id key definitions in EML
there are several problems emerging with the unique key definitions in eml. in
eml.xsd, there is a key definition that requires all instances of the @id
attribute to be unique within the document. when content is to be duplicated
there is to be one instance of the content with an id assigned and all other
instances are to use the <references> tag to point to that id. id's in a
document may be declared as document or system scope, meaning that they are
declared to be unique only within the document or within a broader naming
authority that is identified in the @system attribute.
Here are some of the problems:
1. there no enforced unique constraint on the @system attribute, although it is
implicit. Thus it is possible to create a dataset element usint an id with
system=cesdataset and a creator with system=asupersonnel. when those systems
have each assigned a similar value, you get conflicts. to avoid them, users are
forced to change identifiers and break the pointers back to the original source
of the content.
2. the spirit of the id and references tags was to insert some degree of
normalization that xml inherently lacks. however, it can be a rather arbitrary
choice with in the document which instance is the one that gets the content and
which ones get the reference pointer. This makes it very difficult for people
trying to write tools to edit eml documents since one could easily drop an
element that contains elements that contain content that other elements are
pointing to. This gratuitously complicates programming for EML and is likely to
discourage potential contributors of tools for working with the standard.
3. EML method allows you to embed the eml of related datasets that were used to
produce the current one in the methods discussion. conflicts can arise between
the identifiers of the embedded datasets. attemping to resolve conflicts between
documents using references could mean you have to edit those documents rather
than embed them.
#1 Updated by Peter McCartney about 17 years ago
I checked in changes to eml-resource.xsd and buildDocbook.xsl as a proposed fix
to some of this bug. The fix introduces an optional system attribute to the
references element, which corresponds to the existing system attribute
accompanying the id element through out EML. if used, it qualifys that the
matching id pointed to by the references tag has a scope defined by the system
attribute. Thus, EML documents can contain more than one id attribute with the
same value provided that they are scoped with different system attributes.
Changes to the "reusable content" section reflect this. if system is defined, it
must be defined for both the target and the referencing element and it must
match. Otherwise it must be absent from both.
I think there is still some ambiguity in the wording of the docs that could
benefit from some additional input. the eml root has an attribute called scope
which is "document" by default and "system" by option. We now allow there to be
more than one system referenced. if system tags are used once, does this mean
they must be provided in all cases? or, can i write an eml where the ids for
attribute are document scope, but the ids supplied for literature cited are
scoped to a particular system? Do we really need the "scope" attribute? if it is
left as default, is it invalid to use the system atrribute anywhere in the
#4 Updated by Margaret O'Brien over 13 years ago
The consensus on this bug seems to be that it will require more testing before implementation can proceed, and that it is not appropriate for 2.1.0. So the r1.78 has replaced it in the head. I also merged the changes that Peter committed (in r1.79) into the branch which is currently called eml_2_1_0.
So now there's a problem; this branch name needs to change for obvious reasons -- eml2.1.0 is on the trunk. The branch contains fixes which seem to now be mostly target-unspecified: 1132 (access control issues), 1152 (an element name change that may be dropped), and at least one (#1031) that appears to be already in 2.0.1. Retagging it with any version number seems confusing until a plan for future versions has evolved. Should this branch be renamed eml_target_unspecified, and its contents re-evaluated?
#5 Updated by Matt Jones about 13 years ago
After reviewing this bug, I still concur that it is an important problem that should be addressed as it causes validation problems for people trying to link to external systems. However, the exact implementation will require a more sophisticated EML Parser that can check if IDs are unique within the context of a particular system attribute. So, I think this additional work requires that we move this fix to the next release so that EML 2.1 can be released without delay. I have retargeted this for EML 2.2.0, and it should be triaged then when we make plans for that release.
One other note: In his original report, McCartney worries that it will be difficult to build applications that handle references in arbitrary places. With experience doing this for Metacat and Morpho, it now dow not seem to be such a big problem, so I think that is a non-issue.
#8 Updated by gastil gastil over 10 years ago
Prompted by a discussion about id, scope, and system in the lno-nis, I asked
Margaret for clarification of this issue. Attached are two example EML docs,
one using the current 2.1.0 and one that may work when this feature is added
perhaps in 2.2.0
In EML 2.1.0 the way it is now,
some elements can have none or all of the attributes
And the value of the scope attribute can be either
Currently, the EML parser requires that the value of the id attribute be unique
within an EML document, irregardless of the presence and values of the scope
and system attributes.
The feature request in bug 1662 asks that in EML 2.2.0 that the combination of
(id, scope, system) be unique, rather than requiring id alone to be unique.
It would still require that id values with the scope of document be unique.
There could not be two identical id values both with scope document.
The "example_id_system_scope.xml" (attached and also available as
http://dev.nceas.ucsb.edu/knb/metacat/knb-lter-xxx.1) parses. The
"example_id_system_scope_future.xml" does not parse in EML 2.1.0 but we suggest
it would parse if this feature is added. The same id, janedoe, is used twice,
but with different scopes. The use of that id in a reference raises the issue
of how that reference would be disambiguated.
Is that clarification accurate?
#9 Updated by Margaret O'Brien almost 10 years ago
This would also mean that only ids with the scope="document" could be used in <references>, since ids with scope="systeM" might not be unique, and unique ids for referencing is very reasonable.
So in the example above (id_system_scope_future.xml),
this id could be used in a reference:
<creator id="foo" scope="document">
but this one could not:
<contact id="janedoe" scope="system" system="LTER">
Another task for the parser.