The credentials for systems are stored in the ``external.system`` model, and are to
be configured by the user. This model is the unified interface for the underlying
adapters.

Using the Interface
~~~~~~~~~~~~~~~~~~~

Given an ``external.system`` singleton called ``external_system``, you would do the
following to get the underlying system client:

.. code-block:: python

   with external_system.client() as client:
       client.do_something()

The client will be destroyed once the context has completed. Destruction takes place
in the adapter's ``external_destroy_client`` method.

The only unified aspect of this interface is the client connection itself. Other more
opinionated interface/adapter mechanisms can be implemented in other modules, such as
the file system interface in `OCA/server-tools/external_file_location
<https://github.com/OCA/server-tools/tree/9.0/external_file_location>`_.

Creating an Adapter
~~~~~~~~~~~~~~~~~~~

Modules looking to add an external system adapter should inherit the
``external.system.adapter`` model and override the following methods:

* ``external_get_client``: Returns a usable client for the system
* ``external_destroy_client``: Destroy the connection, if applicable. Does not need
  to be defined if the connection destroys itself.