diff --git a/devdocs/develop/nailgun/customization/extensions.rst b/devdocs/develop/nailgun/customization/extensions.rst
new file mode 100644
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+Nailgun Extensions
+__________________
+
+
+Overview of extensions
+======================
+
+Nailgun extensions provide a capability to extend Fuel features.
+Extensions were introduced to provide *pythonic way* for adding integrations
+with external services, extending existing features, or adding new features
+**without** changing the Nailgun source code.
+
+A Nailgun extension can execute its method on specific events
+such as ``on_node_create`` or ``on_cluster_delete`` (more about event handlers
+in the `Available Events`_ section) and also to change deployment and
+provisioning data just before it is sent to orchestrator by the means of
+Data Pipelines classes.
+
+
+Required properties
+===================
+
+All Nailgun extensions must populate the following class variables:
+
+* ``name`` - a string which will be used inside Nailgun to identify the
+  extension. It should consist only of lowercase letters with "_" (underscore)
+  separator and digits.
+
+* ``version`` - a string which should look like *X.Y.Z* where X is a major
+  version, Y is minor, and Z is bug fix update or build number.
+
+* ``description`` - a short text which briefly describes the actions that the
+  extension performs.
+
+
+Available events
+================
+
+Extension can execute event handlers on specific events. There is
+a list of available handlers:
+
+.. list-table::
+   :widths: 10 10
+   :header-rows: 1
+
+   * - Method
+     - Event
+   * - ``on_node_create``
+     - Node has been created
+   * - ``on_node_update``
+     - Node has been updated
+   * - ``on_node_reset``
+     - Node has been reseted
+   * - ``on_node_delete``
+     - Node has been deleted
+   * - ``on_node_collection_delete``
+     - Collection of nodes has been deleted
+   * - ``on_cluster_delete``
+     - Cluster has been deleted
+   * - ``on_before_deployment_check``
+     - Called right before running "before deployment check task"
+
+
+REST API handlers
+=================
+Nailgun Extensions also provide a way to add additional API endpoints.
+To add an extension-specific handler sub-class from::
+
+  nailgun.api.v1.handlers.base.BaseHandler
+
+The second step is to register the handler by providing the ``urls`` list in
+Extension class:
+
+.. code-block:: python
+
+  urls = [
+      {'uri': r'/example_extension/(?P<node_id>\d+)/?$',
+       'handler': ExampleNodeHandler},
+      {'uri': r'/example_extension/(?P<node_id>\d+)/properties/',
+       'handler': NodeDefaultsDisksHandler},
+  ]
+
+
+As you can see you need to provide a list of dicts with keys:
+
+.. list-table::
+   :widths: 10 10
+   :header-rows: 1
+
+   * - key
+     - value
+   * - ``uri``
+     - a regular expression (string) for the URL path
+   * - ``handler``
+     - handler class
+
+
+Database interaction
+====================
+
+There is a possibility to use the Nailgun database to store the data needed by
+a Nailgun extension. To use it, provide alembic migration scripts which should
+be placed in::
+
+  extension_module/alembic_migrations/migrations/
+
+Where ``extension_module`` is the one where the file with your extension class
+is placed.
+
+You can also change this directory by overriding the classmethod::
+
+  alembic_migrations_path
+
+It should return an absolute path (string) to alembic migrations
+directory.
+
+.. important::
+   Do not use the direct db calls to Nailgun core tables in the extension
+   class. Use the ``nailgun.objects`` module which ensures compatibility
+   between the Nailgun DB and the configuration implemented in your extension.
+
+
+Extension Data Pipelines
+========================
+
+If you want to change the deployment or provisioning data just before it is
+sent to an orchestrator use Extension Data Pipelines.
+
+Data Pipeline is a class which inherits from::
+
+  nailgun.extensions.BasePipeline
+
+BasePipeline provides two methods which you can override:
+
+* ``process_provisioning``
+
+* ``process_deployment``
+
+Both methods take the following parameters:
+
+* ``data`` - serialized data which will be sent to orchestrator. Data
+  **does not include** nodes data which was defined by User in
+  ``replaced_deployment_info`` or in ``replaced_provisioning_info``.
+
+* ``cluster`` - a cluster instance for which the data was serialized.
+
+* ``nodes`` - nodes instances for which the data was serialized. Nodes list
+  **does not include** node instances which were filtered out in ``data``
+  parameter.
+
+* ``**kwargs`` - additional kwargs - must be in method definition to provide
+  backwards-compatibility for future (small) changes in extensions API.
+
+Both methods must return the ``data`` dict so it can be processed by other
+pipelines.
