1 files changed, 35 insertions, 0 deletions
diff --git a/charms/trusty/cassandra/hooks/coordinator.py b/charms/trusty/cassandra/hooks/coordinator.py
new file mode 100644
index 0000000..c353671
--- /dev/null
+++ b/charms/trusty/cassandra/hooks/coordinator.py
@@ -0,0 +1,35 @@
+# Copyright 2015 Canonical Ltd.
+#
+# This file is part of the Cassandra Charm for Juju.
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 3, as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranties of
+# MERCHANTABILITY, SATISFACTORY QUALITY, or FITNESS FOR A PARTICULAR
+# PURPOSE.  See the GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+from charmhelpers.coordinator import BaseCoordinator
+
+
+class CassandraCoordinator(BaseCoordinator):
+    def default_grant(self, lock, unit, granted, queue):
+        '''Grant locks to only one unit at a time, regardless of its name.
+
+        This lets us keep separate locks like repair and restart,
+        while ensuring the operations do not occur on different nodes
+        at the same time.
+        '''
+        # Return True if this unit has already been granted a lock.
+        if self.grants.get(unit):
+            return True
+
+        # Otherwise, return True if the unit is first in the queue.
+        return queue[0] == unit and not granted
+
+
+coordinator = CassandraCoordinator()