Dev:identities: Difference between revisions
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{{InheritIdConstraints}} | {{InheritIdConstraints}} | ||
== Export of {{rml}} files == | == Export of {{rml|tm=no}} files == | ||
Software tools, which create {{rml}} files, are free to define {{Attr|id}} values according to the above mentioned constraints. | Software tools, which create {{rml}} files, are free to define {{Attr|id}} values according to the above mentioned constraints. | ||
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<pre><formation id="d2e717" name="TGV-ICE3">...</pre> | <pre><formation id="d2e717" name="TGV-ICE3">...</pre> | ||
=== Other workable methods === | |||
;serial numbers with letter prefixes | |||
e.g. | |||
<pre><formation id="rsf0002" name="TGV-ICE3">...</pre> | <pre><formation id="rsf0002" name="TGV-ICE3">...</pre> | ||
;UUID's | |||
<pre><formation id="_f36014b1-055d-4171-91a4-2a32a3210f28" name="TGV-ICE3">...</pre>''Please note: a {{external|https://en.wikipedia.org/wiki/Universally_unique_identifier#Nil_UUID|Nil UUID}} shall not be used; {{external|https://en.wikipedia.org/wiki/Universally_unique_identifier#Version_4_(random)|UUID's Version 4}} is commonly used. Due to the limitations of the ID type by www.w3.org in {{rml}} 2 the ID shall begin with a letter. Since this is not the case for all UUIDs, some tools are adding the prefix "_" before the UUID to fufil the constraints.'' | (see {{wiki|Universally_unique_identifier|UUID}}), e.g. | ||
<pre><formation id="_f36014b1-055d-4171-91a4-2a32a3210f28" name="TGV-ICE3">...</pre>''Please note: a {{external|https://en.wikipedia.org/wiki/Universally_unique_identifier#Nil_UUID|Nil UUID}} shall not be used; {{external|https://en.wikipedia.org/wiki/Universally_unique_identifier#Version_4_(random)|UUID's Version 4}} is commonly used. Due to the limitations of the ID type by www.w3.org in {{rml}} 2 the ID shall begin with a letter. <!--Since this is not the case for all UUIDs, some tools are adding the prefix "_" before the UUID to fufil the constraints.'' | |||
-->Since UUIDs can start as well with letters as with numbers and in {{rml|2.x}} IDs are XML IDs (which, according to the specification, must not start with numbers), the UUIDs in {{rml|2.x}} had to be started with an underscore. In {{rml|3}}, the ID was defined differently so that this necessity no longer exists. | |||
{{deu|Da UUIDs sowohl mit Buchstaben als mit Zahlen beginnen können und in {{rml|2.x}} IDs XML-IDs sind (die laut Spezifikation nicht mit Zahlen beginnen dürfen) mussten die UUIDs in {{rml|2.x}} mit einem Unterstrich begonnen werden. In {{rml|3}} wurde die ID anders definiert, sodass diese Notwendigkeit nicht mehr besteht.}} | |||
=== Bad style === | === Bad style === | ||
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= Differentiation from other indications = | = Differentiation from other indications = | ||
Many elements that have the {{attr|id}} attribute will also support the attributes {{attr|code}}, {{attr|name}} and {{attr|description}}. Each of these attributes has a separate purpose. | Many elements that have the {{attr|id}} attribute will also support the attributes {{attr|code}}, {{attr|name}} and {{attr|description}}. Each of these attributes has a separate purpose. | ||
* {{attr|code}}: Like {{attr|id}} it is a machine readable identifier, but while an {{attr|id}} only identifies an element within a single {{rml}} file, so that it can be referenced from other elements in the same file, a {{attr|code}} is designed for interoperability between data exchange partners. An {{attr|id}} ''is not'' a stable reference: the same physical object will likely have different {{attr|id}}s in different files (unless you are using [[# | * {{attr|code}}: Like {{attr|id}} it is a machine readable identifier, but while an {{attr|id}} only identifies an element within a single {{rml}} file, so that it can be referenced from other elements in the same file, a {{attr|code}} is designed for interoperability between data exchange partners. An {{attr|id}} ''is not'' a stable reference: the same physical object will likely have different {{attr|id}}s in different files (unless you are using [[#Export_of_railML_files|UUIDs]]). A {{attr|code}} ''is'' a stable identifier for a given element across files. While an {{attr|id}} is syntactically required to be unique within a file, there is no such syntactical constraint on a {{attr|code}}. However, as it is an identifier, a {{attr|code}} should help the data exchange partners correctly and unambiguously identify an object. As a consequence, use case specific uniqueness requirements may be agreed between exchange partners. As an example, a {{attr|code}} may be valid and unique within a national network. | ||
