[ckan-discuss] Thoughts on a plugin system

[Rufus Pollock]

Going forward we probably want some kind of plugin system in ckan to
provide for easy extensibility. I've started a collaborative pad for
working on this here:

<http://ckan.okfnpad.org/plugins>

Excerpted below. Comments/suggestions/criticism etc *very* welcome --
either comment here or just edit the pad ...

Regards,

Rufus

= Introduction =

Creating a plugin system for CKAN is key to making CKAN (easily) extensible.

= Examples of Plugins =

  1. Custom forms (with extra fields) for CKAN (we already have this)
     1. Custom form widgets (e.g. geo field)

  2. Theming (we already have this but this sort of a plugin)

  3. Widgets e.g.
    1. Visualizing/Previewing a package resouce (e.g. show sample rows
for a csv/gdocs spreadsheet) on the CKAN package page
    2. Wikipedia like flags on pages based on tags
    3. Sidebar customization (widgets)

  4. External workers executing on package create/update etc (now in
place with new messaging framework). E.g.
    * Calculate resource hashes
    * On RDF resource/dataset update calculate attributes (number of
triples, type of triples etc)
    * Validation of resource (is it a 404, is the mime-type it claims etc)
    * Automated storage

= Requirements =

A plugin system must provide 3 things:

  1. Plugin entry/extension points (hooks where plugins get
utilized/called from)
  2. Plugin interface
  3. Plugin manager (registry + dispatch)

  * [ckan.plugin]: setuptools inversion of control for plugin registration
    * List of plugins activated must be stored in config
  * model entry points
    * model_create(session)
    * model_upgrade(session, version) method taking a session argument
  * templates entry points
    * template path (use pkgutil)
    * templates directroy as plugin/{plugin-name}
  * event hooks, integration points
    * ckan has to expose a set of interfaces
  * settings access

= Research =

0. http://stackoverflow.com/questions/932069/building-a-minimal-plugin-architecture-in-python
1. http://wehart.blogspot.com/2009/01/python-plugin-frameworks.html
2. Zope Component Architecture http://pypi.python.org/pypi/grokcore.component
3. Trac Component Architecture
http://trac.edgewall.org/wiki/TracDev/ComponentArchitecture
4. Envisage (enthough):
https://svn.enthought.com/enthought/wiki/EnvisageThree/core.html
5. http://pypi.python.org/pypi/pyutilib.component.core/
  * Based on trac.core. Looks well-factored and very promising
6. https://bespin.mozillalabs.com/docs/pluginguide/index.html
  * Javascript but useful