Knowledge Catalog

This chapter discusses the Knowledge Catalog and how to use it.

Page Contents

1. Overview
2. Configuration Options
3. Usage
   1. Data Model Classes
   2. Example SPARQL Queries
   3. Data Source Linking
   4. Explorer
   5. Explorer Query Builder
   6. Simplified Model

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Overview

The Knowledge Catalog allows users to query virtual graph and data source metadata using SPARQL. The Knowledge Catalog is enabled by default. It watches for changes to virtual graphs and data sources and adds or updates the related metadata in the catalog database.

The Knowledge Catalog automatically creates a catalog database when it starts. Set the catalog.database server property to use a different name or prevent the Knowledge Catalog from using an existing database named catalog.

To prevent the Knowledge Catalog database from being created automatically set catalog.auto.create.db to false.

Configuration Options

The Knowledge Catalog can be configured with the following options in stardog.properties:

Usage

The capture and storage of Stardog metadata happens automatically and without user interaction. Users can interact with the Knowledge Catalog with SPARQL queries and Explorer (to visually explore the metadata). The Explorer query builder can also be used to query the metadata model.

To query only metadata, run SPARQL queries against the catalog database. To query metadata in addition to another database, the local database service can be used.

Data Model Classes

This table contains the classes used for modeling the Knowledge Catalog metadata.

Example SPARQL Queries

The following are some query examples that demonstrate how virtual graph metadata can be queried.

• What catalogs are available?

    prefix dcterms: <http://purl.org/dc/terms/>
    prefix dcat: <http://www.w3.org/ns/dcat#>
    select ?src ?lbl where {
        graph <tag:stardog:api:catalog:local> {
        ?src a dcat:Catalog ; dcterms:title ?lbl
    }}
  

• What datasets are in the catalog?

    prefix dcterms: <http://purl.org/dc/terms/>
    prefix dcat: <http://www.w3.org/ns/dcat#>
    select ?ds where {
        graph <tag:stardog:api:catalog:local> {
        ?src a dcat:Catalog ; dcat:dataset ?ds .
    }}
  

• Query for all table columns across all datasets

    prefix : <tag:stardog:api:catalog:>
    select *
    from stardog:context:local 
    where {
        ?t a :Table ;
           :hasColumn ?c .
        ?c :columnName ?n .
    } 
  

• Dump the contents of a catalog

    prefix : <tag:stardog:api:catalog:>
    prefix rr: <http://www.w3.org/ns/r2rml#>
    select * 
    from stardog:context:local
    where {
        ?ds a :DataSource ;
            :hasCatalog ?cata .
        ?cata :hasTable ?table .
        ?table :hasColumn ?column .
        ?vg :connectsTo ?ds ;
            :hasMapping ?map .
        ?map rr:predicateObjectMap ?pomap .
        ?pomap rr:objectMap ?omap ;
            rr:predicateMap ?predmap .
        ?omap rr:column ?col .
        ?omap rr:datatype ?dtype .
        ?predmap rr:constant ?con .
        ?fld rr:logicalTable ?logtbl .
        ?logtbl rr:tableName ?tblname .
        ?fld rr:subjectMap ?subjmap .
        ?subjmap rr:template ?template ;
            rr:termType ?termtype .
    }
  

Data Source Linking

When data sources and virtual graphs are added to the Knowledge Catalog, their source data and graph maps are scanned for table and column references. Those references are then linked together through a tag:stardog:api:catalog:mapsColumn attribute.

This can be useful information for finding how a triple is mapped to an external database and which tables and columns were used.

• Example query to see all mapped columns:

    select * 
    from stardog:context:local {
        ?s stardog:catalog:mapsColumn ?o .
    }
  

• Example path query from a known predicate to its mapping:

    PATHS shortest
    from stardog:context:local
    start ?s = <http://example.org/comvendor#nr>
    end ?e = <tag:stardog:api:catalog:bsbm:benchmark:vendor:nr>
    via {
        ?pomap <http://www.w3.org/ns/r2rml#constant> ?s ;
               stardog:catalog:mapsColumn ?e .
    }
  

Explorer

After you have the Knowledge Catalog configured and running on your server, you can log into Explorer and select your catalog database to begin exploring.

Explorer caches the catalog data when you log in. If you make changes to virtual graphs in Studio or on the command line, you will need to refresh Explorer to see the changes.

Explorer Query Builder

Properties have been added to enable the Explorer query builder functionality with the Knowledge Catalog.

Simplified Model

To simplify navigation of Stardog data source columns to mapped virtual graphs, new rules have been added to the catalog model starting with Stardog 9.0.0.

In this screenshot the theme property has been added by the simplified model rules. Note that reasoning must be turned on for these rules to apply.

• External Catalogs - adding metadata from external catalog systems

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