+
+To enable pipelines, add the ``data_pipelines`` variable in your extensions
+class:
+
+.. code-block:: python
+
+  class ExamplePipelineOne(BasePipeline):
+
+      @classmethod
+      def process_provisioning(cls, data, cluster, nodes, **kwargs):
+          data['new_field'] = 'example_value'
+          return data
+
+      @classmethod
+      def process_deployment(cls, data, cluster, nodes, **kwargs):
+          data['new_field'] = 'example_value'
+          return data
+
+
+  class ExamplePipelineTwo(BasePipeline):
+
+      @classmethod
+      def process_deployment(cls, data, cluster, nodes, **kwargs):
+          data['new_field2'] = 'example_value2'
+          return data
+
+
+  class ExampleExtension(BaseExtension):
+      ...
+      data_pipelines = [
+          ExamplePipelineOne,
+          ExamplePipelineTwo,
+      ]
+      ...
+
+
+Pipeline classes will be executed **in the order they are defined** in the
+``data_pipelines`` variable.
+
+How to install and plug in extensions
+=====================================
+
+To use extensions system in Nailgun, implement an extension class which will
+be the subclass of::
+
+  nailgun.extensions.BaseExtension
+
+The class must be placed in a separate module which defines ``entry_points`` in
+its ``setup.py`` file.
+
+Extension entry point should use Nailgun extensions namespace which is::
+
+  nailgun.extensions
+
+Example ``setup.py`` file with ``ExampleExtension`` may look like this:
+
+.. code-block:: python
+
+  from setuptools import setup, find_packages
+
+  setup(
+         name='example_package',
+         version='1.0',
+         description='Demonstration package for Nailgun Extensions',
+         author='Fuel Nailgman',
+         author_email='fuel@nailgman.com',
+         url='http://example.com',
+         classifiers=['Development Status :: 3 - Alpha',
+                     'License :: OSI Approved :: Apache Software License',
+                     'Programming Language :: Python',
+                     'Programming Language :: Python :: 2',
+                     'Environment :: Console',
+                     ],
+         packages=find_packages(),
+         entry_points={
+            'nailgun.extensions': [
+                'ExampleExtension = example_package.nailgun_extensions.ExampleExtension',
+             ],
+         },
+  )
+
+
+Now to enable the extension it is enough to run::
+
+  python setup.py install
+
+or::
+
+  pip install .
+
+Now extension will be discovered by Nailgun automatically after restart.
+
+
+Example Extension with Pipeline - additional logging
+====================================================
+
+.. code-block:: python
+
+  import datetime
+  import logging
+
+  from nailgun.extensions import BaseExtension
+  from nailgun.extensions import BasePipeline
+
+  logger = logging.getLogger(__name__)
+
+
+  class TimeStartedPipeline(BasePipeline):
+
+      @classmethod
+      def process_provisioning(cls, data, cluster, nodes, **kwargs):
+          now = datetime.datetime.now()
+          data['time_started'] = 'provisioning started at {}'.format(now)
+          return data
+
+      @classmethod
+      def process_deployment(cls, data, cluster, nodes, **kwargs):
+          now = datetime.datetime.now()
+          data['time_started'] = 'deployment started at {}'.format(now)
+          return data
+
+
+  class ExampleExtension(BaseExtension):
+      name = 'additional_logger'
+      version = '1.0.0'
+      description = 'Additional Logging Extension '
+
+      data_pipelines = [
+          TimeStartedPipeline,
+      ]
+
+      @classmethod
+      def on_node_create(cls, node):
+          logging.debug('Node %s has been created', node.id)
+
+      @classmethod
+      def on_node_update(cls, node):
+          logging.debug('Node %s has been updated', node.id)
+
+      @classmethod
+      def on_node_reset(cls, node):
+          logging.debug('Node %s has been reseted', node.id)
+
+      @classmethod
+      def on_node_delete(cls, node):
+          logging.debug('Node %s has been deleted', node.id)
+
+      @classmethod
+      def on_node_collection_delete(cls, node_ids):
+          logging.debug('Nodes %s have been deleted', ', '.join(node_ids))
+
+      @classmethod
+      def on_cluster_delete(cls, cluster):
+          logging.debug('Cluster %s has been deleted', cluster.id)
+
+      @classmethod
+      def on_before_deployment_check(cls, cluster):
+          logging.debug('Cluster %s will be deployed soon', cluster.id)
diff --git a/devdocs/develop/nailgun/tree.rst b/devdocs/develop/nailgun/tree.rst
index e2557992f..5bfc42496 100644
--- a/devdocs/develop/nailgun/tree.rst
+++ b/devdocs/develop/nailgun/tree.rst
@@ -23,3 +23,4 @@ Nailgun Customization Instructions
     customization/roles
     customization/settings
     customization/bonding_in_ui
+    customization/extensions