ckanext-harvest - Remote harvesting extension

This extension provides a common harvesting framework for ckan extensions and adds a CLI and a WUI to CKAN to manage harvesting sources and jobs.

Installation

The harvest extension uses Message Queuing to handle the different gather stages.

You will need to install the RabbitMQ server:

sudo apt-get install rabbitmq-server

Clone the repository and set up the extension:

git clone https://github.com/okfn/ckanext-harvest
cd ckanext-harvest
pip install -r pip-requirements.txt
python setup.py develop

Make sure the CKAN configuration ini file contains the harvest main plugin, as well as the harvester for CKAN instances (included with the extension):

ckan.plugins = harvest ckan_harvester

Configuration

Run the following command to create the necessary tables in the database:

paster --plugin=ckanext-harvest harvester initdb --config=mysite.ini

The extension needs a user with sysadmin privileges to perform the harvesting jobs. You can create such a user running this command:

paster --plugin=ckan sysadmin add harvest

After installation, the harvest interface should be available under /harvest if you're logged in with sysadmin permissions, eg.

http://localhost:5000/harvest

Command line interface

The following operations can be run from the command line using the paster --plugin=ckanext-harvest harvester command:

harvester initdb
  - Creates the necessary tables in the database

harvester source {url} {type} [{active}] [{user-id}] [{publisher-id}]
  - create new harvest source

harvester rmsource {id}
  - remove (inactivate) a harvester source

harvester sources [all]
  - lists harvest sources
    If 'all' is defined, it also shows the Inactive sources

harvester job {source-id}
  - create new harvest job

harvester jobs
  - lists harvest jobs

harvester run
  - runs harvest jobs

harvester gather_consumer
  - starts the consumer for the gathering queue

harvester fetch_consumer
  - starts the consumer for the fetching queue

harvester import [{source-id}]
  - perform the import stage with the last fetched objects, optionally
    belonging to a certain source.
    Please note that no objects will be fetched from the remote server.
    It will only affect the last fetched objects already present in the
    database.

harvester job-all
  - create new harvest jobs for all active sources.

The commands should be run with the pyenv activated and refer to your sites configuration file (mysite.ini in this example):

paster --plugin=ckanext-harvest harvester sources --config=mysite.ini

Authorization Profiles

Starting from CKAN 1.6.1, the harvester extension offers the ability to use different authorization profiles. These can be defined in your ini file as:

ckan.harvest.auth.profile = <profile_name>

The two available profiles right now are:

• `default`: This is the default profile, the same one that this extension has used historically. Basically, only sysadmins can manage anything related to harvesting, including creating and editing harvest sources or running harvest jobs.

• `publisher`: When using this profile, sysadmins can still perform any harvesting related action, but in addition, users belonging to a publisher (with role admin) can manage and run their own harvest sources and jobs. Note that this requires CKAN core to also use the publisher authorization profile, i.e you will also need to add:

  ckan.auth.profile = publisher

To know more about the CKAN publisher auth profile, visit:

http://oldwiki.ckan.org/Working_with_the_publisher_auth_profile

The CKAN harvester

The plugin includes a harvester for remote CKAN instances. To use it, you need to add the ckan_harvester plugin to your options file:

ckan.plugins = harvest ckan_harvester

After adding it, a 'CKAN' option should appear in the 'New harvest source' form.

The CKAN harvesters support a number of configuration options to control their behaviour. Those need to be defined as a JSON object in the configuration form field. The currently supported configuration options are:

• api_version: You can force the harvester to use either version '1' or '2' of the CKAN API. Default is '2'.
• default_tags: A list of tags that will be added to all harvested datasets. Tags don't need to previously exist.
• default_groups: A list of groups to which the harvested datasets will be added to. The groups must exist. Note that you must use ids or names to define the groups according to the API version you defined (names for version '1', ids for version '2').
• default_extras: A dictionary of key value pairs that will be added to extras of the harvested datasets. You can use the following replacement strings, that will be replaced before creating or updating the datasets:
  • {dataset_id}
  • {harvest_source_id}
  • {harvest_source_url} # Will be stripped of trailing forward slashes (/)
  • {harvest_source_title} # Requires CKAN 1.6
  • {harvest_job_id}
  • {harvest_object_id}
• override_extras: Assign default extras even if they already exist in the remote dataset. Default is False (only non existing extras are added).
• user: User who will run the harvesting process. Please note that this user needs to have permission for creating packages, and if default groups were defined, the user must have permission to assign packages to these groups.
• api_key: If the remote CKAN instance has restricted access to the API, you can provide a CKAN API key, which will be sent in any request.
• read_only: Create harvested packages in read-only mode. Only the user who performed the harvest (the one defined in the previous setting or the 'harvest' sysadmin) will be able to edit and administer the packages created from this harvesting source. Logged in users and visitors will be only able to read them.
• force_all: By default, after the first harvesting, the harvester will gather only the modified packages from the remote site since the last harvesting. Setting this property to true will force the harvester to gather all remote packages regardless of the modification date. Default is False.

