.. -*- coding: utf-8 -*-

Sessions
========

Sessions are objects linked to an authenticated user.  The `Session.new_cnx`
method returns a new Connection linked to that session.

Connections
===========

Connections provide the `.execute` method to query the data sources, along with
`.commit` and `.rollback` methods for transaction management.

Kinds of connections
--------------------

There are two kinds of connections.

* `normal connections` are the most common: they are related to users and
  carry security checks coming with user credentials

* `internal connections` have all the powers; they are also used in only a
  few situations where you don't already have an adequate session at
  hand, like: user authentication, data synchronisation in
  multi-source contexts

Normal connections are typically named `_cw` in most appobjects or
sometimes just `session`.

Internal connections are available from the `Repository` object and are
to be used like this:

.. sourcecode:: python

   with self.repo.internal_cnx() as cnx:
       do_stuff_with(cnx)
       cnx.commit()

Connections should always be used as context managers, to avoid leaks.


Python/RQL API
~~~~~~~~~~~~~~

The Python API developped to interface with RQL is inspired from the standard db-api,
but since `execute` returns its results directly, there is no `cursor` concept.

.. sourcecode:: python

   execute(rqlstring, args=None, build_descr=True)

:rqlstring: the RQL query to execute (unicode)
:args: if the query contains substitutions, a dictionary containing the values to use

The `Connection` object owns the methods `commit` and `rollback`. You
*should never need to use them* during the development of the web
interface based on the *CubicWeb* framework as it determines the end
of the transaction depending on the query execution success. They are
however useful in other contexts such as tests or custom controllers.

.. note::

  If a query generates an error related to security (:exc:`Unauthorized`) or to
  integrity (:exc:`ValidationError`), the transaction can still continue but you
  won't be able to commit it, a rollback will be necessary to start a new
  transaction.

  Also, a rollback is automatically done if an error occurs during commit.

.. note::

   A :exc:`ValidationError` has a `entity` attribute. In CubicWeb,
   this atttribute is set to the entity's eid (not a reference to the
   entity itself).

Executing RQL queries from a view or a hook
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When you're within code of the web interface, the Connection is handled by the
request object. You should not have to access it directly, but use the
`execute` method directly available on the request, eg:

.. sourcecode:: python

   rset = self._cw.execute(rqlstring, kwargs)

Similarly, on the server side (eg in hooks), there is no request object (since
you're directly inside the data-server), so you'll have to use the execute method
of the Connection object.

Proper usage of `.execute`
~~~~~~~~~~~~~~~~~~~~~~~~~~

Let's say you want to get T which is in configuration C, this translates to:

.. sourcecode:: python

   self._cw.execute('Any T WHERE T in_conf C, C eid %s' % entity.eid)

But it must be written in a syntax that will benefit from the use
of a cache on the RQL server side:

.. sourcecode:: python

   self._cw.execute('Any T WHERE T in_conf C, C eid %(x)s', {'x': entity.eid})

The syntax tree is built once for the "generic" RQL and can be re-used
with a number of different eids.  The rql IN operator is an exception
to this rule.

.. sourcecode:: python

   self._cw.execute('Any T WHERE T in_conf C, C name IN (%s)'
                    % ','.join(['foo', 'bar']))

Alternatively, some of the common data related to an entity can be
obtained from the `entity.related()` method (which is used under the
hood by the ORM when you use attribute access notation on an entity to
get a relation. The initial request would then be translated to:

.. sourcecode:: python

   entity.related('in_conf', 'object')

Additionally this benefits from the fetch_attrs policy (see :ref:`FetchAttrs`)
optionally defined on the class element, which says which attributes must be
also loaded when the entity is loaded through the ORM.

.. _resultset:

