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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+===================================
+Adding scope for Watcher data model
+===================================
+
+https://blueprints.launchpad.net/watcher/+spec/scope-for-watcher-datamodel
+
+
+Problem description
+===================
+
+For a large cloud infrastructure, such as CERN, there are more than
+10k servers, retrieving data from Nova to build Watcher compute
+data model may take a long time. If the audit is just for a subset
+of all nodes, it's better to get the data from the nodes that audit needs.
+
+Use Cases
+----------
+
+As a Watcher user, I want that Watcher doesn't build compute data model before
+creating audit.
+As a Watcher user, I want that Watcher build compute data model according to
+the scope of audit.
+
+
+Proposed change
+===============
+
+As now, Watcher builds compute data model when starting the Decision Engine.
+And there is a periodic task to rebuild the data model.
+To avoid building the data model before creating audit, we need to check a
+flag before building the data model.
+for example:
+
+.. code-block:: python
+
+    def execute(self):
+        """Build the compute cluster data model"""
+        if self._audit_scope_handler is None:
+            LOG.debug("No audit, Don't Build compute data model")
+            return
+
+        builder = ModelBuilder(self.osc)
+        return builder.execute(self._data_model_scope)
+
+
+Audit scope is a optional parameter when creating audit. If user don't
+set a scope ,the default scope is empty, it means this audit used for
+all nodes.
+An example of the audit scope:
+
+.. code-block:: python
+
+    {"compute":
+        [{"host_aggregates": [
+                              {"id": 1},
+                              {"id": 2},
+                              {"id": 3}]},
+         {"availability_zones": [
+                              {"name": "AZ1"},
+                              {"name": "AZ2"}]},
+    }
+
+When building the data model according to audit scope, there are some cases
+need to be considered:
+
+no data model, audit scope is empty
+-----------------------------------
+* It's the first time to build the data model. Because audit scope is empty,
+  the data model should include all the nodes.
+
+no data model, audit scope is not empty
+---------------------------------------
+* It's the first time to build the data model according to audit scope.
+
+Existing data model, new audit scope is empty
+---------------------------------------------
+* If the data model has included all nodes, it will not be rebuilt.
+
+* If the data model doesn't include all nodes, it will be rebuilt.
+
+existing data model, new audit scope is not empty
+-------------------------------------------------
+* If the nodes specified in scope are already included in the data model,
+  it will not be rebuilt.
+
+* If the nodes specified in the scope aren't included in the data model,
+  it will be rebuilt.
+
+
+Alternatives
+------------
+
+None
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+This will reduce the impact on system performance, especially for
+large cloud infrastructure.
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  <licanwei>
+
+Work Items
+----------
+
+* Add a check to the audit to create before building the data model.
+
+* Add the model scope to record the audit scope in the ModelBuilder class.
+
+* Update the data model according to the audit scope.
+
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+Add unit tests.
+
+
+Documentation Impact
+====================
+
+Update Watcher developer documents.
+
+
+References
+==========
+
+None
+
+
+History
+=======
+
+.. list-table:: Revisions
+   :header-rows: 1
+
+   * - Release Name
+     - Description
+   * - Stein
+     - Introduced
+