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Work in progress: CKAN + DCAT
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ckanext-dcat

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This extension provides plugins that allow CKAN to expose and consume metadata from other catalogs using RDF documents serialized using DCAT. The Data Catalog Vocabulary (DCAT) is "an RDF vocabulary designed to facilitate interoperability between data catalogs published on the Web". More information can be found on the following W3C page:

http://www.w3.org/TR/vocab-dcat

Contents

Overview

With the emergence of Open Data initiatives around the world, the need to share metadata across different catalogs has became more evident. Sites like http://publicdata.eu aggregate datasets from different portals, and there has been a growing demand to provide a clear and standard interface to allow incorporating metadata into them automatically.

There is growing consensus around DCAT being the right way forward, but actual implementations are needed. This extension aims to provide tools and guidance to allow publishers to publish and share DCAT based metadata easily.

In terms of CKAN features, this extension offers:

These are implemented internally using:

Installation

  1. Install ckanext-harvest (https://github.com/ckan/ckanext-harvest#installation) (Only if you want to use the RDF harvester)

  2. Install the extension on your virtualenv:

    (pyenv) $ pip install -e git+https://github.com/ckan/ckanext-dcat.git#egg=ckanext-dcat

  3. Install the extension requirements:

    (pyenv) $ pip install -r ckanext-dcat/requirements.txt

  4. Enable the required plugins in your ini file:

    ckan.plugins = dcat dcat_rdf_harvester dcat_json_harvester dcat_json_interface

RDF DCAT endpoints

When the dcat plugin is enabled, the following RDF endpoints are available on your CKAN instance. The schema used on the serializations can be customized using profiles.

Dataset endpoints

RDF representations of a particular dataset can accessed using the following endpoint:

https://{ckan-instance-host}/dataset/{dataset-id}.{format}

The extension will determine the RDF serialization format returned. The currently supported values are:

Extension Format Media Type
xml RDF/XML application/rdf+xml
ttl Turtle text/turtle
n3 Notation3 text/n3
jsonld JSON-LD application/ld+json

The fallback rdf format defaults to RDF/XML.

Here's an example of the different formats available (links might not be live as they link to a demo site):

RDF representations will be advertised using <link rel="alternate"> tags on the <head> sectionon the dataset page source code, eg:

<head>

    <link rel="alternate" type="application/rdf+xml" href="http://demo.ckan.org/dataset/34315559-2b08-44eb-a2e6-ebe9ce1a266b.rdf"/>
    <link rel="alternate" type="text/ttl" href="http://demo.ckan.org/dataset/34315559-2b08-44eb-a2e6-ebe9ce1a266b.ttl"/>
    <!-- ... -->

</head>

Check the RDF DCAT Serializer section for more details about how these are generated and how to customize the output using profiles.

Note: When using this plugin, the above endpoints will replace the old deprecated ones that were part of CKAN core.

Catalog endpoint

Additionally to the individual dataset representations, the extension also offers a catalog-wide endpoint for retrieving multiple datasets at the same time (the datasets are paginated, see below for details):

https://{ckan-instance-host}/catalog.{format}?[page={page}]&[modified_since={date}]

This endpoint can be customized if necessary using the ckanext.dcat.catalog_endpoint configuration option, eg:

ckanext.dcat.catalog_endpoint = /dcat/catalog/{_format}

The custom endpoint must start with a backslash (/) and contain the {_format} placeholder.

As described previously, the extension will determine the RDF serialization format returned.

RDF representations will be advertised using <link rel="alternate"> tags on the <head> sectionon the homepage and the dataset search page source code, eg:

<head>

    <link rel="alternate" type="application/rdf+xml" href="http://demo.ckan.org/catalog.rdf"/>
    <link rel="alternate" type="application/rdf+xml" href="http://demo.ckan.org/catalog.xml"/>
    <link rel="alternate" type="text/ttl" href="http://demo.ckan.org/catalog.ttl"/>
    <!-- ... -->

</head>

The number of datasets returned is limited. The response will include paging info, serialized using the Hydra vocabulary. The different terms are self-explanatory, and can be used by clients to iterate the catalog:

@prefix hydra: <http://www.w3.org/ns/hydra/core#> .

<http://example.com/catalog.ttl?page=1> a hydra:PagedCollection ;
    hydra:firstPage "http://example.com/catalog.ttl?page=1" ;
    hydra:itemsPerPage 100 ;
    hydra:lastPage "http://example.com/catalog.ttl?page=3" ;
    hydra:nextPage "http://example.com/catalog.ttl?page=2" ;
    hydra:totalItems 283 .

