The Knora Base Ontology

Overview

The Knora base ontology is the main built-in Knora ontology. Each project that uses Knora must describe its data model by creating ontologies that extend this ontology.

The Knora base ontology is identified by the IRI http://www.knora.org/ontology/knora-base. In the Knora documentation in general, it is identified by the prefix knora-base, but for brevity, in this document, we use kb or omit the prefix entirely.

The Knora Data Model

The Knora data model is based on the observation that, in the humanities, a value or literal is often itself structured and can be highly complex. Moreover, a value may have its own metadata, such as its creation date, information about permissions, and so on. Therefore, the Knora base ontology describes structured value types that can store this type of metadata. In the diagram below, a book (ex:book2) has a title (identified by the predicate ex:title) and a publication date (ex:pubdate), each of which has some metadata.

Figure 1

Projects

In Knora, each item of data belongs to some particular project. Each project using Knora must define a kb:knoraProject, which has these properties (cardinalities are indicated in parentheses after each property name):

projectShortname (1)

A short name that can be used to identify the project in configuration files and the like.

projectLongname (1)

The full name of the project.

projectShortcode (1)

A hexadecimal code that uniquely identifies the project. These codes are assigned to projects by the DaSCH.

projectDescription (1-n)

A description of the project.

belongsToInstitution (0-1)

The kb:Institution that the project belongs to.

Ontologies, resources, and valuesare associated with a project by means of the kb:attachedToProject property, as described in Ontologies and Properties of Resource). Users are associated with a project by means of the kb:isInProject property, as described in Users and Groups.

Ontologies

Each user-created ontology must be defined as an owl:Ontology with the properties rdfs:label and kb:attachedToProject.

Resources

All the content produced by a project (e.g. digitised primary source materials or research data) must be stored in objects that belong to subclasses of kb:Resource, so that Knora can query and update that content. Each project using the Knora base ontology must define its own OWL classes, derived from kb:Resource, to represent the types of data it deals with. A subclass of kb:Resource may additionally be a subclass of any other class, e.g. an industry-standard class such as foaf:Person; this can facilitate searches across projects.

Resources have properties that point to different parts of the content they contain. For example, a resource representing a book could have a property called hasAuthor, pointing to the author of the book. There are two possible kinds of content in a Knora resource: Knora values (see Values) or links to other resources (see Links Between Resources). Properties that point to Knora values must be subproperties of kb:hasValue, and properties that point to other resources must be subproperties of kb:hasLinkTo. Either of these two types of properties may also be a subproperty of any other property, e.g. an industry-standard property such as foaf:name; this can facilitate searches across projects. Each property definition must specify the types that its subjects and objects must belong to (see Constraints on the Types of Property Subjects and Objects for details).

Each user-created resource class definition must use OWL cardinality restrictions to specify the properties that resources of that class can have (see OWL Cardinalities for details).

Resources are not versioned; only their values are versioned (see Values).

Every resource is required to have an rdfs:label. The object of this property is an xsd:string, rather than a Knora value; hence it is not versioned. A user who has modify permission on a resource (see Authorisation) can change its label.

A resource can be marked as deleted; Knora does this by adding the predicate kb:isDeleted true to the resource. An optional kb:deleteComment may be added to explain why the resource has been marked as deleted. Deleted resources are normally hidden. They cannot be undeleted, because even though resources are not versioned, it is necessary to be able to find out when a resource was deleted. If desired, a new resource can be created by copying data from a deleted resource.

Properties of Resource

creationDate (1)

The time when the resource was created.

attachedToUser (1)

The user who owns the resource.

attachedToProject (1)

The project that the resource is part of.

lastModificationDate (0-1)

A timestamp indicating when the resource (or one of its values) was last modified.

seqnum (0-1)

The sequence number of the resource, if it is part of an ordered group of resources, such as the pages in a book.

isDeleted (1)

Indicates whether the resource has been deleted.

deleteDate (0-1)

If the resource has been deleted, indicates when it was deleted.

deleteComment (0-1)

If the resource has been deleted, indicates why it was deleted.

Resources can have properties that point to other resources; see Links Between Resources. A resource grants permissions to groups of users; see Authorisation.

Representations

It is not practical to store all data in RDF. In particular, RDF is not a good storage medium for binary data such as images. Therefore, Knora stores such data outside the triplestore, in ordinary files. A resource can have metadata about a file attached to it. The technical term for such a resource in Knora is a Representation. For each file, there is a kb:FileValue in the triplestore containing metadata about the file (see FileValue). Knora uses Sipi to store files. The Knora APIs provide ways to create file values using Knora and Sipi.

A resource that has a file value must belong to one of the subclasses of kb:Representation. Its subclasses, which are intended to be used directly in data, include:

StillImageRepresentation

A representation containing a still image file.

MovingImageRepresentation

A representation containing a video file.

AudioRepresentation

A representation containing an audio file.

DDDrepresentation

A representation containing a 3D image file.

TextRepresentation

A representation containing a formatted text file, such as an XML file.

DocumentRepresentation

A representation containing a document (such as a PDF file) that is not a text file.

The base class of all these classes is Representation, which is not intended to be used directly. It has this property, which its subclasses override:

hasFileValue (1-n)

Points to one or more file values.

There are two ways for a project to design classes for representations. The simpler way is to create a resource class that represents a thing in the world (such as ex:Painting) and also belongs to a subclass of Representation. This is adequate if the class can have only one type of file attached to it. For example, if paintings are represented only by still images, ex:Painting could be a subclass of StillImageRepresentation. This is the only approach supported in Knora API v1.