The `ResultSet` API
~~~~~~~~~~~~~~~~~~~

ResultSet instances are a very commonly manipulated object. They have
a rich API as seen below, but we would like to highlight a bunch of
methods that are quite useful in day-to-day practice:

* `__str__()` (applied by `print`) gives a very useful overview of both
  the underlying RQL expression and the data inside; unavoidable for
  debugging purposes

* `printable_rql()` returns a well formed RQL expression as a
  string; it is very useful to build views

* `entities()` returns a generator on all entities of the result set

* `get_entity(row, col)` gets the entity at row, col coordinates; one
  of the most used result set methods

.. autoclass:: cubicweb.rset.ResultSet
   :members:


Authentication and management of sessions
-----------------------------------------

The authentication process is a ballet involving a few dancers:

* through its `get_session` method the top-level application object (the
  `CubicWebPublisher`) will open a session whenever a web request
  comes in; it asks the `session manager` to open a session (giving
  the web request object as context) using `open_session`

  * the session manager asks its authentication manager (which is a
    `component`) to authenticate the request (using `authenticate`)

    * the authentication manager asks, in order, to its authentication
      information retrievers, a login and an opaque object containing
      other credentials elements (calling `authentication_information`),
      giving the request object each time

      * the default retriever (named `LoginPasswordRetriever`)
        will in turn defer login and password fetching to the request
        object (which, depending on the authentication mode (`cookie`
        or `http`), will do the appropriate things and return a login
        and a password)

    * the authentication manager, on success, asks the `Repository`
      object to connect with the found credentials (using `connect`)

      * the repository object asks authentication to all of its
        sources which support the `CWUser` entity with the given
        credentials; when successful it can build the cwuser entity,
        from which a regular `Session` object is made; it returns the
        session id

        * the source in turn will delegate work to an authentifier
          class that defines the ultimate `authenticate` method (for
          instance the native source will query the database against
          the provided credentials)

    * the authentication manager, on success, will call back _all_
      retrievers with `authenticated` and return its authentication
      data (on failure, it will try the anonymous login or, if the
      configuration forbids it, raise an `AuthenticationError`)

Writing authentication plugins
------------------------------

Sometimes CubicWeb's out-of-the-box authentication schemes (cookie and
http) are not sufficient. Nowadays there is a plethora of such schemes
and the framework cannot provide them all, but as the sequence above
shows, it is extensible.

Two levels have to be considered when writing an authentication
plugin: the web client and the repository.

We invented a scenario where it makes sense to have a new plugin in
each side: some middleware will do pre-authentication and under the
right circumstances add a new HTTP `x-foo-user` header to the query
before it reaches the CubicWeb instance. For a concrete example of
this, see the `trustedauth`_ cube.

.. _`trustedauth`: http://www.cubicweb.org/project/cubicweb-trustedauth

Repository authentication plugins
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

On the repository side, it is possible to register a source
authentifier using the following kind of code:

.. sourcecode:: python

 from cubicweb.server.sources import native

 class FooAuthentifier(native.LoginPasswordAuthentifier):
     """ a source authentifier plugin
     if 'foo' in authentication information, no need to check
     password
     """
     auth_rql = 'Any X WHERE X is CWUser, X login %(login)s'

     def authenticate(self, session, login, **kwargs):
         """return CWUser eid for the given login
         if this account is defined in this source,
         else raise `AuthenticationError`
         """
         session.debug('authentication by %s', self.__class__.__name__)
         if 'foo' not in kwargs:
             return super(FooAuthentifier, self).authenticate(session, login, **kwargs)
         try:
             rset = session.execute(self.auth_rql, {'login': login})
             return rset[0][0]
         except Exception, exc:
             session.debug('authentication failure (%s)', exc)
         raise AuthenticationError('foo user is unknown to us')

Since repository authentifiers are not appobjects, we have to register
them through a `server_startup` hook.

.. sourcecode:: python

 class ServerStartupHook(hook.Hook):
     """ register the foo authenticator """
     __regid__ = 'fooauthenticatorregisterer'
     events = ('server_startup',)

     def __call__(self):
         self.debug('registering foo authentifier')
         self.repo.system_source.add_authentifier(FooAuthentifier())

Web authentication plugins
~~~~~~~~~~~~~~~~~~~~~~~~~~

.. sourcecode:: python

 class XFooUserRetriever(authentication.LoginPasswordRetriever):
     """ authenticate by the x-foo-user http header
     or just do normal login/password authentication
     """
     __regid__ = 'x-foo-user'
     order = 0

     def authentication_information(self, req):
         """retrieve authentication information from the given request, raise
         NoAuthInfo if expected information is not found
         """
         self.debug('web authenticator building auth info')
         try:
            login = req.get_header('x-foo-user')
            if login:
                return login, {'foo': True}
            else:
                return super(XFooUserRetriever, self).authentication_information(self, req)
         except Exception, exc:
            self.debug('web authenticator failed (%s)', exc)
         raise authentication.NoAuthInfo()

     def authenticated(self, retriever, req, cnx, login, authinfo):
         """callback when return authentication information have opened a
         repository connection successfully. Take care req has no session
         attached yet, hence req.execute isn't available.

         Here we set a flag on the request to indicate that the user is
         foo-authenticated. Can be used by a selector
         """
         self.debug('web authenticator running post authentication callback')
         cnx.foo_user = authinfo.get('foo')

In the `authenticated` method we add (in an admitedly slightly hackish
way) an attribute to the connection object. This, in turn, can be used
to build a selector dispatching on the fact that the user was
preauthenticated or not.

.. sourcecode:: python

 @objectify_selector
 def foo_authenticated(cls, req, rset=None, **kwargs):
     if hasattr(req.cnx, 'foo_user') and req.foo_user:
         return 1
     return 0

Full Session and Connection API
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. autoclass:: cubicweb.server.session.Session
.. autoclass:: cubicweb.server.session.Connection