The default number of datasets returned (100) can be modified by CKAN site maintainers using the following configuration option on your ini file:

ckanext.dcat.datasets_per_page = 20

The catalog endpoint also supports a modified_since parameter to restrict datasets to those modified from a certain date. The parameter value should be a valid ISO-8601 date:

http://demo.ckan.org/catalog.xml?modified_since=2015-07-24

URIs

Whenever possible, URIs are generated for the relevant entities. To try to generate them, the extension will use the first found of the following for each entity:

Note that if you are using the RDF DCAT harvester to import datasets from other catalogs and these define a proper URI for each dataset or resource, these will be stored as uri fields in your instance, and thus used when generating serializations for them.

Content negotiation

The extension supports returning different representations of the datasets based on the value of the Accept header (Content negotiation).

When enabled, client applications can request a particular format via the Accept header on requests to the main dataset page, eg:

curl https://{ckan-instance-host}/dataset/{dataset-id} -H Accept:text/turtle

curl https://{ckan-instance-host}/dataset/{dataset-id} -H Accept:"application/rdf+xml; q=1.0, application/ld+json; q=0.6"

This is also supported on the catalog endpoint, in this case when making a request to the CKAN root URL (home page). This won't support the pagination and filter parameters:

curl https://{ckan-instance-host} -H Accept:text/turtle

Note that this feature overrides the CKAN core home page and dataset page controllers, so you probably don't want to enable it if your own extension is also doing it.

To enable content negotiation, set the following configuration option on your ini file:

ckanext.dcat.enable_content_negotiation = True

RDF DCAT harvester

The RDF parser described in the previous section has been integrated into a harvester, to allow automatic import of datasets from remote sources. To enable the RDF harvester, add the dcat_rdf_harvester plugin to your CKAN configuration file:

ckan.plugins = ... dcat_rdf_harvester

The harvester will download the remote file, extract all datasets using the parser and create or update actual CKAN datasets based on that. It will also handle deletions, ie if a dataset is not present any more in the DCAT dump anymore it will get deleted from CKAN.

The harvester will look at the content-type HTTP header field to determine the used RDF format. Any format understood by the RDFLib library can be parsed. It is possible to override this functionality and provide a specific format using the harvester configuration. This is useful when the server does not return the correct content-type or when harvesting a file on the local file system without a proper extension. The harvester configuration is a JSON object that you fill into the harvester configuration form field.

{"rdf_format":"text/turtle"}

TODO: configure profiles.

Transitive harvesting

In transitive harvesting (i.e., when you harvest a catalog A, and a catalog X harvests your catalog), you may want to provide the original catalog info for each harvested dataset.

By setting the configuration option ckanext.dcat.expose_subcatalogs = True in your ini file, you'll enable the storing and publication of the source catalog for each harvested dataset.

The information contained in the harvested dcat:Catalog node will be stored as extras into the harvested datasets. When serializing, your Catalog will expose the harvested Catalog using the dct:hasPart relation. This means that your catalog will have this structure:

Extending the RDF harvester

The DCAT RDF harvester has extension points that allow to modify its behaviour from other extensions. These can be used by extensions implementing the IDCATRDFHarvester interface. Right now it provides the following methods:

To know more about these methods, please check the source of ckanext-dcat/ckanext/dcat/interfaces.py.

JSON DCAT harvester

The DCAT JSON harvester supports importing JSON objects that are based on DCAT terms but are not defined as JSON-LD. The exact format for these JSON files is the one described in the spec.datacatalogs.org site. There are example files in the examples folder.

To enable the JSON harvester, add the dcat_json_harvester plugin to your CKAN configuration file:

ckan.plugins = ... dcat_json_harvester

TODO: align the fields created by this harvester with the base mapping (ie the ones created by the RDF harvester).

RDF DCAT to CKAN dataset mapping

The following table provides a generic mapping between the fields of the dcat:Dataset and dcat:Distribution classes and their equivalents on the CKAN model. In most cases this mapping is deliberately a loose one. For instance, it does not try to link the DCAT publisher property with a CKAN dataset author, maintainer or organization, as the link between them is not straight-forward and may depend on a particular instance needs. When mapping from CKAN metadata to DCAT though, there are in some cases fallback fields that are used if the default field is not present (see RDF Serializer for more details on this.

This mapping is compatible with the DCAT-AP v1.1.

