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Open Data Schema Map

This module provides a flexible way to expose your Drupal content via APIs following specific Open Data schemas. Currently, the CKAN, Project Open Data and DCAT-AP schemas are provided, but new schemas can be easily added through your own modules. A user interface is in place to create endpoints and map fields from the chosen schema to Drupal content using tokens.

This module was developed as part of the DKAN project, but will work on an Drupal 7 site. A separate module exists for DKAN-specific implementation.

Note that serious performance issues can result if you do not follow recommendations in the ODSM File Cache section.

Basic concepts

Schema

A schema is a list of field definitions, usually representing a community specification for presenting machine-readable data. The core Open Data Schema Map module does not include any schemas; they are provided by additional modules. A schema module includes:

• a standard Drupal .module file -- with an implementation of to expose the schema to the core Open Data Schema Map module, plus _alter functions for any needed modifications of the UI form or the data output itself.
• the schema itself, expressed as a .json file. For instance, see the Project Open Data schema file to see how these schema are defined in JSON

API

An API in this module is a configuration set that exposes a specific set of machine-readable data at a specific URL (known as the API's endpoint). This module allows you to create multiple APIs that you save as database records and/or export using Features. An API record will contain:

• an endpoint URL
• a schema (chosen from the available schemas provided by the additional modules as described above)
• a mapping of fields defined in that schema to Drupal tokens (usually referencing fields from a node)
• optionally, one or more arguments passed through the URL to filter the result set

Usage

Installation

Enable the main Open Data Schema Map module as usual, and additionally enable any schema modules you will need to create your API.

Creating APIs

Navigate to admin/config/services/odsm and click "Add API."

Give the API a title, machine name, choose which entity type (usually node) and bundle (in DKAN, this is usually Dataset).

You will need to create the API record before adding arguments and mappings.

Arguments

The results of the API call can be filtered by a particular field via arguments in the URL. To add an argument, first choose the schema field then, if you are filtering by a custom field API field (ie, a field whose machine name begins with "field_"), identify the database column that would contain the actual argument value. Leave off the field name prefix; for instance, if filtering by a DKAN tag (a term reference field), the correct column is field_tags_tid, so you would enter "tid". Which Drupal field to use will be extrapolated from the token you map to that schema field.

Field Mapping

The API form presents you with a field for each field in your schema. Map the fields using Drupal's token system. Note: using more than one token in a single field may produce unexpected results and is not recommended.

Multi-value fields

For Drupal multi-value entity reference fields, the schema can use an array to instruct the API to iterate over each value and map the referenced data to multiple schema fields. For instance, in the CKAN schema, tags are described like this in schema_ckan.json:

You can choose which of the available multivalue fields on your selected bundle to map to the "tags" array, exposing all of the referenced "tag" entities (taxonomy terms in this example) to use as the context for your token mappings on the schema fields within that array. First, simply choose the multivalue field, leaving the individual field mappings blank, and save the form.

When you return to the tags section of the form after saving, you will now see a special token navigator you can use to find tokens that will work with this iterative approach (using "Nth" in place of the standard delta value in the token):

Data Federation Filtering

To exclude specific groups from the API, navigate to /admin/config/services/odsm/settings and choose the groups you wish to include.

You must then edit your API and check the Apply Data Federation Filters checkbox for the filters to apply to your endpoint.

Customizing

Adding new schemas

You are not limited by the schemas included with this module; any Open Data schema may be defined in a custom module. Use the open_data_schema_ckan module as a model to get started.

Date format

Date formats can be chanaged manually by changing the "Medium" date time format in "admin/config/regional/date-time" or in code by using one of the alter hooks:

A Note on XML Output

Open Data Schema Map provides an XML output format. This is provided via a separate submodule in the modules/ folder for historical reasons, but should be refactored into the main ODSM module in a future release.

XML endpoints still require a schema defined in JSON. Defining your own XML endpoint may be less than intuitive for the time beind, but take a look at the DCAT schema module for a model.

The ODSM File Cache

Open Data Schema Map endpoints that list a large number of entities -- Project Open Data (data.json), the CKAN Package List (/api/3/action/package_list) and DCAT-AP Catalog (catalog.xml) -- perform a full entity load for each record listed in order to perform the token replacements. This can cause a major performance hit each time any of these URLs is hit on a site with more than a few dozen datasets, and on a site with thousands the response time can be two minutes or more.

Open Data Schema Map includes a file caching function to save a snapshot of any endpoint as a static file to be served up quickly, with very few hits to the database.

File caches can be generated either via a Drush command, or an admin UI. The recommended usage on a production website is to set up a cron job or use a task runner like Jenkins to regenerate the file caches for your performance-intensive endpoints daily (usin the drush command), at whatever time your site experiences the least amount of traffic. The trade-off of course is that any additions or changes to your site will not be reflected on these endpoints until they are regenerated.

Drush Use

The Drush command supplied by Open Data Schema Map is odsm-filecache (also available simply as the alias odsmfc). This command takes as its argument the machine name for an ODSM endpoint. For example:

This will render the full data_json_1_1 endpoint (which is the data.json implementation that ships with DKAN) to the filesystem, saving it to:

Now a hit to /data.json will be routed to this file, which in most cases will actually live at /sites/default/files/odsm_cache_data_json_1_1.

UI Use

An administrative UI to regenerate file caches manually is also included. This interface is useful in cases where manual creation of the cache files is sufficient.

To use, navigate to admin/config/services/odsm where there is a column called "Cache" with links to the individual admin pages for specific enpoint caches. If there is no cache the link is labled "none", otherwise the link is labled with the age of the cache in hours. From the cache admin pages you can create, delete or regenerate the cache.

Schema Validation

Both the Project Open Data and DCAT-AP schemas ship with validation tools you can access from the Drupal admin menu. More documentation on this feature coming soon...

Community

We are accepting issues for Open Data Schema Map in the DKAN issue queue only. Please label your issue as "Component: ODSM" after submitting so we can identify problems and feature requests faster.

If submitting a pull request to this project, please try to link your PR to the corresponding issue in the DKAN issue thread.