Update documentation regarding DataSource for strategies

Change-Id: Ifc82fc06858a4e5a2badef074cd6bb5ce4e216cb

.gitignore

@@ -75,3 +75,6 @@ releasenotes/build
 
 # Autogenerated sample config file
 etc/watcher/watcher.conf.sample
+
+# Atom
+.remote-sync.json

doc/source/contributor/plugin/strategy-plugin.rst

@@ -245,15 +245,16 @@ Querying metrics
 
 A large set of metrics, generated by OpenStack modules, can be used in your
 strategy implementation. To collect these metrics, Watcher provides a
-`Helper`_ for two data sources which are `Ceilometer`_ and `Monasca`_. If you
-wish to query metrics from a different data source, you can implement your own
-and directly use it from within your new strategy. Indeed, strategies in
-Watcher have the cluster data models decoupled from the data sources which
-means that you may keep the former while changing the latter.
-The recommended way for you to support a new data source is to implement a new
-helper that would encapsulate within separate methods the queries you need to
-perform. To then use it, you would just have to instantiate it within your
-strategy.
+`DataSourceManager`_ for two data sources which are `Ceilometer`_
+(with `Gnocchi`_ as API) and `Monasca`_. If you wish to query metrics from a
+different data source, you can implement your own and use it via
+DataSourceManager from within your new strategy. Indeed, strategies in Watcher
+have the cluster data models decoupled from the data sources which means that
+you may keep the former while changing the latter. The recommended way for you
+to support a new data source is to implement a new helper that would
+encapsulate within separate methods the queries you need to perform. To then
+use it, you would just have to add it to appropriate watcher_strategies.*
+section in config file.
 
 If you want to use Ceilometer but with your own metrics database backend,
 please refer to the `Ceilometer developer guide`_. The list of the available
@@ -263,52 +264,38 @@ requires new metrics not covered by Ceilometer, you can add them through a
 `Ceilometer plugin`_.
 
 
-.. _`Helper`: https://github.com/openstack/watcher/blob/master/watcher/datasource/ceilometer.py
+.. _`DataSourceManager`: https://github.com/openstack/watcher/blob/master/watcher/datasource/manager.py
 .. _`Ceilometer developer guide`: https://docs.openstack.org/ceilometer/latest/contributor/architecture.html#storing-accessing-the-data
 .. _`Ceilometer`: https://docs.openstack.org/ceilometer/latest
 .. _`Monasca`: https://github.com/openstack/monasca-api/blob/master/docs/monasca-api-spec.md
 .. _`here`: https://docs.openstack.org/ceilometer/latest/contributor/install/dbreco.html#choosing-a-database-backend
 .. _`Ceilometer plugin`: https://docs.openstack.org/ceilometer/latest/contributor/plugins.html
 .. _`Ceilosca`: https://github.com/openstack/monasca-ceilometer/blob/master/ceilosca/ceilometer/storage/impl_monasca.py
+.. _`Gnocchi`: https://gnocchi.xyz/
 
 Read usage metrics using the Watcher Datasource Helper
 ------------------------------------------------------
 
-The following code snippet shows how to invoke a Datasource Helper class:
+The following code snippet shows how datasource_backend is defined:
 
 .. code-block:: py
 
-    from watcher.datasource import ceilometer as ceil
-    from watcher.datasource import monasca as mon
+    from watcher.datasource import manager as ds_manager
 
     @property
-    def ceilometer(self):
-        if self._ceilometer is None:
-            self._ceilometer = ceil.CeilometerHelper(osc=self.osc)
-        return self._ceilometer
-
-    @property
-    def monasca(self):
-        if self._monasca is None:
-            self._monasca = mon.MonascaHelper(osc=self.osc)
-        return self._monasca
+    def datasource_backend(self):
+        if not self._datasource_backend:
+            self._datasource_backend = ds_manager.DataSourceManager(
+                config=self.config,
+                osc=self.osc
+            ).get_backend(self.DATASOURCE_METRICS)
+        return self._datasource_backend
 
 Using that you can now query the values for that specific metric:
 
 .. code-block:: py
 
-        if self.config.datasource == "ceilometer":
-            resource_id = "%s_%s" % (node.uuid, node.hostname)
-            return self.ceilometer.statistic_aggregation(
-                resource_id=resource_id,
-                meter_name='compute.node.cpu.percent',
-                period="7200",
-                aggregate='avg',
-            )
-        elif self.config.datasource == "monasca":
-            statistics = self.monasca.statistic_aggregation(
-                meter_name='compute.node.cpu.percent',
-                dimensions=dict(hostname=node.uuid),
-                period=7200,
-                aggregate='avg'
-            )
+    avg_meter = self.datasource_backend.statistic_aggregation(
+        instance.uuid, 'cpu_util', self.periods['instance'],
+        self.granularity,
+        aggregation=self.aggregation_method['instance'])