The more flexible approach, which is supported by Knora API v2, is for each ex:Painting to use the kb:hasRepresentation property to point to other resources containing files that represent the painting. Each of these other resources can extend a different subclass of Representation. For example, a painting could have a StillImageRepresentation as well as a DDDrepresentation.

Standard Resource Classes

In general, each project using Knora must define its own subclasses of kb:Resource. However, the Knora base ontology provides some standard subclasses of kb:Resource, which are intended to be used by any project:

Region

Represents a region of a Representation (see Representations).

Annotation

Represents an annotation of a resource. The hasComment property points to the text of the annotation, represented as a kb:TextValue.

LinkObj

Represents a link that connects two or more resources. A LinkObj has a hasLinkTo property pointing to each resource that it connects, as well as a hasLinkToValue property pointing to a reification of each of these direct links (see Links Between Resources). A LinkObj is more complex (and hence less convenient and readable) than a simple direct link, but it has the advantage that it can be annotated using an Annotation. For improved readability, a project can make its own subclasses of LinkObj with specific meanings.

Values

The Knora base ontology defines a set of OWL classes that are derived from kb:Value and represent different types of structured values found in humanities data. This set of classes may not be extended by user-created ontologies.

A value is always part of one particular resource, which points to it using some property derived from hasValue. For example, a user-created ontology could specify a Book class with a property hasSummary (derived from hasValue), and that property could have a knora-base:objectClassConstraint of TextValue. This would mean that the summary of each book is represented as a TextValue.

Knora values are versioned. Existing values are not modified. Instead, a new version of an existing value is created. The new version is linked to the old version via the previousValue property.

“Deleting” a value means marking it with kb:isDeleted. An optional kb:deleteComment may be added to explain why the value has been marked as deleted. Deleted values are normally hidden.

Most types of values are marked as deleted without creating a new version of the value. However, link values must be treated as a special case. Before a LinkValue can be marked as deleted, its reference count must be decremented to 0. Therefore, a new version of the LinkValue is made, with a reference count of 0, and it is this new version that is marked as deleted.

To simplify the enforcement of ontology constraints, and for consistency with resource updates, no new versions of a deleted value can be made; it is not possible to undelete. Instead, if desired, a new value can be created by copying data from a deleted value.

Properties of Value

valueCreationDate (1)

The date and time when the value was created.

attachedToUser (1)

The user who owns the value.

attachedToProject (0-1)

The project that the value is part of. If not specified, defaults to the project of the containing resource.

valueHasString (1)

A human-readable string representation of the value’s contents, which is available to Knora’s full-text search index.

valueHasOrder (0-1)

A resource may have several properties of the same type with different values (which will be of the same class), and it may be necessary to indicate an order in which these values occur. For example, a book may have several authors which should appear in a defined order. Hence, valueHasOrder, when present, points to an integer literal indicating the order of a given value relative to the other values of the same property. These integers will not necessarily start at any particular number, and will not necessarily be consecutive.

previousValue (0-1)

The previous version of the value.

isDeleted (1)

Indicates whether the value has been deleted.

deleteDate (0-1)

If the value has been deleted, indicates when it was deleted.

deleteComment (0-1)

If the value has been deleted, indicates why it was deleted.

Each Knora value can grant permissions (see Authorisation).

Subclasses of Value

TextValue

Represents text, possibly including markup. The text is the object of the valueHasString property. A line break is represented as a Unicode line feed character (U+000A). The non-printing Unicode character INFORMATION SEPARATOR TWO (U+001E) can be used to separate words that are separated only by standoff markup (see below), so they are recognised as separate in a full-text search index.

Markup is stored using this property:

valueHasStandoff (0-n)

Points to a standoff markup tag. See Text with Standoff Markup.

valueHasMapping (0-1)

Points to the mapping used to create the standoff markup and to convert it back to the original XML. See Mapping to Create Standoff From XML.

A text value can have a specified language:

valueHasLanguage (0-1)
is an ISO 639-1 code as string specifying the language of the text.
DateValue

Humanities data includes many different types of dates. In Knora, a date has a specified calendar, and is always represented as a period with start and end points (which may be equal), each of which has a precision (DAY, MONTH, or YEAR). An optional ERA indicator term (BCE, CE, or BC, AD) can be added to the date, when no era is provided the default era AD will be considered. Internally, the start and end points are stored as two Julian Day Numbers. This calendar-independent representation makes it possible to compare and search for dates regardless of the calendar in which they were entered. Properties:

valueHasCalendar (1)

The name of the calendar in which the date should be displayed. Currently GREGORIAN and JULIAN are supported.

valueHasStartJDN (1)

The Julian Day Number of the start of the period (an xsd:integer).

valueHasStartPrecision (1)

The precision of the start of the period.

valueHasEndJDN (1)

The Julian Day Number of the end of the period (an xsd:integer).

valueHasEndPrecision (1)

The precision of the end of the period.

IntValue

Represents an integer. Property:

valueHasInteger (1)

An xsd:integer.

ColorValue
valueHasColor (1)

A string representing a color. The string encodes a color as hexadecimal RGB values, e.g. "#FF0000".

DecimalValue

Represents an arbitrary-precision decimal number. Property:

valueHasDecimal (1)

An xsd:decimal.

UriValue

Represents a non-Knora URI. Property:

valueHasUri (1)

An xsd:anyURI.

