Dev:identities

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railML® Identities

Very many railML® elements require an id attribute of the W3C-Type xs:ID.

This has to be provided in order to enable references, that are very much used within the railML® schemas.

On the other hand, software tools may refer to certain railML® elements, that are not referred to within the railML® file.

Semantics

The general railML® description states the following semantics for this attribute.

  • id: XML-file-wide unique, machine-interpretable identity, required for later referencing that element internally. For a detailed explanation see Dev:identities.
    XML-Datei-weit eindeutige, maschineninterpretierbare Identität, die für die spätere interne Referenzierung dieses Elements erforderlich ist. Für eine detaillierte Erklärung siehe Dev:identities.

Constraints

The general railML® description states the following contraints.

  • id: xs:ID, required
    a string, starting with a letter (a..zA..Z) or an underscore (_),
    followed by a non-colonized and non-spaced string consisting of letters, digits, points (.), dashes (-) or underscores (_)

Export of railML® files

Software tools, which create railML® files, are free to define id values according to the above mentioned constraints.

Recommendation

railML® recommends using generic values, e.g.

<operationalPoint id="d2e717">...

or serial numbers with letter prefixes, e.g.

<operationalPoint id="op0002">...

or UUID's (Wiki banner.png), e.g.

<operationalPoint id="04b6f736-23f7-429c-be8a-e286172de2c1">...

Please note: a Nil UUID (Wiki banner.png) shall not be used; UUID's Version 4 (Wiki banner.png) is commonly used.

Bad style

Sometimes exporting software tools create id values, that contain semantic data, like names, designators, train numbers or vehicle family codes.

<operationalPoint id="op-TDM">
  <name name="Tidmouth" language="en-UK"/>
  <designator register="_NWR" entry="TDM"/>
  ...

Please use some more generic coding style in order to not invite the importing software to parse the id values!

Wrong style

Rufzeichen.png If the semantic information is only included in the id, the importing software would have to parse the id value in order to use it.
<operationalPoint id="op-TDM">
  <!-- no designator -->

This coding style hinders interoperability!

Please use a generic coding style for the id value. Look for appropriate railML® attributes in the element and put the semantics there. Otherwise use the xs:anyAttribute mechanism.

The use of this Wrong style prevents certification of the export interface.

Import of railML® files

Recommendation

Software tools, which consume railML® files, should use the id values only for cross-referencing between elements in the imported file. When UUIDs are used, the ids can also provide valid references across files and data sources.

In case of valid railML® files, no problems will occur.

Good style

In case of consuming "good or bad styled" id values the importing software provides routines for getting the semantics from better fitting attributes.

In case of consuming "wrong styled" id values the importing software is not required to extract semantics encoded in the id value.

Wrong style

Rufzeichen.png The importing software expects to parse the id value, e.g. with a special regular expression, in order to get the coded semantics, and fails when such information is not present in the id.


The use of this Wrong style prevents certification of the import interface.

Differentiation from other indications

Many elements that come with an attribute id will also have the child element <name>, with attributes name and description.

  • name: A short, human readable Identifier. Uniqueness is not required.
  • description: Human readable text that gives details. Uniqueness is not required.
Birne.png Please be aware that human-readable attributes are only intended to forward simple remarks/explanations from one end-user to another end-user. They are not to be used to contain computer-processable data, nor shall it be parsed by programmes in any way. In case of semantical overlaps or discrepancies to machine processable file contents, any machine processable contents have priority over possible human interpretable contents. However, these attributes may be used to transmit complementary details to machine processable parameters.

Bitte, berücksichtigen Sie, dass menschenlesbare Attribute nur für einfache Kommentare/Erklärungen von Endbenutzer zu Endbenutzer bestimmt sind. Sie sind nicht für maschineninterpretierbare Daten gedacht und werden nicht geparst. Im Falle semantischer Überschneidungen oder Widersprüche zu maschinenlesbaren Inhalten wird diesen der Vorzug gegenüber durch Menschen lesbaren gegeben. Allerdings können durch Menschen lesbare Felder für Ergänzungen zu maschinenlesbaren verwendet werden.