[hopem,r=gnuoy]

Provide min-cluster-size config option to get
round clustering races until we have leadership
election support in Juju. Also ensure that
cookie is only distributed by the leader.

config.yaml

@@ -166,3 +166,9 @@ options:
       order for this charm to function correctly, the privacy extension must be
       disabled and a non-temporary address must be configured/available on
       your network interface.
+  min-cluster-size:
+    type: int
+    default:
+    description: |
+      Minimum number of units expected to exist before charm will attempt to
+      form a rabbitmq cluster.

hooks/rabbitmq_server_relations.py

@@ -194,6 +194,26 @@ def amqp_changed(relation_id=None, remote_unit=None):
                        relation_settings=relation_settings)
 
 
+def is_sufficient_peers():
+    """If min-cluster-size has been provided, check that we have sufficient
+    number of peers to proceed with creating rabbitmq cluster.
+    """
+    min_size = config('min-cluster-size')
+    if min_size:
+        size = 0
+        for rid in relation_ids('cluster'):
+            size = len(related_units(rid))
+
+        # Include this unit
+        size += 1
+        if min_size > size:
+            log("Insufficient number of peer units to form cluster "
+                "(expected=%s, got=%s)" % (min_size, size), level=INFO)
+            return False
+
+    return True
+
+
 @hooks.hook('cluster-relation-joined')
 def cluster_joined(relation_id=None):
     if config('prefer-ipv6'):
@@ -240,8 +260,13 @@ def cluster_joined(relation_id=None):
         log('erlang cookie missing from %s' % rabbit.COOKIE_PATH,
             level=ERROR)
         return
-    cookie = open(rabbit.COOKIE_PATH, 'r').read().strip()
-    peer_store('cookie', cookie)
+
+    if not is_sufficient_peers():
+        return
+
+    if is_elected_leader('res_rabbitmq_vip'):
+        cookie = open(rabbit.COOKIE_PATH, 'r').read().strip()
+        peer_store('cookie', cookie)
 
 
 @hooks.hook('cluster-relation-changed')
@@ -262,6 +287,11 @@ def cluster_changed():
     whitelist = [a for a in rdata.keys() if a not in blacklist]
     peer_echo(includes=whitelist)
 
+    if not is_sufficient_peers():
+        # Stop rabbit until leader has finished configuring
+        service_stop('rabbitmq-server')
+        return
+
     # sync the cookie with peers if necessary
     update_cookie()
 
@@ -276,6 +306,7 @@ def cluster_changed():
         if rabbit.cluster_with():
             # resync nrpe user after clustering
             update_nrpe_checks()
+
     # If cluster has changed peer db may have changed so run amqp_changed
     # to sync any changes
     for rid in relation_ids('amqp'):

unit_tests/test_rabbitmq_server_relations.py

@@ -129,3 +129,20 @@ class RelationUtil(TestCase):
         mock_peer_store_and_set.assert_called_with(
             relation_settings={'private-address': ipv6_addr},
             relation_id=None)
+
+    @patch.object(rabbitmq_server_relations, 'related_units')
+    @patch.object(rabbitmq_server_relations, 'relation_ids')
+    @patch.object(rabbitmq_server_relations, 'config')
+    def test_is_sufficient_peers(self, mock_config, mock_relation_ids,
+                                 mock_related_units):
+        _config = {'min-cluster-size': None}
+        mock_config.side_effect = lambda key: _config.get(key)
+        self.assertTrue(rabbitmq_server_relations.is_sufficient_peers())
+
+        mock_relation_ids.return_value = ['cluster:0']
+        mock_related_units.return_value = ['test/0']
+        _config = {'min-cluster-size': 3}
+        self.assertFalse(rabbitmq_server_relations.is_sufficient_peers())
+
+        mock_related_units.return_value = ['test/0', 'test/1']
+        self.assertTrue(rabbitmq_server_relations.is_sufficient_peers())