Upgrading to nornir 3.x from 2.x
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If you find some change not documented below don't hesitate to open a PR or issue about it :)

Plugin Register
===============

Introduced `plugin register <../api/nornir/core/plugins/register.html#nornir.core.plugins.register.PluginRegister>`_

Starting with nornir3 some plugins need to be registered. See `plugins section <../plugins/index.html#registering-plugins>`_ for details.

Inventory
=========

#. Remove inventory deserializer
#. Fixed ``mypy``
#. ``ParentGroups`` is simplified
#. ``__init__`` functions are more explicit in order to improve correctness
#. Removed ``add_host`` and ``add_group``
#. Removed ``get_inventory_dict``, ``get_defaults_dict``, ``get_groups_dict``, ``get_hosts_dict``. Only ``dict`` remains
#. Inventory plugins need to be registered
#. Transform functions need to be registered

InitNornir
==========

#. Passing callables as inventory plugin and transform functions is no longer supported
#. ``configure_logging`` has been removed (it used to indicate it was to be deprecated)

Configuration
=============

#. Order of resolution for parameters is now InitNornir > config > environment
#. ``jinja2`` can no longer be configured using the configuration
#. ``num_workers`` is no longer a ``core`` option
#. ``logging.file`` is now ``logging.log_file``
#. new ``runner`` section introduced


``num_workers``
---------------

Nornir used to accept ``num_workers`` under the core configuration section. This used to instruct whether to use threads or not and how many. This has now changed with the introduction of `runners <../plugins/index.html#runners>`_. Now if you want to avoid using threads you would instruct nornir to use the ``SerialRunner`` like this::

    #config.yaml
    runners:
        plugin: serial

To tweak the number of threads::

    #config.yaml
    runners:
        plugin: threaded
        options:
            num_workers: 100

For this same reason ``NORNIR_CORE_NUM_WORKERS`` doesn't work anymore. You should be able to do ``NORNIR_RUNNER_OPTIONS='{"num_workers": 100}'`` instead.