Full text and metadata extractor
by City of Karlsruhe, Germany



A CKAN extension for automatically extracting text and metadata from datasets.

ckanext-extractor automatically extracts text and metadata from your resources and adds them to the search index so that they can be used to find your data.


ckanext-extractor has been developed and tested with CKAN 2.5.2. Other versions may or may not work, please share your experiences by creating an issue.


Note: The following steps assume a standard CKAN source installation.

Install Python Package

Activate your CKAN virtualenv:

. /usr/lib/ckan/default/bin/activate

Install the latest development version of ckanext-extractor and its dependencies:

cd /usr/lib/ckan/default
pip install -e git+https://github.com/stadt-karlsruhe/ckanext-extractor#egg=ckanext-extractor
pip install -r src/ckanext-extractor/requirements.txt

On a production system you’ll probably want to pin a certain release version of ckanext-extractor instead:

pip install -e git+https://github.com/stadt-karlsruhe/[email protected]#egg=ckanext-extractor

Configure CKAN

Open your CKAN configuration file (e.g. /etc/ckan/default/production.ini) and add extractor to the list of plugins:

ckan.plugins = ... extractor

Initialize the database:

paster --plugin=ckanext-extractor init -c /etc/ckan/default/production.ini

Start Celery Daemon

ckanext-extractor uses Celery background tasks to perform the extraction asynchronously so that they do not block the web server. You therefore need to make sure that Celery is running, for example using

paster --plugin=ckan celeryd -c /etc/ckan/default/production.ini

See the CKAN documentation for more information on Celery.

Configure Solr

For the actual extraction CKAN’s Apache Solr server is used. However, the necessary Solr plugins are deactivated by default. To enable them, find your main Solr configuration file (usually /etc/solr/conf/solrconfig.xml) and add/uncomment the following lines:

<lib dir="../../dist/" regex="apache-solr-cell-\d.*\.jar" />
<lib dir="../../contrib/extraction/lib" regex=".*\.jar" />

Note: The Solr packages on Ubuntu are broken and do not contain the necessary files. You can simply download an official release of the same version, unpack it to a suitable location (without installing it) and adjust the dir arguments in the configuration lines above accordingly. For example, if you have unpacked the files to /var/lib/apache-solr, then you would need to put the following lines into solrconfig.xml:

<lib dir="/var/lib/apache-solr/dist/" regex="apache-solr-cell-\d.*\.jar" />
<lib dir="/var/lib/apache-solr/contrib/extraction/lib" regex=".*\.jar" />

Once the text and metadata have been extracted they need to be added to the Solr index, which requires appropriate Solr fields. To set them up add the following lines to your Solr schema configuration (usually /etc/solr/conf/schema.xml):

# Directly before the line that says "</fields>"
<dynamicField name="ckanext-extractor_*" type="text" indexed="true" stored="false"/>

# Directly before the line that says "</schema>"
<copyField source="ckanext-extractor_*" dest="text"/>

Make sure to restart Solr after you have applied the changes. For example, if you’re using Jetty as an application server for Solr, then

sudo service jetty restart

Restart CKAN

Finally, restart your CKAN server:

sudo service apache2 restart

Test your Installation

The installation is now complete. To verify that everything is working open the URL /api/3/action/extractor_list, e.g. via

wget -qO - http://localhost/api/3/action/extractor_list

The output should look like this (in particular, success should true):

{"help": "http://localhost/api/3/action/help_show?name=extractor_list", "success": true, "result": []}

You’re Done!

Your installation of ckanext-extractor is now complete, and new/updated resources will have their metadata automatically indexed. You may want to adapt the configuration to your needs, see below for details. Once that is done you may also want to extract metadata from your existing resources:

. /usr/lib/ckan/default/bin/activate
paster --plugin=ckanext-extractor extract all -c /etc/ckan/default/production.ini

This and other paster administration commands are explained below in more detail.


ckanext-extractor can be configured via the usual CKAN configuration file (e.g. /etc/ckan/default/production.ini). You must restart your CKAN server after updating the configuration.

Formats for Extraction

While Solr can extract text and metadata from many file formats not all of them might be of interest to you. You can therefore configure for which formats extraction is performed via the ckanext.extractor.indexed_formats option. It takes a list of space-separated formats, where the format is the one specified in a resource’s CKAN metadata (and not the file extension or MIME type):

ckanext.extractor.indexed_formats = pdf txt

Formats are case-insensitive. You can use wildcards (* and ?) to match multiple formats. To extract data from all formats simply set

ckanext.extractor.indexed_formats = *

By default, extraction is only enabled for the PDF format:

ckanext.extractor.indexed_formats = pdf

Fields for Indexing

Once text and metadata have been extracted they can be added to the search index. Again, Solr supports more metadata fields than one usually needs. You can therefore configure which fields are indexed via the ckanext.extractor.indexed_fields option. It takes a space-separated list of field names:

ckanext.extractor.indexed_fields = fulltext author

The full text of a document is available via the fulltext field. Field names are case-insensitive. You can use wildcards (* and ?) to match multiple field names. To index all fields simply set

ckanext.extractor.indexed_fields = *

By default, only the full text of a document is indexed:

ckanext.extractor.indexed_fields = fulltext

Note: ckanext-extractor normalizes the field names reported by Solr by replacing underscores (_) with hyphens (-).