BooleanValue

Represents a boolean value. Property:

valueHasBoolean (1)

An xsd:boolean.

GeomValue

Represents a geometrical object as a JSON string, using normalized coordinates. Property:

valueHasGeometry (1)

A JSON string.

GeonameValue

Represents a geolocation, using the identifiers found at GeoNames. Property:

valueHasGeonameCode (1)

the identifier of a geographical feature from GeoNames, represented as an xsd:string.

IntervalValue

Represents a time interval, with precise start and end times on a timeline, e.g. relative to the beginning of an audio or video file. Properties:

valueHasIntervalStart (1)

An xsd:decimal representing the start of the interval in seconds.

valueHasIntervalEnd (1)

An xsd:decimal representing the end of the interval in seconds.

ListValue

Projects often need to define lists or hierarchies of categories that can be assigned to many different resources. Then, for example, a user interface can provide a drop-down menu to allow the user to assign a category to a resource. The ListValue class provides a way to represent these sorts of data structures. It can represent either a flat list or a tree.

A ListValue has this property:

valueHasListNode (1)

Points to a ListNode.

Each ListNode can have the following properties:

isRootNode (0-1)

Set to true if this is the root node.

hasSubListNode (0-n)

Points to the node’s child nodes, if any.

hasRootNode (0-1)

Points to the root node of the list (absent if isRootNode is true).

listNodePosition (0-1)

An integer indicating the node’s position in the list of its siblings (absent if isRootNode is true).

listNodeName (0-1)

The node’s human-readable name (absent if isRootNode is true).

FileValue

Knora stores certain kinds of data outside the triplestore, in files (see Representations). Each digital object that is stored outside the triplestore has associated metadata, which is stored in the triplestore in a kb:FileValue. The base class FileValue, which is not intended to be used directly, has these properties:

internalFilename (1)

The name of the file as stored by Knora.

internalMimeType (1)

The MIME type of the file as stored by Knora.

originalFilename (0-1)

The original name of the file when it was uploaded to the Knora API server.

originalMimeType (0-1)

The original MIME type of the file when it was uploaded to the Knora API server.

isPreview (0-1)

A boolean indicating whether the file is a preview, i.e. a small image representing the contents of the file. A preview is always a StillImageFileValue, regardless of the type of the enclosing Representation.

The subclasses of FileValue, which are intended to be used directly in data, include:

StillImageFileValue

Contains metadata about a still image file.

MovingImageFileValue

Contains metadata about a video file.

AudioFileValue

Contains metadata about an audio file.

DDDFileValue

Contains metadata about a 3D image file.

TextFileValue

Contains metadata about a text file.

DocumentFileValue

Contains metadata about a document (such as PDF) that is not a text file.

Each of these classes contains properties that are specific to the type of file it describes. For example, still image files have dimensions, video files have frame rates, and so on.

FileValue objects are versioned like other values, and the actual files stored by Knora are also versioned. Version 1 of the Knora API does not provide a way to retrieve a previous version of a file, but this feature will be added in a subsequent version of the API.

LinkValue

A LinkValue is an RDF “reification” containing metadata about a link between two resources. It is therefore a subclass of rdf:Statement as well as of Value. It has these properties:

rdf:subject (1)

The resource that is the source of the link.

rdf:predicate (1)

The link property.

rdf:object (1)

The resource that is the target of the link.

valueHasRefCount (1)

The reference count of the link. This is meaningful when the LinkValue describes resource references in Standoff text markup (see StandoffLinkTag). Otherwise, the reference count will always be 1 (if the link exists) or 0 (if it has been deleted).

For details about how links are created in Knora, see Links Between Resources.

ExternalResValue

Represents a resource that is not stored in the RDF triplestore managed by Knora, but instead resides in an external repository managed by some other software. The ExternalResValue contains the information that Knora needs in order to access the resource, assuming that a suitable gateway plugin is installed.

extResAccessInfo (1)

The location of the repository containing the external resource (e.g. its URL).

extResId (1)

The repository-specific ID of the external resource.

extResProvider (1)

The name of the external provider of the resource.

Links Between Resources

A link between two resources is expressed, first of all, as a triple, in which the subject is the resource that is the source of the link, the predicate is a “link property” (a subproperty of kb:hasLinkTo), and the object is the resource that is the target of the link.

It is also useful to store metadata about links. For example, Knora needs to know who owns the link, who has permission to modify it, when it was created, and so on. Such metadata cannot simply describe the link property, because then it would refer to that property in general, not to any particular instance in which that property is used to connect two particular resources. To attach metadata to a specific link in RDF, it is necessary to create an RDF “reification”. A reification makes statements about a particular triple (subject, predicate, object), in this case the triple that expresses the link between the resources. Knora uses reifications of type kb:LinkValue (described in LinkValue to store metadata about links.

For example, suppose a project describes paintings that belong to collections. The project can define an ontology as follows (expressed here in Turtle format, and simplified for the purposes of illustration):

@prefix kb <http://www.knora.org/ontology/knora-base#> .
@prefix : <http://www.knora.org/ontology/paintings#> .

:Painting rdf:type owl:Class ;
    rdfs:subClassOf kb:Resource ,
        [ rdf:type owl:Restriction ;
            owl:onProperty :hasArtist ;
            owl:cardinality 1 ] ,
        [ rdf:type owl:Restriction ;
              owl:onProperty :hasTitle ;
              owl:cardinality 1 ] ;
        [ rdf:type owl:Restriction ;
              owl:onProperty :isInCollection ;
              owl:minCardinality 1 ] ;
        [ rdf:type owl:Restriction ;
              owl:onProperty :isInCollectionValue ;
              owl:minCardinality 1 ] .