* {{attr|name}}: A short, human readable identifier. Uniqueness is not required. Its value should help a human reader separate the object from other (similar) objects. For instance, it can be displayed as a label to the user of a graphical software tool, a header in printed information or as a column in tabular representations. | * {{attr|name}}: A short, human readable identifier. Uniqueness is not required. Its value should help a human reader separate the object from other (similar) objects. For instance, it can be displayed as a label to the user of a graphical software tool, a header in printed information or as a column in tabular representations. | ||
* {{attr|description}}: Human readable text that gives additional details not given by the {{attr|name}}. Uniqueness is not required. It may contain remarks, explanations and hints about the contents of the object. | * {{attr|description}}: Human readable text that gives additional details not given by the {{attr|name}}. Uniqueness is not required. It may contain remarks, explanations and hints about the contents of the object. | ||
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[[Category:GeneralDescription]] | [[Category:GeneralDescription]] | ||
[[category:Common Concept]] | [[category:Common Concept]] | ||
{{interwiki}} | |||
Latest revision as of 14:33, 10 June 2024
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.
<formation id="d2e717" name="TGV-ICE3">...
Other workable methods
- serial numbers with letter prefixes
e.g.
<formation id="rsf0002" name="TGV-ICE3">...
- UUID's
(see UUID (
)), e.g.
<formation id="_f36014b1-055d-4171-91a4-2a32a3210f28" name="TGV-ICE3">...
Please note: a Nil UUID (external link) shall not be used; UUID's Version 4 (external link) is commonly used. Due to the limitations of the ID type by www.w3.org in railML® 2 the ID shall begin with a letter. Since UUIDs can start as well with letters as with numbers and in railML® 2.x IDs are XML IDs (which, according to the specification, must not start with numbers), the UUIDs in railML® 2.x had to be started with an underscore. In railML® 3, the ID was defined differently so that this necessity no longer exists.
Da UUIDs sowohl mit Buchstaben als mit Zahlen beginnen können und in railML® 2.x IDs XML-IDs sind (die laut Spezifikation nicht mit Zahlen beginnen dürfen) mussten die UUIDs in railML® 2.x mit einem Unterstrich begonnen werden. In railML® 3 wurde die ID anders definiert, sodass diese Notwendigkeit nicht mehr besteht.
Bad style
Sometimes exporting software tools create id values, that contain semantic data, like train numbers or vehicle family codes.
<formation id="fTGV-ICE3" name="TGV-ICE3">...
Please use some more generic coding style in order to not invite the importing software to parse the id values!
Wrong style
|
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
|
The use of this Wrong style prevents certification of the import interface.
Differentiation from other indications
Many elements that have the id attribute will also support the attributes code, name and description. Each of these attributes has a separate purpose.
- code: Like id it is a machine readable identifier, but while an id only identifies an element within a single railML® file, so that it can be referenced from other elements in the same file, a code is designed for interoperability between data exchange partners. An id is not a stable reference: the same physical object will likely have different ids in different files (unless you are using UUIDs). A code is a stable identifier for a given element across files. While an id is syntactically required to be unique within a file, there is no such syntactical constraint on a code. However, as it is an identifier, a code should help the data exchange partners correctly and unambiguously identify an object. As a consequence, use case specific uniqueness requirements may be agreed between exchange partners. As an example, a code may be valid and unique within a national network.
- name: A short, human readable identifier. Uniqueness is not required. Its value should help a human reader separate the object from other (similar) objects. For instance, it can be displayed as a label to the user of a graphical software tool, a header in printed information or as a column in tabular representations.
- description: Human readable text that gives additional details not given by the name. Uniqueness is not required. It may contain remarks, explanations and hints about the contents of the object.
|