DCAT class DCAT property CKAN dataset field CKAN fallback fields Stored as
dcat:Dataset - extra:uri text See note about URIs
dcat:Dataset dct:title title text
dcat:Dataset dct:description notes text
dcat:Dataset dcat:keyword tags text
dcat:Dataset dcat:theme extra:theme list See note about lists
dcat:Dataset dct:identifier extra:identifier extra:guid, id text
dcat:Dataset adms:identifier extra:alternate_identifier text
dcat:Dataset dct:issued extra:issued metadata_created text
dcat:Dataset dct:modified extra:modified metadata_modified text
dcat:Dataset owl:versionInfo version extra:dcat_version text
dcat:Dataset adms:versionNotes extra:version_notes text
dcat:Dataset dct:language extra:language list See note about lists
dcat:Dataset dcat:landingPage url text
dcat:Dataset dct:accrualPeriodicity extra:frequency text
dcat:Dataset dct:conformsTo extra:conforms_to list See note about lists
dcat:Dataset dct:accessRights extra:access_rights text
dcat:Dataset foaf:page extra:documentation list See note about lists
dcat:Dataset dct:provenance extra:provenance text
dcat:Dataset dct:type extra:dcat_type text As of DCAT-AP v1.1 there's no controlled vocabulary for this field
dcat:Dataset dct:hasVersion extra:has_version list See note about lists. It is assumed that these are one or more URIs referring to another dcat:Dataset
dcat:Dataset dct:isVersionOf extra:is_version_of list See note about lists. It is assumed that these are one or more URIs referring to another dcat:Dataset
dcat:Dataset dct:source extra:source list See note about lists. It is assumed that these are one or more URIs referring to another dcat:Dataset
dcat:Dataset adms:sample extra:sample list See note about lists. It is assumed that these are one or more URIs referring to dcat:Distribution instances
dcat:Dataset dct:spatial extra:spatial_uri text If the RDF provides them, profiles should store the textual and geometric representation of the location in extra:spatial_text and extra:spatial respectively
dcat:Dataset dct:temporal extra:temporal_start + extra:temporal_end text None, one or both extras can be present
dcat:Dataset dct:publisher extra:publisher_uri text See note about URIs
foaf:Agent foaf:name extra:publisher_name text
foaf:Agent foaf:mbox extra:publisher_email organization:title text
foaf:Agent foaf:homepage extra:publisher_url text
foaf:Agent dct:type extra:publisher_type text
dcat:Dataset dcat:contactPoint extra:contact_uri text See note about URIs
vcard:Kind vcard:fn extra:contact_name maintainer, author text
vcard:Kind vcard:hasEmail extra:contact_email maintainer_email, author_email text
dcat:Dataset dcat:distribution resources text
dcat:Distribution - resource:uri text See note about URIs
dcat:Distribution dct:title resource:name text
dcat:Distribution dcat:accessURL resource:url text If accessURL is not present, downloadURL will be used as resource url
dcat:Distribution dcat:downloadURL resource:download_url text
dcat:Distribution dct:description resource:description text
dcat:Distribution dcat:mediaType resource:mimetype text
dcat:Distribution dct:format resource:format text This is likely to require extra logic to accommodate how CKAN deals with formats (eg ckan/ckanext-dcat#18)
dcat:Distribution dct:license resource:license text See note about dataset license
dcat:Distribution adms:status resource:status text
dcat:Distribution dcat:byteSize resource:size number
dcat:Distribution dct:issued resource:issued text
dcat:Distribution dct:modified resource:modified text
dcat:Distribution dct:rights resource:rights text
dcat:Distribution foaf:page resource:documentation list See note about lists
dcat:Distribution dct:language resource:language list See note about lists
dcat:Distribution dct:conformsTo resource:conforms_to list See note about lists
spdx:Checksum spdx:checksumValue resource:hash text
spdx:Checksum spdx:algorithm resource:hash_algorithm text

Notes

RDF DCAT Parser

The ckanext.dcat.processors.RDFParser class allows to read RDF serializations in different formats and extract CKAN dataset dicts. It will look for DCAT datasets and distributions and create CKAN datasets and resources, as dictionaries that can be passed to package_create or package_update.

Here is a quick overview of how it works:


    from ckanext.dcat.processors import RDFParser, RDFParserException

    parser = RDFParser()

    # Parsing a local RDF/XML file

    with open('datasets.rdf', 'r') as f:
        try:
            parser.parse(f.read())

            for dataset in parser.datasets():
                print('Got dataset with title {0}'.format(dataset['title'])

        except RDFParserException, e:
            print ('Error parsing the RDF file: {0}'.format(e))

    # Parsing a remote JSON-LD file

    import requests

    parser = RDFParser()

    content = requests.get('https://some.catalog.org/datasets.jsonld').content

    try:
        parser.parse(content, _format='json-ld')

        for dataset in parser.datasets():
            print('Got dataset with title {0}'.format(dataset['title'])

    except RDFParserException, e:
        print ('Error parsing the RDF file: {0}'.format(e))

The parser is implemented using RDFLib, a Python library for working with RDF. Any RDF serialization format supported by RDFLib can be parsed into CKAN datasets. The examples folder contains serializations in different formats including RDF/XML, Turtle or JSON-LD.

