An IVOA Identifier is a globally unique name for a resource. This
name can be used to retrieve a unique description of the resource
from an IVOA-compliant registry. This document describes the syntax
for IVOA identifiers as well as how they are created.
An IVOA identifier has two separable components that can appear in
two equivalent formats: an XML-tagged form and a URI-compliant form.
The syntax has been defined to encourage global-uniqueness naturally
and to maximize the freedom of resource providers to control the
character content of an identifier.
This is a Proposed Recommendation. The first release of this document
was 2004 February 9. It deprecates
Proposed Recommendation: IVOA Identifiers v1.0.
Comments on this document are due for consideration in the next version of this
document. They should be sent to
registry@ivoa.net,
a mailing list with a
public
archive. General discussion of related technology is also welcome
on the Rwp03
wiki site.
This is an IVOA Proposed Recommendation made available for public
review. It is appropriate to reference this document only as a
recommended standard that is under review and which may be changed
before it is accepted as a full recommendation.
A list of current IVOA
Recommendations and other technical documents can be found at
http://www.ivoa.net/Documents/.
This document builds on the concept of a Uniform Resource Identifier
as described in the IETF RFC 2396 by Berners-Lee, Fielding, Irvine,
& Masinter [RFC 2396] and the
subsequent IETF Internet Draft revisions
[Berners-Lee et al. 2003].
This document has been developed with support from the
National Science Foundation's
Information Technology Research Program under Cooperative Agreement
AST0122449 with The Johns Hopkins University, from the
UK Particle Physics and Astronomy
Research Council (PPARC), and from the
European Commission's Sixth
Framework Program via the
Optical Infrared Coordination Network (OPTICON).
Conformance-related definitions
The words "MUST", "SHALL", "SHOULD", "MAY", "RECOMMENDED", and
"OPTIONAL" (in upper or lower case) used in this document are to be
interpreted as described in IETF standard, RFC 2119
[RFC 2119].
The Virtual Observatory (VO) is
general term for a collection of federated resources that can be used
to conduct astronomical research, education, and outreach.
The International
Virtual Observatory Alliance (IVOA) is a global
collaboration of separately funded projects to develop standards and
infrastructure that enable VO applications.
Syntax Notation
This document uses the Augmented Backus-Naur Form (ABNF) notation
(RFC 2234) to
formally define syntax for identifier components. It references the
following core ABNF syntax productions (defined in section 6.1):
ALPHA, DIGIT.
There will be many occasions in Virtual Observatory applications where
metadata will need to refer to some other data or concept
unambiguously which is described elsewhere. In these cases, it is
valuable to use a global identifier (ID) as a reference to the
external item. An unambiguous reference within the entire IVOA
community requires that the identifier be globally unique. Ensuring
this uniqueness inevitably will require oversight by a moderating
authority; however, a flexible framework can minimize the opportunity
for duplicated IDs.
Many data providers in the VO have been creating
and using IDs for a long time. Their choices of identifiers were made
presumably to best fit the needs of the data. If an IVOA ID framework
is to minimize the cost of adoption, then it needs to maximize the
control of providers have to reuse the IDs they already have in place
as well as create new IDs that are consistent with their overall
organization. In addition, providers will need full flexibility over
what an ID refers to. It could be a dataset that can be transmitted
over the network, or a scientific instrument which cannot, or an
abstract concept or organization.
Identifiers are very important to registries as they aid users in
discovering data and services. In general, a registry stores
descriptions of data and services in a searchable form, and it
distinguishes them by a unique ID. Furthermore, when users
encounter an ID, they should be able to go to a registry and find
out something about the thing it refers to.
It is important to distinguish between two forms of referencing. The
first is a reference to a specific instance of something. Typically,
we think of this thing having a specific location; however, in the
framework proposed here, we think of it as being held and managed by a
specific organization even though its physical location may be
undefined. The framework recognizes that entities like datasets do
not always remain in the control of a single organization forever; thus,
necessitating a second form of referencing that is
location-independent--or more precisely, organization-independent.
