Architecture and Technologies

The software stack in KARL reflects history as a Plone application, as well as the backgrounds of the team involved throughout development.

At a high level, the technology stack as of KARL 3.0 uses:

  • *nix. Linux, FreeBSD, and other flavors of Unix such as OS X are in use by the KARL developers. Linux is used for the current hosting of KARL3 for OSI.
  • Python. The Python programming language is used for the implementation of KARL.
  • Pyramid. Although much of the web framework used by KARL comes from a number of popular Python tools, Pyramid is the web microframework that glues the application together.
  • ZODB object database. Content is stored and searched using the object database from the Zope project, also used by Plone.

More detail is available on in the architecture guide.


In development and deployment, KARL 3.0 uses a number of technologies from two basic backgrounds: Python and Zope. KARL itself has very little “framework” or architecture, which was certainly a design goal (KARL isn’t a platform, it is an end-user application.)

Python Technologies

This group of technologies are used beyond the Zope/Plone/KARL community and should be familiar to people familiar with basic Python web development.

  • WSGI for generic Python web applications.
  • WebOb for handling web requests and responses in a standard API.
  • Paste to wire together application configurations in a simple, declarative format.
  • virtualenv to give each deployment a reproducible, isolated Python environment.
  • Eggs as the package distribution facility.
  • repoze.who for generic authentication and goup handling.
  • Supervisor to manage server processes.
  • TinyMCE as a widely-used (beyond Python) browser-based editor.
  • jQuery and jQuery UI as a widely-used (beyond Python) JavaScript framework for Ajax.
  • mod_wsgi as one deployment choice for running KARL directly inside Apache.
  • nose for a unit testing framework.
  • Twill to give functional testing of web requests.

Zope Technologies

Reflecting the project’s background in Plone and Zope, a number of battle-tested technologies were used from the world of Zope.

  • Interfaces as a “type” system for objects and behavior.
  • Adapters to provide pluggable customization points.
  • Utilities to provide pluggable services across the system.
  • Traversal to represent hierarchies in URLs as containment in the object system.
  • Views to interact with web requests and return a response.
  • Events to provide uniform actions on certain kinds of operations.
  • Security as a very rich architecture for access control, including filtering of search results and other queries.
  • ZODB (including ZEO, middleware transactions, and blobs) to store, retrieve, index, and query schemaless objects.
  • ZPT as the templating language, via the Chameleon implementation.
  • Catalog as an indexing and searching facility.
  • PAS to manage information about users and groups.
  • ZCML to allow customization points by different deployments of KARL.
  • buildout to give repeatable, reliable installation and ongoing updating of the software.