Paster Commands

In general, ckanext-extractor works automatically: whenever a new resource is created or an existing resource changes, its metadata is extracted and indexed. However, for administration purposes, metadata can also be managed from the command line using the paster tool.

Note: You have to activate your virtualenv before you can use these commands:

. /usr/lib/ckan/default/bin/activate

The general form for a paster command is

paster --plugin=ckanext-extractor COMMAND ARGUMENTS --config=/etc/ckan/default/production.ini

Replace COMMAND and ARGUMENTS as described below. For example:

paster --plugin=ckanext-extractor extract all --config=/etc/ckan/default/production.ini
  • delete (all | ID [ID [...]]): Delete metadata. You can specify one or more resource IDs or a single all argument (in which case all metadata is deleted).
  • extract [--force] (all | ID [ID [...]]): Extract metadata. You can specify one or more resource IDs or a single all argument (in which case metadata is extracted from all resources with appropriate formats). An optional --force argument can be used to force extraction even if the resource is unchanged, or if another extraction job already has been scheduled for that resource.

    Note that this command only schedules the necessary extraction background tasks. The Celery daemon has to be running for the extraction to actually happen.

  • init: Initialize the database tables for ckanext-extractor. You only need to use this once (during the installation).
  • list: List the IDs of all resources for which metadata has been extracted.
  • show (all | ID [ID [...]]): Show extracted metadata. You can specify one or more resource IDs or a single all argument (in which case all metadata is shown).


Metadata can be managed via the standard CKAN API. Unless noted otherwise all commands are only available via POST requests to authenticated users.


Delete metadata.

Only available to administrators.


ID of the resource for which metadata should be deleted.


Extract metadata.

This function schedules a background task for extracting metadata from a resource.

Only available to administrators.


ID of the resource for which metadata should be extracted.

Optional boolean flag to force extraction even if the resource is unchanged, or if an extraction task has already been scheduled for that resource.

Returns a dict with the following entries:

A string describing the state of the metadata. This can be one of the following:

if no metadata for the resource existed before

if metadata existed but is going to be updated

if metadata existed but won’t get updated (for example because the resource’s URL did not change since the last extraction)

if a background extraction task for this resource is already in progress

if the resource format is configured to be ignored

Note that if force is true then an extraction job will be scheduled regardless of the status reported, unless that state is ignored.

The ID of the background task. If state is new or update then this is the ID of a newly created task. If state is inprogress then it’s the ID of the existing task. Otherwise it is null.

If force is true then this is the ID of the new extraction task.


List resources with metadata.

Returns a list with the IDs of all resources for which metadata has been extracted.

Available to all (even anonymous) users via GET and POST.


Show the metadata for a resource.


ID of the resource for which metadata should be extracted.

Returns a dict with the resource’s metadata and information about the last extraction.

Available to all (even anonymous) users via GET and POST.

Postprocessing Extraction Results

The ckanext.extractor.interfaces.IExtractorPostprocessor interface can be used to hook into the extraction process. It allows you to postprocess extraction results and to automatically trigger actions that use the extraction results for other purposes.

The interface offers 3 hooks:

  • extractor_after_extract(resource_dict, extracted) is called right after the extraction before the extracted metadata extracted is filtered and stored. You can modify extracted (in-place) and the changes will end up in the database.
  • extractor_after_save(resource_dict, metadata_dict) is called after the metadata has been filtered and stored in the database but before it is indexed. metadata_dict is a dict-representation of a ckanext.extractor.model.ResourceMetadata instance and contains both the extracted metadata and information about the extraction process (meta-metadata, so to speak).
  • extractor_after_index(resource_dict, metadata_dict) is called at the very end of the extraction process, after the metadata has been extracted, filtered, stored and indexed.


. /usr/lib/ckan/default/bin/activate
git clone https://github.com/stadt-karlsruhe/ckanext-extractor.git
cd ckanext-extractor
python setup.py develop
pip install -r dev-requirements.txt

Running the Tests

To run the tests, activate your CKAN virtualenv and do:


Any additional arguments are passed on to nosetests.


Copyright (C) 2016 Stadt Karlsruhe (www.karlsruhe.de)

Distributed under the GNU Affero General Public License. See the file LICENSE for details.



  • Added IExtractorPostprocessor interface for postprocessing extraction results
  • Fixed logging problems in paster commands


  • First release

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