:Collection rdf:type owl:Class ;
    rdfs:subClassOf kb:Resource ,
        [ rdf:type owl:Restriction ;
            owl:onProperty :hasCollectionName ;
            owl:cardinality 1 ] .

:hasArtist rdf:type owl:ObjectProperty ;
    rdfs:label "Name of artist" ;
    kb:subjectClassConstraint :Painting ;
    kb:objectClassConstraint kb:TextValue .

:hasTitle rdf:type owl:ObjectProperty ;
    rdfs:label "Title of painting"
    kb:subjectClassConstraint :Painting ;
    kb:objectClassConstraint kb:TextValue .

:hasCollectionName rdf:type owl:ObjectProperty ;
    rdfs:label "Name of collection" ;
    kb:subjectClassConstraint :Collection ;
    kb:objectClassConstraint kb:TextValue .

To link the paintings to the collection, we must add a “link property” to the ontology. In this case, the link property will point from a painting to the collection it belongs to. Every link property must be a subproperty of kb:hasLinkTo.

:isInCollection rdf:type owl:ObjectProperty ;
    rdfs:subPropertyOf kb:hasLinkTo ;
    kb:subjectClassConstraint :Painting ;
    kb:objectClassConstraint :Collection .

We must then add a “link value property”, which will point from a painting to a kb:LinkValue (described in LinkValue), which will contain metadata about the link between the property and the collection. In particular, the link value specifies the creator of the link, the date when it was created, and the permissions that determine who can view or modify it. The name of the link value property is constructed using a simple naming convention: the word Value is appended to the name of the link property. In this case, since our link property is called :isInCollection, the link value property must be called :isInCollectionValue. Every link value property must be a subproperty of kb:hasLinkToValue.

:isInCollectionValue rdf:type owl:ObjectProperty ;
    rdfs:subPropertyOf kb:hasLinkToValue ;
    kb:subjectClassConstraint :Painting ;
    kb:objectClassConstraint kb:LinkValue .

Given this ontology, we can create some RDF data describing a painting and a collection:

@prefix paintings <http://www.knora.org/ontology/paintings#> .
@prefix data <http://www.knora.org/ontology/paintings/data#> .

data:dali_4587 rdf:type paintings:Painting ;
    paintings:hasTitle data:value_A ;
    paintings:hasArtist data:value_B .

data:value_A rdf:type kb:TextValue ;
    kb:valueHasString "The Persistence of Memory" .

data:value_B rdf:type kb:TextValue ;
    kb:valueHasString "Salvador Dali" .

data:pompidou rdf:type paintings:Collection ;
    paintings:hasCollectionName data:value_C .

data:value_C rdf:type kb:TextValue ;
    kb:valueHasString "Centre Pompidou, Paris" .

We can then state that the painting is in the collection:

data:dali_4587 paintings:isInCollection data:pompidou ;
    paintings:isinCollectionValue data:value_D .

data:value_D rdf:type kb:LinkValue ;
    rdf:subject data:dali_4587 ;
    rdf:predicate paintings:isInCollection ;
    rdf:object data:pompidou ;
    kb:valueHasRefCount 1 .

This creates a link (paintings:isInCollection) between the painting and the collection, along with a reification containing metadata about the link. We can visualise the result as the following graph:

Figure 2

Knora allows a user to see a link if the requesting user has permission to see the source and target resources as well as the kb:LinkValue.

Text with Standoff Markup

Knora is designed to be able to store text with markup, which can indicate formatting and structure, as well as the complex observations involved in transcribing handwritten manuscripts. One popular way of representing text in the humanities is to encode it in XML using the Text Encoding Initiative (TEI) guidelines. In Knora, a TEI/XML document can be stored as a file with attached metadata, but this is not recommended, because it does not allow Knora to perform searches across multiple documents.

The recommended way to store text with markup in Knora is to use Knora’s built-in support for “standoff” markup, which is stored separately from the text. This has some advantages over embedded markup such as XML. While XML requires markup to have a hierarchical structure, and does not allow overlapping tags, standoff nodes do not have these limitations (see Using Standoff Properties for Marking-up Historical Documents in the Humanities). A standoff tag can be attached to any substring in the text by giving its start and end positions. Unlike in corpus linguistics, we do not use any tokenisation resulting in a form of predefined segmentation, which would limit the user’s ability to freely annotate any ranges in the text.

For example, suppose we have the following text:

This sentence has overlapping visual attributes.
This would require just two standoff tags: (italic, start=5, end=29) and (bold, start=14, end=36).

Moreover, standoff makes it possible to mark up the same text in different, possibly incompatible ways, allowing for different interpretations without making redundant copies of the text. In the Knora base ontology, any text value can have standoff tags.

By representing standoff as RDF triples, Knora makes markup searchable across multiple text documents in a repository. For example, if a repository contains documents in which references to persons are indicated in standoff, it is straightforward to find all the documents mentioning a particular person. Knora’s standoff support is intended to make it possible to convert documents with embedded, hierarchical markup, such as TEI/XML, into RDF standoff and back again, with no data loss, thus bringing the benefits of RDF to existing TEI-encoded documents.