When several copies of a dataset exist at several locations around
the VO, one can refer to all of them collectively, deferring the choice
of a particular instance until it is actually needed. Also, the
curation of a dataset may be transferred from one organization to
another; an organization-independent reference thus serves as a
persistent pointer to data that can be resolved to a new
location when it moves. This is very important to journal publishers
that wish to refer to data in publications (whose useful life might be
measured in decades) without worry that the references will become
obsolete.
This proposal defines identifiers of the first type describe
above--that is, organization-dependent identifiers.
Persistent, organization- and location-independent identifiers are
not currently defined as part of this proposal, because it is
not clear under what conditions it can be claimed that a resource is
unchanged when its management is changed or duplicated from one
organization to another. It is expected that a standard for
organization-independent identifiers will be based on this proposal
and the standard registry framework.
Nevertheless, these two forms of referencing are precisely what are
addressed by the IETF standards for URIs [RFC 2396]
and URNs [RFC 2141]. Thus, the framework proposed
in this document builds directly on these standards. They provide an
explicit mechanism for communities to add additional syntactic
restrictions onto URIs and URNs and to define rules for interpreting
them, all while still remaining compatible with the generic standards.
This proposal uses this mechanism to enable the retrieval of
descriptions of referenced items given their specific identifier and
the resolving of organization-independent identifiers to specific
ones.
A Uniform Resource Identifier
(URI) is defined by RFC 2396 as "a compact string of
characters for identifying an abstract or physical resource" which
complies with the syntax specification of that document (Berners-Lee et al. 1998). It can point to an
actual retrievable resource (i.e. a URL is a type of URI); however, it
need not. The use of the term URI in this document typically refers
to an abstract identifier that cannot be used as a URL to
retrieve things directly via HTTP. A Uniform Resource
Name (URN) is a type of URI that serves as a "persistent,
location-independent" identifier (Moats 1997).
In the Internet world, a resource
is essentially anything that can be referenced by a URI. This
document refines this definition by adding that it is describable by
the generic resource metadata defined in the IVOA Recommendation on
Resource Metadata (Hanisch et al. 2004, here on
referred to as RM).
We also refer to organizations and providers in the sense that they
are defined in the RM:
An organization is a specific
type of resource that brings people together to pursue
participation in VO applications. Organizations can be hierarchical
and range greatly in size and scope. At a high-level, it could be a
university, observatory, or government agency. At a finer level, it
could be a specific scientific project, space mission, or individual
researcher. A provider is an
organization that makes data and/or services
available to users over the network.
Definitions of other types of resources, including data collection
and service, are also defined in the RM, and
are assumed by this document.
As discussed above, identifiers are critical to registries.
A registry is a
resource that stores metadata about other resources,
including organizations, data collections, and
services, and makes that information accessible through a set of
services. Typically, one gets at this information either by doing a
search against the metadata or through a look-up operation given an
identifier. Registration is an
operation that a provider carries out to tell a
registry that a resource exists and can be
referred to by a particular identifier; this is typically done through
a registry service. An IVOA-compliant registry
is a registry that implements the minimum, IVOA standard registry
services (currently under development), including description look-up
by ID. Upon registering a
resource, its provider becomes
known as the resource's publisher.
This proposal follows on various requirement studies for VO
identifiers and registries in general (e.g.
NVO ID
requirements). This section highlights a few of the important
ones that guided the design of the ID framework.
-
A single framework should be used to identify anything a VO
application can refer to, including organizations, projects
(mission/telescope), data collections, and services.
-
It should be easy to compare two instances of an identifier to
determine if they refer to the same object.
-
It should be possible to use an identifier to access a unique
description of the resource it identifies.
-
The framework should maximize the freedom of data providers to
choose identifiers for resources and collections under their
control.
The IVOA identifier framework supports two forms of an identifier: an XML
form and a URI form. An example of the XML form looks like this:
<ResourceID>
<AuthorityID>adil.ncsa</AuthorityID>
<ResourceKey>surveys/96.JC.01</ResourceKey>
</ResourceID>
The same identifier can be expressed as a URI:
ivo://adil.ncsa/surveys/96.JC.01
In both forms, the identifier has two components: an authority ID and
a resource key. The former establishes a namespace within which the
rest of the ID, the resource key, can be considered unique.
