======================
CKAN code architecture
======================

This section documents our CKAN-specific coding standards, which are guidelines
for writing code that is consistent with the intended design and architecture
of CKAN.

  .. image:: /images/architecture.png
     :alt: CKAN architecture diagram


----------
Blueprints
----------

CKAN is based on Flask and built using Blueprints.

Default blueprints are defined along views in ``ckan.views`` and extended with the
:class:`ckan.plugins.interfaces.IBlueprint` plugin interface.


-----
Views
-----

Views process requests by reading and updating data with action
function and return a response by rendering Jinja2 templates.
CKAN views are defined in ``ckan.views`` and templates in
``ckan.templates``.

Views and templates may use ``logic.check_access`` or
:func:`ckan.lib.helpers.check_access` to hide links or render
helpful errors but action functions, not views, are responsible for
actually enforcing permissions checking.

Plugins define new views by adding or updating routes. For adding
templates or helper functions from a plugin see
:doc:`../theming/index` and
:ref:`custom template helper functions`.


Template helper functions
#########################

Template helper functions are used for code that is reused frequently or
code that is too complicated to be included in the templates themselves.

Template helpers should never perform expensive queries or update data.

``ckan.lib.helpers`` contains helper functions that can be used from
``ckan.controllers`` or from templates. When developing for ckan core, only use
the helper functions found in ``ckan.lib.helpers.__allowed_functions__``.


.. _always use action functions:

Always go through the action functions
######################################

Whenever some code, for example in ``ckan.lib`` or ``ckan.controllers``, wants
to get, create, update or delete an object from CKAN's model it should do so by
calling a function from the ``ckan.logic.action`` package, and *not* by
accessing ``ckan.model`` directly.


Use ``get_action()``
####################

Don't call ``logic.action`` functions directly, instead use ``get_action()``.
This allows plugins to override action functions using the ``IActions`` plugin
interface. For example::

    ckan.logic.get_action('group_activity_list')(...)

Instead of ::

    ckan.logic.action.get.group_activity_list(...)

Views and templates may check authorization to avoid rendering

Don't pass ORM objects to templates
###################################

Don't pass SQLAlchemy ORM objects (e.g. :py:class:`ckan.model.User` objects)
to templates (for example by adding them to :py:data:`c`, passing them to
:py:func:`~ckan.lib.base.render` in the ``extra_vars`` dict, returning them
from template helper functions, etc.)

Using ORM objects in the templates often creates SQLAlchemy "detached instance"
errors that cause 500 Server Errors and can be difficult to debug.


-----
Logic
-----

Logic includes action functions, auth functions, background tasks
and business logic.

Action functions have a uniform interface accepting a dictionary of simple
strings lists, dictionaries or files (wrapped in a ``cgi.FieldStorage``
objects). They return simple dictionaries or raise one of a small number of
exceptions including ``ckan.logic.NotAuthorized``, ``ckan.logic.NotFound``
and ``ckan.logic.ValidationError``.

Plugins override action functions with the
:class:`ckan.plugins.interfaces.IActions` interface and auth functions
with the :class:`ckan.plugins.interfaces.IAuthFunctions` interface.


Action functions are exposed in the API
#######################################

The functions in ``ckan.logic.action`` are exposed to the world as the
:doc:`/api/index`.  The API URL for an action function is automatically generated
from the function name, for example
``ckan.logic.action.create.package_create()`` is exposed at
``/api/action/package_create``. See `Steve Yegge's Google platforms rant
<https://plus.google.com/112678702228711889851/posts/eVeouesvaVX>`_ for some
interesting discussion about APIs.

**All** publicly visible functions in the
``ckan.logic.action.{create,delete,get,update}`` namespaces will be exposed
through the :doc:`/api/index`. **This includes functions imported** by those
modules, **as well as any helper functions** defined within those modules.  To
prevent inadvertent exposure of non-action functions through the action api,
care should be taken to:

1. Import modules correctly (see :ref:`imports`).  For example::

     import ckan.lib.search as search

     search.query_for(...)