In the Knora base ontology, a TextValue can have one or more standoff tags. Each standoff tag indicates the start and end positions of a substring in the text that has a particular attribute. The OWL class kb:StandoffTag, which is the base class of all standoff node classes, has these properties:

standoffTagHasStart (1)

The index of the first character in the text that has the attribute.

standoffTagHasEnd (1)

The index of the last character in the text that has the attribute, plus 1.

standoffTagHasUUID (1)

A UUID identifying this instance and those corresponding to it in later versions of the TextValue it belongs to. The UUID is a means to maintain a reference to a particular range of a text also when new versions are made and standoff tag IRIs change.

standoffTagHasOriginalXMLID (0-1)

The original id of the XML element that the standoff tag represents, if any.

standoffTagHasStartIndex (1)

The start index of the standoff tag. Start indexes are numbered from 0 within the context of a particular text. When several standoff tags share the same start position, they can be nested correctly with this information when transforming them to XML.

standoffTagHasEndIndex (1)

The end index of the standoff tag. Start indexes are numbered from 0 within the context of a particular text. When several standoff tags share the same end position, they can be nested correctly with this information when transforming them to XML.

standoffTagHasStartParent (0-1)

Points to the parent standoff tag. This corresponds to the original nesting of tags in XML. If a standoff tag has no parent, it represents the XML root element. If the original XML element is a CLIX tag, it represents the start of a virtual (non syntactical) hierarchy.

standoffTagHasEndParent (0-1)

Points to the parent standoff tag if the original XML element is a CLIX tag and represents the end of a virtual (non syntactical) hierarchy.

The StandoffTag class is not used directly in RDF data; instead, its subclasses are used. A few subclasses are currently provided in standoff-onto.ttl, and more will be added to support TEI semantics. Projects are able to define their own custom standoff tag classes (direct subclasses of StandoffTag or one of the standoff data type classes or subclasses of one of the standoff classes defined in standoff-onto.ttl).

Subclasses of StandoffTag

Standoff Data Type Tags

Associates data in some Knora value type with a substring in a text. Standoff data type tags are subclasses of ValueBase classes.

  • StandoffLinkTag Indicates that a substring refers to another kb:Resource. See StandoffLinkTag.
  • StandoffInternalReferenceTag Indicates that a substring refers to another standoff tag in the same text value. See Internal Links in a TextValue.
  • StandoffUriTag Indicates that a substring is associated with a URI, which is stored in the same form that is used for kb:UriValue. See UriValue.
  • StandoffDateTag Indicates that a substring represents a date, which is stored in the same form that is used for kb:DateValue. See DateValue.
  • StandoffColorTag Indicates that a substring represents a color, which is stored in the same form that is used for kb:ColorValue. See ColorValue.
  • StandoffIntegerTag Indicates that a substring represents an integer, which is stored in the same form that is used for kb:IntegerValue. See IntValue.
  • StandoffDecimalTag Indicates that a substring represents a number with fractions, which is stored in the same form that is used for kb:DecimalValue. See DecimalValue.
  • StandoffIntervalTag Indicates that a substring represents an interval, which is stored in the same form that is used for kb:IntervalValue. See IntervalValue.
  • StandoffBooleanTag Indicates that a substring represents a Boolean, which is stored in the same form that is used for kb:BooleanValue. See BooleanValue.
StandoffLinkTag

A StandoffLinkTag Indicates that a substring is associated with a Knora resource. For example, if a repository contains resources representing persons, a text could be marked up so that each time a person’s name is mentioned, a StandoffLinkTag connects the name to the Knora resource describing that person. Property:

standoffTagHasLink (1)

The IRI of the resource that is referred to.

One of the design goals of the Knora ontology is to make it easy and efficient to find out which resources contain references to a given resource. Direct links are easier and more efficient to query than indirect links. Therefore, when a text value contains a resource reference in its standoff nodes, Knora automatically creates a direct link between the containing resource and the target resource, along with an RDF reification (a kb:LinkValue) describing the link, as discussed in Links Between Resources. In this case, the link property is always kb:hasStandoffLinkTo, and the link value property (which points to the LinkValue) is always kb:hasStandoffLinkToValue.

Knora automatically updates direct links and reifications for standoff resource references when text values are updated. To do this, it keeps track of the number of text values in each resource that contain at least one standoff reference to a given target resource. It stores this number as the reference count of the LinkValue (see LinkValue) describing the direct link. Each time this number changes, it makes a new version of the LinkValue, with an updated reference count. When the reference count reaches zero, it removes the direct link and makes a new version of the LinkValue, marked with kb:isDeleted.

For example, if data:R1 is a resource with a text value in which the resource data:R2 is referenced, the repository could contain the following triples:

data:R1 ex:hasComment data:V1 .

data:V1 rdf:type kb:TextValue ;
    kb:valueHasString "This link is internal." ;
    kb:valueHasStandoff data:SO1 .

data:SO1 rdf:type kb:StandoffLinkTag ;
    kb:standoffTagHasStart: 5 ;
    kb:standoffTagHasEnd: 9 ;
    kb:standoffTagHasLink data:R2 .

data:R1 kb:hasStandoffLinkTo data:R2 .
data:R1 kb:hasStandoffLinkToValue data:LV1 .

data:LV1 rdf:type kb:LinkValue ;
    rdf:subject data:R1 ;
    rdf:predicate kb:hasStandoffLinkTo ;
    rdf:object data:R2 ;
    kb:valueHasRefCount 1 .