Identifiers are considered case-insensitive; however, the preferred
rendering of character case in the ID is determined when its resource
is registered.
An IVOA Resource Identifier
(or IVOA identifier or IVOA ID for short) is a
globally unique reference to a resource
represented in a compact, ASCII-text format, as prescribed in this
document. This document defines two formats: an XML format and a URI
format. An IVOA identifier MUST always refer to a resource that
has been registered with an
IVOA-compliant registry; that is, it should be
possible to use the ID to get a description of the resource from a
compliant registry somewhere in the VO environment.
An IVOA ID has two required logical components: an authority
identifier and a resource key.
A naming authority is an
organization (usually a
provider) that has been granted the right by
the IVOA to create IVOA-compliant identifiers for resources it
registers. (See Creating Identifiers for
details on how this right is granted.) The naming authority creates
IDs within the scope of one or more authority identifiers.
An authority identifier is a
compact string of ASCII text that defines a globally unique namespace
controlled by a single naming authority. The
authority ID string is a compliant URI authority component
(RFC 2396, section 3.2) with the following
restrictions:
- it must be at least three (3) characters long,
- it must begin with an alpha-numeric character, and
- it must not contain any of the reserved characters ";", ":",
"@", "?", "#", ",", "[", "]", "<", ">", nor "/".
- Note:
- While the syntax for the authority ID
allows it to look just like a DNS hostname, current convention
discourages this practice to avoid the suggestion that an IVOA
Identifier can be resolved like a URL. As of this writing, the
convention of the National
Virtual Observatory (NVO) is hierarchical naming that
combines the publishing organization name with the project or
archive (e.g. "adil.ncsa") while leaving out fields like ".edu"
or ".org". In the AstroGrid
project, the convention is to use a DNS name in reverse order
(e.g. "org.astrogrid.www"); this practice has the advantage of
reducing the probability that two
authority ID.
|
- Recommendation:
- To make an authority ID more
recognizable, the following characters are discouraged from
being a part of the authority ID: "!", "~", "*", "'", "(", and
")". It is also recommended that authority IDs avoid the
multiple, sequential occurrences of periods, ".".
|
In ABNF notation, the syntax for an authority ID
is as follows:
authorityid = alphanum 2*unreserved
alphanum = ALPHA / DIGIT
reserved = "/" / "?" / "#" / "[" / "]" / ";" / ":" / "@" /
"&" / "$" / "," / "<" / ">"
unreserved = alphanum / mark / discouraged
mark = "-" / "_" / "."
discouraged = "!" / "~" / "*" / "'" / "(" / ")"
A naming authority is allowed to control multiple
authority IDs to organize related resources into different
namespaces. For example, an organization may
choose to control two authority IDs, one for research-related
resources and one for education/outreach resources, even though they
are all maintained by the same organization and perhaps made available
through the same machine.
VO applications should be case-insensitive when handling
authority IDs (see
"Comparing Identifiers" below). In practice,
applications are encouraged to present identifiers using all
lower-case characters.
3.1.2. Resource Key
A resource key is a localized
name for a resource that is unique within the namespace of an
authority ID. The naming authority creates keys for its namespaces
and has complete control their form beyond the syntax constraints
specified here.
A resource key must conform to the syntax of a
URI path component (RFC 2396, section 3.2); that
is, it is a slash ("/") delimited ASCII string. In addition, it must
not contain any of the other reserved characters, ";", ":", "@", "?",
"#", ",", "[", "]", "<", ">".
In ABNF notation, the syntax for a resource key is
as follows:
resourcekey = segment *( "/" segment)
segment = *unreserved
Naming authorities are discouraged from creating
segments matching either "." or "..". Empty
segments, resulting in two or more consecutive slashes or a trailing
slash, is also discouraged. As described in the section
"Comparing Identifiers", such segments do not have the
special meaning they have in traditional file system pathnames; that
is, a resource key cannot be transformed to remove any "." or ".."
segments and still reference the same resource. Such segments are
interpreted as a literal component of the path.
- Note:
- The reserved characters may be
employed in the future to differentiate additional components
of an identifier beyond the authority ID and resource key.
|
The naming authority is free to create a
resource key that suggests something about the resource it refers to.