RDF DCAT Serializer

The ckanext.dcat.processors.RDFSerializer class generates RDF serializations in different formats from CKAN dataset dicts, like the ones returned by package_show or package_search.

Here is an example of how to use it:


    from ckanext.dcat.processors import RDFSerializer

    # Serializing a single dataset

    dataset = get_action('package_show')({}, {'id': 'my-dataset'})

    serializer = RDFserializer()

    dataset_ttl = serializer.serialize_dataset(dataset, _format='turtle')

    # Serializing the whole catalog (or rather part of it)

    datasets = get_action('package_search')({}, {'q': '*:*', 'rows': 50})

    serializer = RDFserializer()

    catalog_xml = serializer.serialize_catalog({'title': 'My catalog'},
                                               dataset_dicts=datasets,
                                               _format='xml')

    # Creating and RDFLib graph from a single dataset

    dataset = get_action('package_show')({}, {'id': 'my-dataset'})

    serializer = RDFserializer()

    dataset_reference = serializer.graph_from_dataset(dataset)

    # serializer.g now contains the full dataset graph, an RDFLib Graph class

The serializer uses customizable profiles to generate an RDF graph (an RDFLib Graph class). By default these use the mapping described in the previous section.

In some cases, if the default CKAN field that maps to a DCAT property is not present, some other fallback values will be used instead. For instance, if the contact_email field is not found, maintainer_email and author_email will be used (if present) for the email property of the adms:contactPoint property.

Note that the serializer will look both for a first level field or an extra field with the same key, ie both the following values will be used for dct:accrualPeriodicity:

{
    "name": "my-dataset",
    "frequency": "monthly",
    ...
}

{
    "name": "my-dataset",
    "extras": [
        {"key": "frequency", "value": "monthly"},
    ]
    ...
}

Once the dataset graph has been obtained, this is serialized into a text format using RDFLib, so any format it supports can be obtained (common formats are 'xml', 'turtle' or 'json-ld').

Profiles

Both the parser and the serializer use profiles to allow customization of how the values defined in the RDF graph are mapped to CKAN and viceversa.

Profiles define :

They essentially define the mapping between DCAT and CKAN.

In most cases the default profile will provide a good mapping that will cover most properties described in the DCAT standard. If you want to extract extra fields defined in the RDF, are using a custom schema or need custom logic, you can write a custom to profile that extends or replaces the default one.

The default profile is mostly based in the DCAT application profile for data portals in Europe. It is actually fully-compatible with DCAT-AP v1.1. As mentioned before though, it should be generic enough for most DCAT based representations.

To define which profiles to use you can:

  1. Set the ckanext.dcat.rdf.profiles configuration option on your CKAN configuration file:

    ckanext.dcat.rdf.profiles = euro_dcat_ap sweden_dcat_ap

  2. When initializing a parser or serializer class, pass the profiles to be used as a parameter, eg:


   parser = RDFParser(profiles=['euro_dcat_ap', 'sweden_dcat_ap'])

   serializer = RDFSerializer(profiles=['euro_dcat_ap', 'sweden_dcat_ap'])

Note that in both cases the order in which you define them is important, as it will be the one that the profiles will be run on.

Writing custom profiles

Internally, profiles are classes that define a particular set of methods called during the parsing process. For instance, the parse_dataset method is called on each DCAT dataset found when parsing an RDF file, and should return a CKAN dataset. Conversely, the graph_from_dataset will be called when requesting an RDF representation for a dataset, and will need to generate the necessary RDF graph.

Custom profiles should always extend the ckanext.dcat.profiles.RDFProfile class. This class has several helper functions to make getting metadata from the RDF graph easier. These include helpers for getting fields for FOAF and VCard entities like the ones used to define publishers or contact points. Check the source code of ckanex.dcat.profiles.py to see what is available.

Profiles can extend other profiles to avoid repeating rules, or can be completely independent.

