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A.  XFN Composite Names Composite Name Resolution Explicit NNSPs: Junctions  Previous   Contents   Next 
   
 

Implicit NNSPs

When a context does not want to use part of its namespace for junctions, it uses implicit NNSPs for federating subordinate naming systems. An implicit NNSP is named using the XFN component separator. For example, the name wiz.com/ names the implicit NNSP of wiz.com. Each context can have one implicit NNSP.

Composite name resolution involving implicit NNSPs proceeds as follows, depending on whether the context supports strong or weak separation.

A context that supports strong separation and resolves composite names using an implicit NNSP consumes the first component of the composite name supplied to it. Any remaining components are resolved in the context pointed to by the implicit NNSP of the first component.

A context that supports weak separation and implicit NNSPs in its implementation needs to distinguish the use of the XFN component separator character as an XFN component separator or an atomic separator. This means that such a context needs to know when to exit the current (native) naming system and follow the NNSP. This can be achieved using a static, syntactic policy or a dynamic, resolution-based policy.

With the syntactic policy, a context syntactically discovers where the boundary between its naming system and the subordinate naming system files. This can impose certain restrictions on the syntax of subordinate naming systems. Subordinate naming systems must not permit, as valid top-level names, that are syntactically indistinguishable from names allowed in the superior naming system. For example, assume the superior naming system has a name syntax whose distinguishing feature is that each atomic part must have an equal sign (=). The superior naming system might impose as a policy that subordinate naming systems must not have top-level names with an equal sign in them. Resolution in the superior naming system continues until all leading components of the supplied composite name fitting the syntactic rule are consumed. Any remaining components are resolved in the context of the NNSP of the last component fitting the syntactic rule.

If a context is not able to syntactically differentiate between atomic components and composite name components, or does not want to impose any syntactic restrictions, it might be able to determine the naming system boundary at runtime, during resolution. The policy is to continue resolution in the current naming system until resolution fails, at which point the implicit NNSP associated with the last context at which resolution succeeded is used to continue the resolution. A conflict arises if the same atomic name is bound both in the last context and the context pointed to by the last context's implicit NNSP. In this case, the binding in the last context takes precedence. This way of supporting weak separation requires the context to have the capability of returning remaining unresolved parts of a given name.

Coexistence of Explicit and Implicit NNSPs

Naming systems that implement either technique can coexist in a federation. A naming system that supports composite name resolution using junctions can be federated with one that supports implicit NNSPs, and vice versa.

XFN Links

An XFN link affects name resolution in the following way. Suppose lname is a link bound to the atomic name aname in the context ctx. If at some point resolution of a composite name cname reaches the context ctx and the next atomic name is aname, resolution of aname results in the resolution of the link name lname. This is termed "following the link." If the first component of the link lname is the atomic name ".", the remaining components of lname are resolved relative to ctx; otherwise, lname is resolved from the initial context. The resolution of any remaining portion of the name cname proceeds from the reference that results by resolving lname.

The link name can itself cause resolution to resolve through other links. This gives rise to the possibility of a cycle of links whose resolution could not terminate normally. As a simple means to avoid such nonterminating resolutions, implementations can define limits on the number of XFN links that can may be resolved in any single operation invoked by the caller.

Composite Name Encoding

All XFN implementations are required to support the ISO 646 portable representation (same encoding as ASCII) for XFN composite names. All other representations are optional.

All characters of the string form of an XFN composite name use a single encoding. Characters with different encodings cannot exist in the same name string. This does not preclude component names of a composite name in its structural form from having different encodings. Code set mismatches that occur during the process of converting a composite name structure to its string form are resolved in an implementation-dependent way. Strings with code sets that are determined by the implementation to be compatible are converted without loss of information into a single representation, which is also determined by the implementation. When an implementation discovers that a composite name has components with incompatible code sets, it returns the error code FN_E_INCOMPATIBLE_CODE_SETS.

Backus-Naur Form (BNF)

The following defines the standard string form of XFN composite names in Backus-Naur Form (BNF). All the characters of the string representation of one name must uniformly use the same encoding and locale information. The notations used are as shown in Table A-2:

Table A-2 Backus-Naur Notation

Symbol

Meaning

::=

Is defined to be

|

Alternatively

<text>

Nonterminal element

`' `'

Literal expression

*

The preceding syntactic unit can appear 0 or more times.

+

The preceding syntactic unit can appear 1 or more times.

{}

The enclosed syntactic units are grouped as a single syntactic unit (can be nested).

