Add common plugin framework.

For blueprint novaplugins.

It turns out that most of the plugin framework applies equally well
to other projects, so it's going into Nova via common so that
other projects can pull it in with a minimum of fuss.

Change-Id: Ia4455a8ca0b8c1c3e4b0b9647e8cacaf0a47c914

1 files changed, 87 insertions, 0 deletions

diff --git a/openstack/common/plugin/plugin.py b/openstack/common/plugin/plugin.py
new file mode 100644
index 0000000..9f06342
--- /dev/null
+++ b/openstack/common/plugin/plugin.py
@@ -0,0 +1,87 @@
+# Copyright 2012 OpenStack LLC.
+# All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from openstack.common import log as logging
+from openstack.common.notifier import list_notifier
+
+
+LOG = logging.getLogger(__name__)
+
+
+class Plugin(object):
+    """Defines an interface for adding functionality to an OpenStack service.
+
+    A plugin interacts with a service via the following pathways:
+
+    - An optional set of notifiers, managed by calling add_notifier()
+      or by overriding _notifiers()
+
+    - A set of api extensions, managed via add_api_extension_descriptor()
+
+    - Direct calls to service functions.
+
+    - Whatever else the plugin wants to do on its own.
+
+    This is the reference implementation.
+    """
+
+    # The following functions are provided as convenience methods
+    # for subclasses.  Subclasses should call them but probably not
+    # override them.
+    def _add_api_extension_descriptor(self, descriptor):
+        """Subclass convenience method which adds an extension descriptor.
+
+           Subclass constructors should call this method when
+           extending a project's REST interface.
+
+           Note that once the api service has loaded, the
+           API extension set is more-or-less fixed, so
+           this should mainly be called by subclass constructors.
+        """
+        self._api_extension_descriptors.append(descriptor)
+
+    def _add_notifier(self, notifier):
+        """Subclass convenience method which adds a notifier.
+
+           Notifier objects should implement the function notify(message).
+           Each notifier receives a notify() call whenever an openstack
+           service broadcasts a notification.
+
+           Best to call this during construction.  Notifiers are enumerated
+           and registered by the pluginmanager at plugin load time.
+        """
+        self._notifiers.append(notifier)
+
+    # The following methods are called by OpenStack services to query
+    #  plugin features.  Subclasses should probably not override these.
+    def _notifiers(self):
+        """Returns list of notifiers for this plugin."""
+        return self._notifiers
+
+    notifiers = property(_notifiers)
+
+    def _api_extension_descriptors(self):
+        """Return a list of API extension descriptors.
+
+           Called by a project API during its load sequence.
+        """
+        return self._api_extension_descriptors
+
+    api_extension_descriptors = property(_api_extension_descriptors)
+
+    # Most plugins will override this:
+    def __init__(self, service_name):
+        self._notifiers = []
+        self._api_extension_descriptors = []