The following example shows a complete example of a profile built on top of the default one (euro_dcat_ap):


    from rdflib.namespace import Namespace
    from ckanext.dcat.profiles import RDFProfile

    DCT = Namespace("http://purl.org/dc/terms/")

    class SwedishDCATAPProfile(RDFProfile):
        '''
        An RDF profile for the Swedish DCAT-AP recommendation for data portals

        It requires the European DCAT-AP profile (`euro_dcat_ap`)
        '''

        def parse_dataset(self, dataset_dict, dataset_ref):

            # Spatial label
            spatial = self._object(dataset_ref, DCT.spatial)
            if spatial:
                spatial_label = self.g.label(spatial)
                if spatial_label:
                    dataset_dict['extras'].append({'key': 'spatial_text',
                                                   'value': str(spatial_label)})

            return dataset_dict

        def graph_from_dataset(self, dataset_dict, dataset_ref):

            g = self.g

            spatial_uri = self._get_dataset_value(dataset_dict, 'spatial_uri')
            spatial_text = self._get_dataset_value(dataset_dict, 'spatial_text')

            if spatial_uri:
                spatial_ref = URIRef(spatial_uri)
            else:
                spatial_ref = BNode()

            if spatial_text:
                g.add((dataset_ref, DCT.spatial, spatial_ref))
                g.add((spatial_ref, RDF.type, DCT.Location))
                g.add((spatial_ref, RDFS.label, Literal(spatial_text)))

Note how the dataset dict is passed between profiles so it can be further tweaked.

Extensions define their available profiles using the ckan.rdf.profiles in the setup.py file, as in this example from this same extension:

[ckan.rdf.profiles]
euro_dcat_ap=ckanext.dcat.profiles:EuropeanDCATAPProfile

Command line interface

The parser and serializer can also be accessed from the command line via python ckanext-dcat/ckanext/dcat/processors.py.

You can point to RDF files:

python ckanext-dcat/ckanext/dcat/processors.py consume catalog_pod_2.jsonld -P -f json-ld

python ckanext/dcat/processors.py produce examples/ckan_dataset.json

or pipe them to the script:

http http://localhost/dcat/catalog.rdf | python ckanext-dcat/ckanext/dcat/processors.py consume -P > ckan_datasets.json

http http://demo.ckan.org/api/action/package_show id=afghanistan-election-data | jq .result | python ckanext/dcat/processors.py produce

To see all available options, run the script with the -h argument:

python ckanext-dcat/ckanext/dcat/processors.py -h
usage: processors.py [-h] [-f FORMAT] [-P] [-p [PROFILE [PROFILE ...]]] [-m]
                     mode [file]

DCAT RDF - CKAN operations

positional arguments:
  mode                  Operation mode. `consume` parses DCAT RDF graphs to
                        CKAN dataset JSON objects. `produce` serializes CKAN
                        dataset JSON objects into DCAT RDF.
  file                  Input file. If omitted will read from stdin

optional arguments:
  -h, --help            show this help message and exit
  -f FORMAT, --format FORMAT
                        Serialization format (as understood by rdflib) eg:
                        xml, n3 ... Defaults to 'xml'.
  -P, --pretty          Make the output more human readable
  -p [PROFILE [PROFILE ...]], --profile [PROFILE [PROFILE ...]]
                        RDF Profiles to use, defaults to euro_dcat_ap
  -m, --compat-mode     Enable compatibility mode

Compatibility mode

In compatibility mode, some fields are modified to maintain compatibility with previous versions of the ckanext-dcat parsers (eg adding the dcat_ prefix or storing comma separated lists instead of JSON blobs).

CKAN instances that were using the legacy XML and JSON harvesters (dcat_json_harvester and dcat_xml_harvester) and want to move to the RDF based one may want to turn compatibility mode on to ensure that CKAN dataset fields are created as before. Users are encouraged to migrate their applications to support the new DCAT to CKAN mapping.

To turn compatibility mode on add this to the CKAN configuration file:

ckanext.dcat.compatibility_mode = True

XML DCAT harvester (deprecated)

The old DCAT XML harvester (dcat_xml_harvester) is now deprecated, in favour of the RDF harvester. Loading it on the ini file will result in an exception on startup.

The XML serialization described in the spec.datacatalogs.org site is a valid RDF/XML one, so changing the harvester should have no effect. There might be slight differences in the way CKAN fields are created though, check Compatibility mode for more details.

Running the Tests

To run the tests, do:

nosetests --nologcapture --ckan --with-pylons=test.ini ckanext

Acknowledgements

Work on ckanext-dcat has been made possible by:

If you can fund new developments or contribute please get in touch.

Copying and License

This material is copyright (c) Open Knowledge.

It is open and licensed under the GNU Affero General Public License (AGPL) v3.0 whose full text may be found at:

http://www.fsf.org/licensing/licenses/agpl-3.0.html