How Owl Imports Work

OWL Imports

First it must be understood that the semantics of imports in OWL is a subject of some controversy. In the OWL 1.1 specification there is effort in progress to clear up some of this confusion. Until this is settled we will use the definition of the semantics of imports given in the semantics document of the w3.org specs:

Aside from this local meaning, an owl:imports annotation also imports the contents of another OWL ontology into the current ontology. The imported ontology is the one, if any, that has as name the argument of the imports construct. (This treatment of imports is divorced from Web issues. The intended use of names for OWL ontologies is to make the name be the location of the ontology on the Web, but this is outside of this formal treatment.)

In this note we will describe what this innocent little paragraph is saying and what it means to Protege.

Names of Ontologies

All owl ontologies have a name. The w3.org document describing this naming process can be found at this link. Unfortunately the way that this name is calculated (at least for the RDF/XML OWL syntax) is not well specified. Even so - in most cases it can be calculated in an unambiguous way. In most RDF/XML ontologies there is a single RDF resource that is declared to be of type owl:Ontology. So for instance in the pizza owl ontology, the declaration in question is the following:

 <owl:Ontology rdf:about="">
   <protege:defaultLanguage rdf:datatype="http://www.w3.org/2001/XMLSchema#string"
   >en</protege:defaultLanguage>
   <owl:versionInfo rdf:datatype="http://www.w3.org/2001/XMLSchema#string"
   >version 1.3</owl:versionInfo>
   <rdfs:comment xml:lang="en">An example ontology that contains all constructs required for the various versions of the Pizza Tutorial run by Manchester University (see http://www.co-ode.org/resources/tutorials/)</rdfs:comment>
   <owl:imports rdf:resource="http://protege.stanford.edu/plugins/owl/protege"/>
 </owl:Ontology>

The first line of this declaration defines an RDF resource of type owl:Ontology. The name of this resource is given by the rdf:about statement. In this case the rdf:about string is empty which means that the xml base is used. The xml:base declaration can be found near the top of the pizza ontology in the namespace declarations:

<rdf:RDF
   xmlns:protege="http://protege.stanford.edu/plugins/owl/protege#"
   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
   xmlns:xsd="http://www.w3.org/2001/XMLSchema#"
   xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
   xmlns:owl="http://www.w3.org/2002/07/owl#"
   xmlns="http://www.co-ode.org/ontologies/pizza/2005/10/18/pizza.owl#"
   xmlns:daml="http://www.daml.org/2001/03/daml+oil#"
   xmlns:dc="http://purl.org/dc/elements/1.1/"
 xml:base="http://www.co-ode.org/ontologies/pizza/2005/10/18/pizza.owl">

No other declarations of an owl:Ontology resource occur in the pizza ontology so the name of this resource (e.g. http://www.co-ode.org/ontologies/pizza/2005/10/18/pizza.owl) is the name of the pizza ontology. It is relevant to this discussion that the name of the pizza ontology is working URL and clicking on the link pulls up the pizza ontology. In summary:

• the pizza ontology contains exactly one owl:Ontology declaration,
• this declaration is named by rdf:about="",
• the xm:base is declared and
• the xml:base is a working URL that points to the pizza ontology.

This situation is the simplest case and in this case things tend to work very smoothly.

However none of the assumptions above are guaranteed to hold. Sometimes there are no owl:Ontology declarations in the owl ontology. In this case the xml:base is used as the ontology name if it is present. Otherwise the name of the ontology is given by the url that was used to fetch the ontology. If this is still ambiguous, I am not sure what happens. In addition sometimes the rdf:about statement does not use the empty string. This allows one to give the ontology a name other than the xml:base. Finally the name of the ontology may only be a URI and may not resolve to the right ontology as a URL. Each of these cases adds some difficulty to ontology developers and to some degree motivate the writing of this document.

There is another case that has been troublesome in the past. Occasionally in some cases there are more than one owl:Ontology declaration. In this case tools occasionally get confused and give different results. Protege works by taking the first declaration of an owl:Ontology individual to be the name of the ontology. This would be fine except that the notion of first declaration in RDF/XML is not well defined.

Import By Name

If we look at the owl:Ontology header of the pizza ontology, we see that the declared owl:Ontology resource is related to another resource by the owl:imports property:

 <owl:Ontology rdf:about="">
   <protege:defaultLanguage rdf:datatype="http://www.w3.org/2001/XMLSchema#string"
   >en</protege:defaultLanguage>
   <owl:versionInfo rdf:datatype="http://www.w3.org/2001/XMLSchema#string"
   >version 1.3</owl:versionInfo>
   <rdfs:comment xml:lang="en">An example ontology that contains all constructs required for the various versions of the Pizza Tutorial run by Manchester University (see http://www.co-ode.org/resources/tutorials/)</rdfs:comment>
   <owl:imports rdf:resource="http://protege.stanford.edu/plugins/owl/protege"/>
 </owl:Ontology>

As triples the owl:imports statement would look like this:

http://www.co-ode.org/ontologies/pizza/2005/10/18/pizza.owl owl:imports http://protege.stanford.edu/plugins/owl/protege.

This statement tells tools such as Protege or reasoners that the pizza ontology should import an ontology whose name is

http://protege.stanford.edu/plugins/owl/protege.

So now the problem is "how do we find an ontology with this name?". The easy answer would be to simply use the name of the imported ontology as a URL and fetch the imported ontology in this way. Most of the time this works (e.g. this example) but in general the retrieval is not that simple.

Suppose that you are on a train commuting home from work and you decide to edit the pizza ontology. If Protege uses the URL to try to retrieve the ontology it will fail. In this case, this will not be that inconvenient. The user can still edit. But with more complex ontologies with more critical imports (e.g. the birnlex ontology) not finding the imports may make working on the ontology impossible. In this case, both Protege 3 and Protege 4 (and most other owl ontology tools) use a repository mechanism to find the imported ontology. Loosely speaking the repositories define a set of ontologies to be searched. A typical ontology repository will consist of all the ontologies in a given directory on the disk. When Protege is trying to resolve an import, it will first search in all the defined ontology repositories for an ontology whose name matches the desired imported name. If it doesn't find any matches, then it tries the ontology name as a URL. If this fails then the import will fail. This is not generally a fatal error, it just means that Protege does not have access to the assertions contained in that import.

What Can Go Wrong

There are two main things that can go wrong. First Protege may fail to find the imported ontology and second Protege may follow the above algorithm but end up retrieving an ontology with the wrong name.