Any meaning that is suggested by the resource key is intended only for
human consumption. The character content of a resource key is not
semantically machine-interpretable. Any information about the
resource, apart from whether it is same exact resource (i.e. instance
or copy) as the resource referred to by another identifier (see
"Comparing Identifers"), can only be
determined by examining the resource metadata.
The presence of a resource key is optional. An identifier that
contains only an authority ID refers to the registered namespace
itself and, indirectly, the organization that acts as the naming
authority for that namespace.
VO applications should be case-insensitive when handling a
resource key (see
"Comparing Identifiers" below). In practice,
the preferred use of case is set by the rendering of the key by the
naming authority when the resource is registered. This may contain
one or more capital letters to improve reading.
3.2 Identifier Formats
The identifier components can be combined into
two equivalent formats: an XML-tagged form and a URI-compliant form.
3.2.1. XML Format
An IVOA identifier rendered in XML can be described as an XML complex
type as defined by the XML Schema specification
(XMLSchema-1). This type contains two child
elements. The first element, which must
be present and is non-repeatable, is named
AuthorityID, and its content is a string that
conforms to the syntax of an authority ID. The
second, non-repeatable element is named ResourceKey,
and its content is a string that conforms to the syntax of a
resource key.
Appendix A lists an XML Schema definition which
includes the XML element, Identifier, that is
defined to be an IVOA identifier for a registered resource. Other
schemas may import and use this element directly wherever this
general metadata concept is useful. If a more precise meaning is
needed, e.g. if "PublisherID" is meant to refer to a resource's
publisher, then a schema can create a new
element whose type is "IVOAidentifier".
3.2.2. URI Format
There will be occasions when an application needs to encode an
identifier in a format where the XML encoding defined in the
previous section cannot be readily used but
where a simple string can. This would include a non-XML format,
such as in a FITS keyword. A number of XML-based metadata handling
systems also treat identifiers as strings (e.g. Resource Description
Language, Dublin Core, Open Archives Initiative). In some cases,
identifiers are encoded as XML attributes. To enable greater
compatibility with other metadata technologies, this document also
defines a URI format for an identifier.
This specification defines a new URI scheme called
"ivo." A URI that uses this scheme signals that:
- the identifier and the resource it refers to have been
registered with an IVOA-compliant registry.
- the URI can be parsed into an authority ID and a
resource key according to the syntax defined in this section.
The URI form of an identifier starts with the "ivo" scheme string and
is followed by a colon and two slashes. The double slashes reflect
the URI convention that a URI that refers to a remote resource. After
the double slashes is the authority id which
must match the syntax given in section 3.1.1.
After that is a slash followed by the resource key,
which must match the syntax given in section
3.1.2. Thus, the authority ID is separated from the resource key
by the first slash after the double slashes.
In addition to the syntax to the above syntax rules, the URI format
reserves the question mark ("?") and the pound sign ("#") as "stop"
characters: that is, when one of these two characters appears in a
string beginning the "ivo" scheme, all characters before the first
question mark or pound constitute the complete IVOA
resource identifier, and all characters after and including the "stop"
character are to be ignored when handling the string as an
IVOA resource identifier.
To simplify comparisons identifiers in URI format (see
"Comparing Identifiers"), case-insensitive variations on "ivo" as
the scheme shall be considered equivalent; however, use of variations
other than the all lower-case form are strongly discouraged.
In ABNF format, the URI form is defined as:
ivo-scheme = ("i" / "I") ("v" | "V") ("o" | "O")
uri-form = ivo-scheme "://" authorityid ?( "/" resourcekey )
It is important to note that an IVOA resource
identifier in URI format is not a URL. The ivo scheme does not
imply a transport protocol by which the resource may be accessed.
Agents, in general, should not depend on implicit mappings between
IVOA resource identifiers and URLs
(e.g. replacing the ivo scheme with the http scheme) to derive
a URL for the resource. Resource publishers,
however, may support such a mapping between identifiers and URLs that
they manage; in this case; agents should only assume the mapping
applies within the domain of the publisher.