Here is an example of a configuration object (the one that must be entered in the configuration field):

{
 "api_version":"1",
 "default_tags":["new-tag-1","new-tag-2"],
 "default_groups":["my-own-group"],
 "default_extras":{"new_extra":"Test","harvest_url":"{harvest_source_url}/dataset/{dataset_id}"},
 "override_extras": true,
 "user":"harverster-user",
 "api_key":"<REMOTE_API_KEY>",
 "read_only": true
}

The harvesting interface

Extensions can implement the harvester interface to perform harvesting operations. The harvesting process takes place on three stages:

1. The gather stage compiles all the resource identifiers that need to be fetched in the next stage (e.g. in a CSW server, it will perform a GetRecords operation).
2. The fetch stage gets the contents of the remote objects and stores them in the database (e.g. in a CSW server, it will perform n GetRecordById operations).
3. The import stage performs any necessary actions on the fetched resource (generally creating a CKAN package, but it can be anything the extension needs).

Plugins willing to implement the harvesting interface must provide the following methods:

from ckan.plugins.core import SingletonPlugin, implements
from ckanext.harvest.interfaces import IHarvester

class MyHarvester(SingletonPlugin):
'''
A Test Harvester
'''
implements(IHarvester)

def info(self):
    '''
    Harvesting implementations must provide this method, which will return a
    dictionary containing different descriptors of the harvester. The
    returned dictionary should contain:

    * name: machine-readable name. This will be the value stored in the
      database, and the one used by ckanext-harvest to call the appropiate
      harvester.
    * title: human-readable name. This will appear in the form's select box
      in the WUI.
    * description: a small description of what the harvester does. This will
      appear on the form as a guidance to the user.
    * form_config_interface [optional]: Harvesters willing to store configuration
      values in the database must provide this key. The only supported value is
      'Text'. This will enable the configuration text box in the form. See also
      the ``validate_config`` method.

    A complete example may be::

        {
            'name': 'csw',
            'title': 'CSW Server',
            'description': 'A server that implements OGC\'s Catalog Service
                            for the Web (CSW) standard'
        }

    returns: A dictionary with the harvester descriptors
    '''

def validate_config(self, config):
    '''
    Harvesters can provide this method to validate the configuration entered in the
    form. It should return a single string, which will be stored in the database.
    Exceptions raised will be shown in the form's error messages.

    returns A string with the validated configuration options
    '''

def gather_stage(self, harvest_job):
    '''
    The gather stage will recieve a HarvestJob object and will be
    responsible for:
        - gathering all the necessary objects to fetch on a later.
          stage (e.g. for a CSW server, perform a GetRecords request)
        - creating the necessary HarvestObjects in the database, specifying
          the guid and a reference to its source and job.
        - creating and storing any suitable HarvestGatherErrors that may
          occur.
        - returning a list with all the ids of the created HarvestObjects.

    :param harvest_job: HarvestJob object
    :returns: A list of HarvestObject ids
    '''

def fetch_stage(self, harvest_object):
    '''
    The fetch stage will receive a HarvestObject object and will be
    responsible for:
        - getting the contents of the remote object (e.g. for a CSW server,
          perform a GetRecordById request).
        - saving the content in the provided HarvestObject.
        - creating and storing any suitable HarvestObjectErrors that may
          occur.
        - returning True if everything went as expected, False otherwise.

    :param harvest_object: HarvestObject object
    :returns: True if everything went right, False if errors were found
    '''

def import_stage(self, harvest_object):
    '''
    The import stage will receive a HarvestObject object and will be
    responsible for:
        - performing any necessary action with the fetched object (e.g
          create a CKAN package).
          Note: if this stage creates or updates a package, a reference
          to the package must be added to the HarvestObject.
          Additionally, the HarvestObject must be flagged as current.
        - creating the HarvestObject - Package relation (if necessary)
        - creating and storing any suitable HarvestObjectErrors that may
          occur.
        - returning True if everything went as expected, False otherwise.

    :param harvest_object: HarvestObject object
    :returns: True if everything went right, False if errors were found
    '''

See the CKAN harvester for an example of how to implement the harvesting interface:

ckanext-harvest/ckanext/harvest/harvesters/ckanharvester.py

Here you can also find other examples of custom harvesters:

• https://github.com/okfn/ckanext-pdeu/tree/master/ckanext/pdeu/harvesters
• https://github.com/okfn/ckanext-inspire/ckanext/inspire/harvesters.py

Running the harvest jobs

The harvesting extension uses two different queues, one that handles the gathering and another one that handles the fetching and importing. To start the consumers run the following command (make sure you have your python environment activated):

paster --plugin=ckanext-harvest harvester gather_consumer --config=mysite.ini

On another terminal, run the following command:

paster --plugin=ckanext-harvest harvester fetch_consumer --config=mysite.ini

Finally, on a third console, run the following command to start any pending harvesting jobs:

paster --plugin=ckanext-harvest harvester run --config=mysite.ini

Note: If you don't have the synchronous_search plugin loaded, you will need to update the search index after the harvesting in order for the packages to appear in search results:

paster --plugin=ckan search-index rebuild

Setting up the harvesters on a production server

The previous approach works fine during development or debugging, but it is not recommended for production servers. There are several possible ways of setting up the harvesters, which will depend on your particular infrastructure and needs. The bottom line is that the gather and fetch process should be kept running somehow and then the run command should be run periodically to start any pending jobs.