The result can be visualized like this:

Figure 3

Link values created automatically for resource references in standoff are visible to all users, and the creator of these link values is always kb:SystemUser (see Users and Groups). The Knora API server allows a user to see a standoff link if the user has permission to see the source and target resources.

Internal Links in a TextValue

Internal links in a TextValue can be represented using the data type standoff class StandoffInternalReferenceTag or a subclass of it. It has the following property:

standoffTagHasInternalReference (1)

Points to a StandoffTag that belongs to the same TextValue. It has an objectClassConstraint of StandoffTag.

For links to a kb:Resource, see StandoffLinkTag.

Mapping to Create Standoff From XML

A mapping allows for the conversion of an XML document to RDF-standoff and back. A mapping defines one-to-one relations between XML elements (with or without a class) and attributes and standoff classes and properties (see XML to Standoff Mapping).

A mapping is represented by a kb:XMLToStandoffMapping which contains one or more kb:MappingElement. A kb:MappingElement maps an XML element (including attributes) to a standoff class and standoff properties. It has the following properties:

mappingHasXMLTagname (1)

The name of the XML element that is mapped to a standoff class.

mappingHasXMLNamespace (1)

The XML namespace of the XML element that is mapped to a standoff class. If no namespace is given, noNamespace is used.

mappingHasXMLClass (1)

The name of the class of the XML element. If it has no class, noClass is used.

mappingHasStandoffClass (1)

The standoff class the XML element is mapped to.

mappingHasXMLAttribute (0-n)

Maps XML attributes to standoff properties using MappingXMLAttribute. See below.

mappingHasStandoffDataTypeClass (0-1)

Indicates the standoff data type class of the standoff class the XML element is mapped to.

mappingElementRequiresSeparator (1)

Indicates if there should be an invisible word separator inserted after the XML element in the RDF-standoff representation. Once the markup is stripped, text segments that belonged to different elements may be concatenated.

A MappingXMLAttribute has the following properties:

mappingHasXMLAttributename

The name of the XML attribute that is mapped to a standoff property.

mappingHasXMLNamespace

The namespace of the XML attribute that is mapped to a standoff property. If no namespace is given, noNamespace is used.

mappingHasStandoffProperty

The standoff property the XML attribute is mapped to.

Knora includes a standard mapping used by the SALSAH GUI. It has the IRI http://rdfh.ch/standoff/mappings/StandardMapping and defines mappings for a few elements used to write texts with simple markup.

Standoff in Digital Editions

Knora’s standoff is designed to make it possible to convert XML documents to standoff and back. One application for this feature is an editing workflow in which an editor works in an XML editor, and the resulting XML documents are converted to standoff and stored in Knora, where they can be searched and annotated.

If an editor wants to correct text that has been imported from XML into standoff, the text can be exported as XML, edited, and imported again. To preserve annotations on standoff tags across edits, each tag can automatically be given a UUID. In a future version of the Knora base ontology, it will be possible to create annotations that point to UUIDs rather than to IRIs. When a text is exported to XML, the UUIDs can be included in the XML. When the edited XML is imported again, it can be converted to new standoff tags with the same UUIDs. Annotations that applied to standoff tags in the previous version of the text will therefore also apply to equivalent tags in the new version.

When text is converted from XML into standoff, tags are also given indexes, which are numbered from 0 within the context of a particular text. This makes it possible to order tags that share the same position, and to preserve the hierarchy of the original XML document. An ordinary, hierarchical XML tag is converted to a standoff tag that has one index, as well as the index of its parent tag, if any. The Knora base ontology also supports non-hierarchical markup such as CLIX, which enables overlapping markup to be represented in XML. When non-hierarchical markup is converted to standoff, both the start position and the end position of the standoff tag have indexes and parent indexes.

To support these features, a standoff tag can have these additional properties:

standoffTagHasStartIndex (0-1)

The index of the start position.

standoffTagHasEndIndex (0-1)

The index of the end position, if this is a non-hierarchical tag.

standoffTagHasStartParent (0-1)

The IRI of the tag, if any, that contains the start position.

standoffTagHasEndParent (0-1)

The IRI of the tag, if any, that contains the end position, if this is a non-hierarchical tag.

standoffTagHasUUID (0-1)

A UUID that can be used to annotate a standoff tag that may be present in different versions of a text, or in different layers of a text (such as a diplomatic transcription and an edited critical text).

Querying Standoff in SPARQL

A future version of Knora will provide an API for querying standoff markup. In the meantime, it is possible to query it directly in SPARQL. For example, here is a SPARQL query (using RDFS inference) that finds all the text values texts that have a standoff date tag referring to Christmas Eve 2016, contained in a StandoffItalicTag:

PREFIX knora-base: <http://www.knora.org/ontology/knora-base#>
PREFIX standoff: <http://www.knora.org/ontology/standoff#>

select * where {
    ?standoffTag a knora-base:StandoffDateTag  .

    ?standoffTag knora-base:valueHasStartJDN ?dateStart .
    ?standoffTag knora-base:valueHasEndJDN ?dateEnd .

    FILTER (2457747  <= ?dateEnd && 2457747  >= ?dateStart)

    ?standoffTag knora-base:standoffTagHasStartParent ?parent .
    ?parent a standoff:StandoffItalicTag .

    ?textValue knora-base:valueHasStandoff ?standoffTag .
    ?textValue knora-base:valueHasString ?string .