3.3 Creating Identifiers
An important aim of the process for creating identifiers is to ensure
uniqueness. In the context of IVOA resource
identifiers, "unique" means that a given identifier MUST NOT refer
to two different resources at any instant. Furthermore, the
identifier SHOULD refer to one logical resource over all time; that
is, IVOA identifier should not be reused. The description of the
resource referred to by the identifier, however, MAY change over
time. What constitutes the difference between re-use of an identifier
and an update of its description is left up to the discretion of the
publisher.
Another aim of the identifier creation process is to trace the
delegation of authority over the identifier. In principle, the right
to create identifiers is granted to naming
authorities by the IVOA via a service defined among the standard
registry services. In practice, an identifier is created by a person
or organization when registering a resource.
Thus, only recognized naming authorities (or
persons representing such organizations) may register new resources.
The exact details of the service used to become a recognized
naming authority is not described here; however,
such a service has the following requirements:
-
The service approves an organization as
naming authority by associating one or
more authority identifiers with the
organization.
-
The organization desiring recognition as a naming authority
must be registered with a registry; that is, a registry must
contain a standard IVOA description for that organization.
This registered description must be accessible to the
service.
It is possible that an organization may become registered as
part of the process of requesting an
authority ID. Equivalently, registering an organization
may result in an implicit request for a particular
authority ID.
-
If the service approves the request, the
authority ID is considered controlled by the organization.
-
In processing the request for an authority
ID, the service must check to ensure that the ID is not
already controlled by another organization. This is done by
doing a by-identifier search of a global registry for the
requested ID. If the authority
identifier is already registered, then request is denied.
The registry service specification or a particular implementation may
impose additional requirements for approving a request for an
authority ID. Note, however, that it is
intended that the requirements determining who may become a
naming authority should not be overly
restrictive, nor should it take much more time to obtain approval than
what it takes to determine if the authority ID
is already taken.
Once an organization is recognized as a naming authority, it is free
to register any number of resources with identifiers having an
authority ID that they control. No other
organizations may create identifiers with an
authority ID it does not control. The naming
authority has full control over the creation of a
resource key as long as it conforms to the syntax
and uniqueness constraints described in this specification.
It is the responsibility of the registry that accepts new resource
descriptions to ensure that new descriptions are not associated with
identifiers already referring to other resources. The mechanisms used
to ensure this are not described here.
Since IVOA resource identifiers refer to a
resource that has been registered (see
definition), providers and
software agents should avoid referring to unregistered resources via
an IVOA identifier. In practice, there may be some time lag between
when a resource is available and when it is actually published;
in this case, any use of the identifier should based on the expectation
that the resource will be registered soon.
3.4 Comparing Identifiers
An important use of identifiers is comparing two instances to
determine if they refer to the same resource. This will most commonly
occur when using an identifier to look up the associated resource
description in a registry.
Two resource identifiers are guaranteed to refer to the same resource
if they have identical authority IDs and
identical resource keys. A pair of components is
considered identical if a case-insensitive,
character-by-character comparison indicates they are identical.
Apart from a transformation to handle case-insensitive comparisons, no
other normalizing transformations shall be necessary to test if
two resources are IDs refer to the same resource.
- Note:
- The case-insensitive string comparison test on two identifiers
in URI format is equivalent to parsing to the identifiers into
their two components and comparing them.
|
In general, the string-based comparison of identifiers described
cannot determine definitively if two identifiers refer to different
resources. While it is not intended that a single resource be
registered multiple times with different identifiers, it is not
disallowed by this specification. In particular, it is possible that
two resources with different identifiers may be mirrors of each other;
such a relationship can only be determined by examining the metadata
contained in the descriptions associated with each identifier.
Appendix A: An XML Schema for Identifiers
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:vid="http://www.ivoa.net/xml/VOIdentifier/v1.1"
targetNamespace="http://www.ivoa.net/xml/VOIdentifier/v1.1"
elementFormDefault="qualified" attributeFormDefault="unqualified">
<xs:annotation>
<xs:documentation>Version 0.1</xs:documentation>
<xs:documentation>
This schema defines the XML format for IVOA Identifiers as
specified in the IVOA Identifiers Working Draft, Version 0.1.