The XFN composite name syntax in BNF is shown in Table A-3:

Table A-3 XFN Composite Name Syntax Using BNF

XFN Composite Name

BNF Syntax

NULL ::=

// Empty set

<PCS> ::=

// Portable Character Set
The set consists of the glyphs:
//!"#$%&'()*+,\0123456789:;<=>? 
//@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_'
//abcdefghijklmnopqrstuvwxyz{|}~

<CharSet> ::= 

<PCS>
| Characters from the repertoire of a string representation

<EscapeChar> ::=

\

<ComponentSep>::=

/

<Quote1>::=

"

<Quote2> ::=

`

<MetaChar> ::=

<EscapeChar> | <ComponentSep>

<SimpleChar> ::= 

// any character from <CharSet> with <MetaChar>, <Quote1>,
// and <Quote2> excluded. An<EscapeChar> <MetaChar>, or
// <EscapeChar> <Quote1>, or <EscapeChar> <Quote2> is equivalent to
// <SimpleChar>.

<Component> ::=

<SimpleChar>*
| <SimpleChar>+ {<Quote1> |  <Quote2> | <SimpleChar>}*
| <Quote1> <CharSet>* {<EscapeChar> <Quote1>}* <Quote1>
 // <CharSet> must not contain unescaped <Quote1>
 // (note that <Quote2> can appear unescaped)  
| <Quote2> <CharSet>* {<EscapeChar> <Quote2>}* <Quote2>
 // <CharSet> must not contain unescaped <Quote2>
 // (note that <Quote1> can appear unescaped)

<CompositeName> ::= 

NULL
| <Component> {<ComponentSep> <Component>}*

Decomposing the Composite Name String

The function fn_composite_name_from_string() returns an XFN composite name in its structural form, FN_composite_name_t, given the composite name's string representation. The syntax rules used by fn_composite_name_from_string() are as follows:

  • An XFN composite name is decomposed into an ordered set of components (<Component>).

  • Each component represents a compound name, or a single atomic name of a compound name if the compound name's syntax uses the XFN component separator (/) as a separator for its atomic parts and the compound name is not quoted.

The following are the rules for parsing a composite name.

  1. Any <ComponentSep> character that is neither escaped nor enclosed in quoted strings is considered to be a component separator.

  2. Any string enclosed by component separators is a component (<Component>).

  3. A composite name is parsed and decomposed into components from left to right:

    1. The first component is the string preceding the first occurrence of a component separator.

    2. Empty components are processed as follows:

      1. A leading component separator (the composite name begins with a component separator) means a leading null component.

      2. A trailing component separator (the composite name ends with a component separator) means a trailing null component.

    3. Two consecutive component separators mean a null component.

    4. The name string that immediately follows the last component separator of the composite name is the final component.

  4. A component string is evaluated from left to right and converted into its standard form according to the following rules:

    1. A component string is considered to be quoted if it is enclosed in a pair of matching unescaped quote characters (either a <Quote1> or a <Quote2> pair). The quoted string must represent the full component; that is, a begin quote must immediately be preceded by a component separator or no character, and the end quote must immediately be followed by a component separator or no character.

    2. If a component does not contain a valid begin quote (a <Quote1> or <Quote2> immediately preceded by either a component separator or no character), any occurrence of <Quote1> or <Quote2> within that component is treated just as any other <SimpleChar>.

    3. An unmatched begin quote (missing or misplaced end quote) fails with an FN_E_ILLEGAL_NAME status.

    4. Quotes are considered to be escaped in quoted strings if a matching quote character is preceded immediately by the unescaped <EscapeChar>.

    5. Quoted components are resolved by eliminating the quote characters from the component name and substituting possibly escaped quotes by simple quote characters. <MetaChar>s and the nonmatching quote characters enclosed in quoted strings are treated just as any other <SimpleChar>.

    6. Any of the defined metacharacters (<ComponentSep> and <EscapeChar>) is considered to be escaped in an unquoted component name string if preceded immediately by the unescaped <EscapeChar> (for instance, the sequence <EscapeChar> <EscapeChar> <ComponentSep> denotes an escaped <EscapeChar> but an unescaped <ComponentSep>).

    7. <Quote1> and <Quote2> are considered to be escaped in an unquoted component if and only if <EscapeChar> is preceded by a component separator (that is, sequences <ComponentSep> <EscapeChar> <Quote1> or <ComponentSep> <EscapeChar> <Quote2>). Other occurrences of <Quote1> and <Quote2> in an unquoted component are treated just as any other <SimpleChar>.

    8. Any occurrence of escaped <MetaChar>, escaped <Quote1>, or escaped <Quote2> in unquoted components is substituted by the corresponding unescaped character.

    9. No substitution is done for <EscapeChar> <SimpleChar>. <EscapeChar> <SimpleChar> maps to <EscapeChar> <SimpleChar>.

 		 
 
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