Sessions are object carrying the `.execute` method to query the data

sources.

Kinds of sessions

-----------------

* `normal sessions` are the most common: they are related to users and

  instance : `in memory` or `pyro`.

Normal sessions are typically named `_cw` in most appobjects or

sometimes just `session`.

Internal sessions are available from the `Repository` object and are

to be used like this:

.. sourcecode:: python

   session = self.repo.internal_session()

   try:

       # do stuff

   finally:

       session.close()

.. warning::

  Do not forget to close such a session after use for a session leak

  will quickly lead to an application crash.

Authentication and management of sessions

-----------------------------------------

The authentication process is a ballet involving a few dancers:

* through its `connect` method the top-level application object (the

  `CubicWebPublisher`) will (re-)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 (bizarrely named

        `LoginPaswordRetreiver`) 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 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`)