</xs:documentation>
<xs:documentation>
History:
</xs:documentation>
</xs:annotation>
<xs:element name="Identifier" type="vid:IVOAidentifier">
<xs:annotation>
<xs:documentation>
a global, IVOA-compliant identifier that refers
unambiguously to a resource.
</xs:documentation>
</xs:annotation>
</xs:element>
<xs:complexType name="IVOAidentifier">
<xs:sequence>
<xs:element ref="vid:AuthorityID" />
<xs:element ref="vid:ResourceKey" minOccurs="0"/>
</xs:sequence>
</xs:complexType>
<xs:element name="AuthorityID" type="vid:AuthorityIDType">
<xs:annotation>
<xs:documentation>the identifier a namespace under the control of
a single naming authority</xs:documentation>
</xs:annotation>
</xs:element>
<xs:element name="ResourceKey" type="vid:ResourceKeyType">
<xs:annotation>
<xs:documentation>the identifier a namespace under the control of
a single naming authority</xs:documentation>
</xs:annotation>
</xs:element>
<xs:simpleType name="AuthorityIDType">
<xs:restriction base="xs:string">
<xs:pattern value="[\w\d][\w\d\-_\.!~\*'\(\)\+=]{2,}"/>
</xs:restriction>
</xs:simpleType>
<xs:simpleType name="ResourceKeyType">
<xs:restriction base="xs:string">
<xs:pattern value="[\w\d\-_\.!~\*'\(\)\+=]+(/[\w\d\-_\.!~\*'\(\)\+=])*"/>
</xs:restriction>
</xs:simpleType>
<xs:simpleType name="IVOAIdentifierURI">
<xs:restriction base="xs:anyURI">
<xs:pattern value="ivo://[\w\d][\w\d\-_\.!~\*'\(\)]{2,}(/[\w\d\-_\.!~\*'\(\)]+(/[\w\d\-_\.!~\*'\(\)])*)"/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
Appendix B: Changes from Previous Versions
B.1 Changes from v1.0
- The prohibition of using "+" and "=" within
Identifier components has been
dropped.
- Recommendations for authority ID strings
have been updated to match current practice in AstroGrid and the
NVO.
- In the example schema in App. A, the namespace was altered to conform
with IVOA conventions. A correction was also made to the
allowed pattern for AuthorityIDType to properly comply with the XML
specification defined in section 3.2.1.
- various clarifications based on reviewer comments
B.1 Changes from v0.1
- Resource key is now required except when referring to a naming
authority itself (3.1,
3.1.2).
- support for DNS-like authority IDs clarified
(3.1.1).
- added role of # and ? as "stop" characters in URI form
(3.2.1).
- dropped non-binding Appendix B: Recommended Mechanism for
becoming a Naming authority.
- [RFC 2396]
- Berners-Lee, Tim, Fielding, R., Irvine, U.C., Masinter, L. 1998.
Universal
Resource Identifier (URI): Generic Syntax, IETF RFC 2396,
http://www.ietf.org/rfc/rfc2396.txt
- [Berners-Lee et al. 2003]
- Berners-Lee, Tim, Fielding, R., Masinter, L. 2003.
Universal
Resource Identifier (URI): Generic Syntax, IETF
Internet-Draft,
http://www.ietf.org/internet-drafts/draft-fielding-uri-rfc2396bis-03.txt
- [RFC 2119]
- Bradner, S. 1997.
Key words for use in RFCs to Indicate Requirement
Levels, IETF RFC 2119,
http://www.ietf.org/rfc/rfc2119.txt
- [RFC 2234]
- Crocker, D. and Overell, P. 1997.
Augmented
BNF for Syntax Specifications: ABNF, IETF RFC 2234,
http://www.ietf.org/rfc/rfc2234.txt
- [Hanisch et al. 2004]
- Hanisch, Robert (ed.) 2004.
Resource
Metadata for the Virtual Observatory, IVOA Recommendation,
http://www.ivoa.net/Documents/REC/ResMetadata/RM-20040426.htm
- [RFC 2141]
- Moats, R. 1997.
URN Syntax, IETF RFC 2141,
http://www.ietf.org/rfc/rfc2141.txt
