[hopem,r=]

Provide min-cluster-size config option to get
round clustering races until we have leadership
election support in Juju.

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 cluster.

hooks/rabbitmq_server_relations.py

@@ -276,6 +276,18 @@ def cluster_changed():
         if rabbit.cluster_with():
             # resync nrpe user after clustering
             update_nrpe_checks()
+
+    min_size = config('min-cluster-size')
+    if min_size:
+        size = 0
+        for rid in relation_ids('cluster'):
+            size = len(related_units(rid))
+
+        if min_size != size:
+            log("Not enough units to form cluster (required=%s, got=%s)" %
+                (min_size, size), level=INFO)
+            return
+
     # If cluster has changed peer db may have changed so run amqp_changed
     # to sync any changes
     for rid in relation_ids('amqp'):