The following approach is the one generally used on CKAN deployments, and it will probably suit most of the users. It uses Supervisor, a tool to monitor processes, and a cron job to run the harvest jobs, and it assumes that you have already installed and configured the harvesting extension (See Installation if not).

Note: It is recommended to run the harvest process from a non-root user (generally the one you are running CKAN with). Replace the user okfn in the following steps with the one you are using.

1. Install Supervisor:

   sudo apt-get install supervisor

   You can check if it is running with this command:

   ps aux | grep supervisord

   You should see a line similar to this one:

   root      9224  0.0  0.3  56420 12204 ?        Ss   15:52   0:00 /usr/bin/python /usr/bin/supervisord

2. Supervisor needs to have programs added to its configuration, which will describe the tasks that need to be monitored. This configuration files are stored in /etc/supervisor/conf.d.

   Create a file named /etc/supervisor/conf.d/ckan_harvesting.conf, and copy the following contents:

   ; ===============================
   ; ckan harvester
   ; ===============================
   
   [program:ckan_gather_consumer]
   
   command=/var/lib/ckan/std/pyenv/bin/paster --plugin=ckanext-harvest harvester gather_consumer --config=/etc/ckan/std/std.ini
   
   ; user that owns virtual environment.
   user=okfn
   
   numprocs=1
   stdout_logfile=/var/log/ckan/std/gather_consumer.log
   stderr_logfile=/var/log/ckan/std/gather_consumer.log
   autostart=true
   autorestart=true
   startsecs=10
   
   [program:ckan_fetch_consumer]
   
   command=/var/lib/ckan/std/pyenv/bin/paster --plugin=ckanext-harvest harvester fetch_consumer --config=/etc/ckan/std/std.ini
   
   ; user that owns virtual environment.
   user=okfn
   
   numprocs=1
   stdout_logfile=/var/log/ckan/std/fetch_consumer.log
   stderr_logfile=/var/log/ckan/std/fetch_consumer.log
   autostart=true
   autorestart=true
   startsecs=10

   There are a number of things that you will need to replace with your specific installation settings (the example above shows paths from a ckan instance installed via Debian packages):

   • command: The absolute path to the paster command located in the python virtual environment and the absolute path to the config ini file.
   • user: The unix user you are running CKAN with
   • stdout_logfile and stderr_logfile: All output coming from the harvest consumers will be written to this file. Ensure that the necessary permissions are setup.

   The rest of the configuration options are pretty self explanatory. Refer to the Supervisor documentation to know more about these and other options available.

3. Start the supervisor tasks with the following commands:

   sudo supervisorctl reread
   sudo supervisorctl add ckan_gather_consumer
   sudo supervisorctl add ckan_fetch_consumer
   sudo supervisorctl start ckan_gather_consumer
   sudo supervisorctl start ckan_fetch_consumer

   To check that the processes are running, you can run:

   sudo supervisorctl status
   
   ckan_fetch_consumer              RUNNING    pid 6983, uptime 0:22:06
   ckan_gather_consumer             RUNNING    pid 6968, uptime 0:22:45

   Some problems you may encounter when starting the processes:

   • ckan_gather_consumer: ERROR (no such process)

     Double-check your supervisor configuration file and stop and restart the supervisor daemon:

     sudo service supervisor start; sudo service supervisor stop

   • ckan_gather_consumer: ERROR (abnormal termination)

     Something prevented the command from running properly. Have a look at the log file that you defined in the stdout_logfile section to see what happened. Common errors include:

     `socket.error: [Errno 111] Connection refused`
     RabbitMQ is not running:: 
     
       sudo service rabbitmq-server start

4. Once we have the two consumers running and monitored, we just need to create a cron job that will run the run harvester command periodically. To do so, edit the cron table with the following command (it may ask you to choose an editor):

   sudo crontab -e -u okfn

   Note that we are running this command as the same user we configured the processes to be run with (okfn in our example).

   Paste this line into your crontab, again replacing the paths to paster and the ini file with yours:

   # m  h  dom mon dow   command
   */15 *  *   *   *     /var/lib/ckan/std/pyenv/bin/paster --plugin=ckanext-harvest harvester run --config=/etc/ckan/std/std.ini

   This particular example will check for pending jobs every fifteen minutes. You can of course modify this periodicity, this Wikipedia page has a good overview of the crontab syntax.