "know" that something "is" a number. In the absence of a schema, or some other normative document explicitly defines equivalent representations of a value, a processor of the data should faithfully pass the data, byte-for-byte. Thus, in a schema-free system, "1," "00001", "+001", and "+1" must be considered as distinct, non-equivalent, and non-substitutable values. On the other hand in a schema-based system, there is no problem with substituting one form for another as long as the schema permits it. Those who try to apply the rules of schema-free systems to schema-based systems are doing something very, very dangerous. Because "1" and "001" mean the same thing in a schema-based system, those who argue that the precise original forms of value should be preserved are probably under the false impression that there is some semantics associated with those forms. Yet, unless the schema defines such semantics, there is none and any assumptions concerning such unspecified semantics are likely to be proved wrong -- potentially with disastrous impacts. These disasters often happen when people get sloppy in their interpretation of schemas. For instance a company might have a 5 digit "employee" code that should have leading zeros. Some sloppy coder might decide to map this to an integer... Of course, it is inevitable that some down-stream code will look at an integer like "00001" and convert it to "1"... (i.e. treat it like the schema says it can be treated.) The results could be unpleasant. Of course, what the coder *should have done* was recognize that the employee code is *NOT* an integer, rather it is a five digit value that is composed only of numeric characters and should be padded with leading "0" characters. The field should be defined as such and appropriate types provided in the schema. The distinction between these two environments is recognized in the distinction between the ASN.1 based "Fast Schema" and "Fast Infoset" systems. Fast Schema is only to be used when a schema is present and, depending on the schema, will convert things that are declared to be integers to binary forms with no guarantee of preserving distinctions between things like "1", "001" and "+1". On the other hand, "Fast Infoset" is for use in schema-free processing. Thus, Fast Infoset (X.finfo) would preserve the exact forms of "1", "001", and "+1". This is how it should be. Rules which apply to schema-based systems do not necessarily apply to schema-free systems. Sometimes, but not always, "1," "001," and "+1" are really the same value. bob wyman 
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