    ?standoffTag knora-base:standoffTagHasStart ?startPos .
    ?standoffTag knora-base:standoffTagHasEnd ?endPos .  
}

Authorisation

Users and Groups

Each Knora user is represented by an object belonging to the class kb:User, which is a subclass of foaf:Person, and has the following properties:

userid (1)

A unique identifier that the user must provide when logging in.

password (1)

A cryptographic hash of the user’s password.

email (0-n)

Email addresses belonging to the user.

isInProject (0-n)

Projects that the user is a member of.

isInGroup (0-n)

user-created groups that the user is a member of.

foaf:familyName (1)

The user’s family name.

foaf:givenName (1)

The user’s given name.

Knora’s concept of access control is that an object (a resource or value) can grant permissions to groups of users (but not to individual users). There are four built-in groups:

UnknownUser

Any user who has not logged into Knora is automatically assigned to this group.

KnownUser

Any user who has logged into Knora is automatically assigned to this group.

ProjectMember

When checking a user’s permissions on an object, the user is automatically assigned to this group if she is a member of the project that the object belongs to.

Creator

When checking a user’s permissions on an object, the user is automatically assigned to this group if he is the creator of the object.

A user-created ontology can define additional groups, which must belong to the OWL class kb:UserGroup.

There is one built-in SystemUser, which is the creator of link values created automatically for resource references in standoff markup (see StandoffLinkTag).

Permissions

An object can grant the following permissions, which are stored in a compact format in a single string. This string is the object of the predicate kb:hasPermissions, which is required on every kb:Resource and kb:Value.

  1. Restricted view permission (RV) Allows a restricted view of the object, e.g. a view of an image with a watermark.
  2. View permission (V) Allows an unrestricted view of the object. Having view permission on a resource only affects the user’s ability to view information about the resource other than its values. To view a value, she must have view permission on the value itself.
  3. Modify permission (M) For values, this permission allows a new version of a value to be created. For resources, this allows the user to create a new value (as opposed to a new version of an existing value), or to change information about the resource other than its values. When he wants to make a new version of a value, his permissions on the containing resource are not relevant. However, when he wants to change the target of a link, the old link must be deleted and a new one created, so he needs modify permission on the resource.
  4. Delete permission (D) Allows the item to be marked as deleted.
  5. Change rights permission (CR) Allows the permissions granted by the object to be changed.

Each permission in the above list implies all lower-numbered permissions. A user’s permission level on a particular object is calculated in the following way:

  1. Make a list of the groups that the user belongs to, including Creator and/or ProjectMember if applicable.
  2. Make a list of the permissions that she can obtain on the object, by iterating over the permissions that the object grants. For each permission, if she is in the specified group, add the specified permission to the list of permissions she can obtain.
  3. From the resulting list, select the highest-level permission.
  4. If the result is that she would have no permissions, give her whatever permission UnknownUser would have.

To view a link between resources, a user needs permission to view the source and target resources. He also needs permission to view the LinkValue representing the link, unless the link property is hasStandoffLinkTo (see StandoffLinkTag).

The format of the object of kb:hasPermissions is as follows:

  • Each permission is represented by the one-letter or two-letter abbreviation given above.
  • Each permission abbreviation is followed by a space, then a comma-separated list of groups that the permission is granted to.
  • The IRIs of built-in groups are shortened using the knora-base prefix.
  • Multiple permissions are separated by a vertical bar (|).

For example, if an object grants view permission to unknown and known users, and modify permission to project members, the resulting permission literal would be:

V knora-base:UnknownUser,knora-base:KnownUser|M knora-base:ProjectMember

Consistency Checking

Knora tries to enforce repository consistency by checking constraints that are specified in the Knora base ontology and in user-created ontologies. Three types of consistency rules are enforced:

  • Cardinalities in OWL class definitions must be satisfied.
  • Constraints on the types of the subjects and objects of OWL object properties must be satisfied.
  • A datatype property may not have an empty string as an object.

The implementation of consistency checking is partly triplestore-dependent; Knora may be able to provide stricter checks with some triplestores than with others.

OWL Cardinalities

As noted in Resources, each subclass of Resource must use OWL cardinality restrictions to specify the properties it can have. More specifically, a resource is allowed to have a property that is a subproperty of kb:hasValue or kb:hasLinkTo only if the resource’s class has some cardinality for that property. Similarly, a value is allowed to have a subproperty of kb:valueHas only if the value’s class has some cardinality for that property.

Knora supports, and attempts to enforce, the following cardinality constraints:

owl:cardinality 1

A resource of this class must have exactly one instance of the specified property.

owl:minCardinality 1

A resource of this class must have at least one instance of the specified property.

owl:maxCardinality 1

A resource of this class may have zero or one instance of the specified property.

owl:minCardinality 0

A resource of this class may have zero or more instances of the specified property.

Knora requires cardinalities to be defined using blank nodes, as in the following example from knora-base:

:Representation rdf:type owl:Class ;
    rdfs:subClassOf :Resource ,
        [ rdf:type owl:Restriction ;
          owl:onProperty :hasFileValue ;
          owl:minCardinality "1"^^xsd:nonNegativeInteger ] .

:StillImageRepresentation rdf:type owl:Class ;
    rdfs:subClassOf :Representation ,
        [ rdf:type owl:Restriction ;
          owl:onProperty :hasStillImageFileValue ;
          owl:minCardinality "1"^^xsd:nonNegativeInteger ] .

The cardinality of a link property must be the same as the cardinality of the corresponding link value property.