2. Hide any locally defined helper functions: ::

     def _a_useful_helper_function(x, y, z):
        '''This function is not exposed because it is marked as private```
        return x+y+z

3. Bring imported convenience functions into the module namespace as private
   members: ::

     _get_or_bust = logic.get_or_bust


Auth functions and ``check_access()``
#####################################

Each action function defined in ``ckan.logic.action`` should use its own
corresponding auth function defined in ``ckan.logic.auth``. Instead of calling
its auth function directly, an action function should go through
``ckan.logic.check_access`` (which is aliased ``_check_access`` in the action
modules) because this allows plugins to override auth functions using the
``IAuthFunctions`` plugin interface. For example::

    def package_show(context, data_dict):
        _check_access('package_show', context, data_dict)

``check_access`` will raise an exception if the user is not authorized, which
the action function should not catch. When this happens the user will be shown
an authorization error in their browser (or will receive one in their response
from the API).


``logic.get_or_bust()``
#######################

The ``data_dict`` parameter of logic action functions may be user provided, so
required files may be invalid or absent. Naive Code like::

  id = data_dict['id']

may raise a ``KeyError`` and cause CKAN to crash with a 500 Server Error
and no message to explain what went wrong. Instead do::

  id = _get_or_bust(data_dict, "id")

which will raise ``ValidationError`` if ``"id"`` is not in ``data_dict``. The
``ValidationError`` will be caught and the user will get a 400 Bad Request
response and an error message explaining the problem.


Validation and ``ckan.logic.schema``
####################################

Logic action functions can use schema defined in ``ckan.logic.schema`` to
validate the contents of the ``data_dict`` parameters that users pass to them.

An action function should first check for a custom schema provided in the
context, and failing that should retrieve its default schema directly, and
then call ``_validate()`` to validate and convert the data. For example, here
is the validation code from the ``user_create()`` action function::

 schema = context.get('schema') or ckan.logic.schema.default_user_schema()
 session = context['session']
 validated_data_dict, errors = _validate(data_dict, schema, context)
 if errors:
     session.rollback()
     raise ValidationError(errors)


------
Models
------

Ideally SQLAlchemy should only be used within ``ckan.model`` and not from other
packages such as ``ckan.logic``.  For example instead of using an SQLAlchemy
query from the logic package to retrieve a particular user from the database,
we add a ``get()`` method to ``ckan.model.user.User``::

    @classmethod
    def get(cls, user_id):
        query = ...
        .
        .
        .
        return query.first()

Now we can call this method from the logic package.


-----------
Deprecation
-----------

- Anything that may be used by :doc:`extensions </extensions/index>`,
  :doc:`themes </theming/index>` or :doc:`API clients </api/index>` needs to
  maintain backward compatibility at call-site. For example: action functions,
  template helper functions and functions defined in the plugins toolkit.

- The length of time of deprecation is evaluated on a function-by-function
  basis. At minimum, a function should be marked as deprecated during a point
  release.

- To deprecate a function use the :py:func:`ckan.lib.maintain.deprecated`
  decorator and add "deprecated" to the function's docstring::

    @maintain.deprecated("helpers.get_action() is deprecated and will be removed "
                        "in a future version of CKAN. Instead, please use the "
                        "extra_vars param to render() in your controller to pass "
                        "results from action functions to your templates.")
    def get_action(action_name, data_dict=None):
        '''Calls an action function from a template. Deprecated in CKAN 2.3.'''
        if data_dict is None:
            data_dict = {}
        return logic.get_action(action_name)({}, data_dict)

- Any deprecated functions should be added to an *API changes and deprecations*
  section in the :doc:`/changelog` entry for the next release (do this before
  merging the deprecation into master)

- Keep the deprecation messages passed to the decorator short, they appear in
  logs. Put longer explanations of why something was deprecated in the
  changelog.
