The Knora Base Ontology

Introduction

Resource Description Framework (RDF)

Knora uses a hierarchy of ontologies based on the Resource Description Framework (RDF), RDF Schema (RDFS), and the Web Ontology Language (OWL). Both RDFS and OWL are expressed in RDF. RDF expresses information as a set of statements (called triples). A triple consists of a subject, a predicate, and an object:

The object may be either a literal value (such as a name or number) or another subject. Thus it is possible to create complex graphs that connect many subjects, like this:

In RDF, each subject and predicate has a unique, URL-like identifier called an Internationalized Resource Identifier (IRI). Within a given project, IRIs typically differ only in their last component (the “local part”), which is often the fragment following a # character. Such IRIs share a long “prefix”. In Turtle and similar formats for writing RDF, a short prefix label can be defined to represent the long prefix. Then an IRI can be written as a prefix label and a local part, separated by a colon (:). For example, if the “example” project’s long prefix is http://www.example.org/rdf#, and it contains subjects with IRIs like http://www.example.org/rdf#book, we can define the prefix label ex to represent the prefix label, and write prefixed names for IRIs:

In this document, we use the prefix label kb to represent the Knora base ontology, [1] but we usually omit it for brevity.

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.

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):

shortname (1)

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

basepath (1)

The filesystem path of the directory where the project’s files are stored.

foaf:name (0-1)

The name of the project.

description (0-1)

A description of the project.

belongsTo (0-1)

The kb:Institution that the project belongs to.

Resources and values are associated with a project by means of the kb:attachedToProject property, as described in The Knora Data Model. Users are associated with a project by means of the kb:isInProject property, as described in Users and Groups.

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 the Knora API server 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 project-specific 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 Authorization can change its label.

A resource can be marked as deleted; the Knora API server 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 Authorization.

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 one or more files attached to it. For each file, there is a kb:FileValue in the triplestore containing metadata about the file (see FileValue). A resource that has file values must belong to one of the subclasses of kb:Representation. The base class Representation, which is not intended to be used directly, has this property:

hasFileValue (1-n)

Points to one or more file values.

Its subclasses, which are intended to be used directly in data, include:

StillImageRepresentation

A representation containing still image files.

MovingImageRepresentation

A representation containing video files.

AudioRepresentation

A representation containing audio files.

DDDrepresentation

A representation containing 3D images.

TextRepresentation

A representation containing formatted text files, such as XML files.

DocumentRepresentation

A representation containing documents (such as PDF files) that are not text files.

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 version 1 of the Knora API.

The more flexible approach, which is allowed by the Knora base ontology and will be supported by version 2 of the Knora API, 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 project-specific ontologies.

A value is always part of one particular resource, which points to it using some property derived from hasValue. For example, a project-specific 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 Authorization).

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.

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). 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 the Knora API server.

internalMimeType (1)

The MIME type of the file as stored by the Knora API server.

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.

The files in a given representation must be semantically equivalent, meaning that coordinates that relate to one file must also be valid for other files in the same representation. Coordinates in Knora are expressed as fractions of the size of the object on some dimension; for example, image coordinates are expressed as fractions of its width and height, rather than in pixels. Therefore, the image files in a StillImageRepresentation must have the same aspect ratio, but they need not have the same dimensions in pixels. Similarly, the audio and video files in an AudioRepresentation or MovingImageRepresentation must have the same length in seconds, but may have different bitrates.

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 the Knora API server, but instead resides in an external repository managed by some other software. The ExternalResValue contains the information that the Knora API server 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:

The Knora API server 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. [2] 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. [3] While XML requires markup to have a hierarchical structure, and does not allow overlapping tags, standoff nodes do not have these limitations (Schmidt2016). A standoff tag can be attached to any substring in the text by giving its start and end positions. [4] 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, the Knora API server 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.

The Knora API server 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:

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 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://data.knora.org/projects/standoff/mappings/StandardMapping and defines mappings for a few elements used to write texts with simple markup (see XML to Standoff Mapping).

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 the Knora API server 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 .
}

Authorization

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)

Project-specific 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 the Knora API server is automatically assigned to this group.

KnownUser

Any user who has logged into the Knora API server 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 project-specific 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 project-specific 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 ] .

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 project-specific 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 the Knora API server 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.

Consistency Constraint Example

A project-specific 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 .

Open Questions

Extending Existing Resource Definitions

How should extensions of existing resources be handled? Project B extends a resource defined in the project A ontology, by adding new properties/values which are interesting for project B.

Notes

[1]	http://www.knora.org/ontology/knora-base#

[2]	TEI refers both to an organization and an XML-based markup language (or more precisely: a set of grammar modules – XML schemas – that can be combined to define a markup language). For reasons of clarity, we use the term TEI/XML to refer to the markup language.

[3]	It is also possible to encode standoff markup using XML. For example, the TEI guidelines discuss standoff markup. However, standoff markup is not widely applied in the TEI community. TEI’s main focus is on encoding a hierarchy of elements.

[4]	Unlike in corpus linguistics, we do not use any tokenization resulting in a form of predefined segmentation that would limit the user’s possibility to freely annotate any ranges in the text.

References

[Schmidt2016]

Schmidt, Desmond. 2016. “Using Standoff Properties for Marking-up Historical Documents in the Humanities.” It – Information Technology 58: 1. http://ecdosis.net/papers/schmidt.d.2016.pdf.