Each owl:Restriction may have the predicate salsah-gui:guiOrder to indicate the order in which properties should be displayed in a GUI (see The SALSAH GUI Ontology).

A resource class inherits cardinalities from its superclasses. This follows from the rules of RDFS inference. Also, in Knora, cardinalities in the subclass can override cardinalities that would otherwise be inherited from the superclass. Specifically, if a superclass has a cardinality on a property P, and a subclass has a cardinality on a subproperty of P, the subclass’s cardinality overrides the superclass’s cardinality. In the example above, hasStillImageFileValue is a subproperty of hasFileValue. Therefore, the cardinality on hasStillImageFileValue overrides (i.e. replaces) the one on hasFileValue.

Note that, unlike cardinalities, predicates of properties are not inherited. If :foo rdfs:subPropertyOf :bar, this does not mean that :foo inherits anything from :bar. Any predicates of :foo that are also needed by :bar must be defined explicitly on :bar. This design decision was made because property predicate inheritance is not provided by RDFS inference, and would make it more difficult to check the correctness of ontologies, while providing little practical benefit.

For more information about OWL cardinalities, see the OWL 2 Primer.

Constraints on the Types of Property Subjects and Objects

When a user-created ontology defines a property, it must indicate the types that are allowed as objects (and, if possible, as subjects) of the property. This is done using the following Knora-specific properties:

subjectClassConstraint

Specifies the class that subjects of the property must belong to. This constraint is recommended but not required. Knora will attempt to enforce this constraint.

objectClassConstraint

If the property is an object property, specifies the class that objects of the property must belong to. Every subproperty of kb:hasValue or a kb:hasLinkTo (i.e. every property of a resource that points to a kb:Value or to another resource) is required to have this constraint, because Knora relies on it to know what type of object to expect for the property. Knora will attempt to enforce this constraint.

objectDatatypeConstraint

If the property is a datatype property, specifies the type of literals that can be objects of the property. Knora will not attempt to enforce this constraint, but it is useful for documentation purposes.

Note that it is possible for a subproperty to have a more restrictive contraint than its base property, by specifing a subject or object class that is a subclass of the one specified in the base property. However, it is not possible for the subproperty to make the base property’s constraint less restrictive.

See also Why doesn’t Knora use rdfs:domain and rdfs:range for consistency checking?

Consistency Constraint Example

A user-created ontology could define consistency constraints as in this simplified example:

:book rdf:type owl:Class ;
    rdfs:subClassOf knora-base:Resource ,
        [ rdf:type owl:Restriction ;
          owl:onProperty :hasTitle ;
          owl:cardinality "1"^^xsd:nonNegativeInteger ] ,
        [ rdf:type owl:Restriction ;
          owl:onProperty :hasAuthor ;
          owl:minCardinality "0"^^xsd:nonNegativeInteger ] .

:hasTitle rdf:type owl:ObjectProperty ;
    knora-base:subjectClassConstraint :book ;
    knora-base:objectClassConstraint knora-base:TextValue .

:hasAuthor rdf:type owl:ObjectProperty ;
    knora-base:subjectClassConstraint :book ;
    knora-base:objectClassConstraint knora-base:TextValue .

Summary of Restrictions on User-Created Ontologies

An ontology can refer to a Knora ontology in another project only if the other ontology is built-in or shared (see Shared Ontologies).

Restrictions on Classes

  • Each class must be a subclass of either kb:Resource or kb:StandoffTag, but not both (note that this forbids user-created subclasses of kb:Value).
  • All the cardinalities that a class defines directly (i.e. does not inherit from kb:Resource) must be on properties that are defined in the triplestore.
  • Within the cardinalities of a class, there must be a link value property for each link property and vice versa.
  • The cardinality of a link property must be the same as the cardinality of the corresponding link value property.
  • A cardinality on a property with a boolean value must be owl:cardinality 1 or owl:maxCardinality 1.
  • Each class must be a subclass of all the classes that are subject class constraints of the properties in its cardinalities.
  • If it’s a resource class, all its directly defined cardinalities must be on Knora resource properties (subproperties of kb:hasValue or kb:hasLinkTo), and all its base classes with Knora IRIs must also be resource classes. A cardinality on kb:resourceProperty or kb:hasValue is forbidden. It must also have an rdfs:label.
  • If it’s a standoff class, none of its cardinalities may be on Knora resource properties, and all its base classes with Knora IRIs must also be standoff classes.
  • A class cannot have a cardinality on property P as well as a cardinality on a subproperty of P.

Restrictions on properties

  • The property’s subject class constraint, if provided, must be a subclass of kb:Resource or kb:StandoffTag, and must be a subclass of the subject class constraints of all its base properties.
  • Its object class constraint, if provided, must be a subclass of the object class constraints of all its base properties.
  • If the property is a Knora resource property, it must have an object class constraint and an rdfs:label.
  • It can’t be a subproperty of both kb:hasValue and kb:hasLinkTo.
  • It can’t be a subproperty of kb:hasFileValue.
  • Each of its base properties that has a Knora IRI must also be a Knora resource property.

Standardisation

The DaSCH intends to coordinate the standardisation of generally useful entities proposed in user-created ontologies. We envisage a process in which two or more projects would initiate the process by starting a public discussion on proposed entities to be shared. Once a consensus was reached, the DaSCH would publish these entities in an ontology that could be used by multiple projects, and would ensure that such ontologies are not subsequently modified in ways that break compatibility with existing data. Functionality may be added to Knora to facilitate this process.