Combine batches of successive roles for same nodes,

We can speed-up the application of (n+1)th role if both(n,n+1)
roles are being applied on the same node. This speedup of deployment of
ceilometer by atleast 1m20s (measured: 90sec) and swift by ~20s.

eg. In our 2node deployment ceilometer{server,central} are always
applied on the same node, given that they have different priorities, they are
to be applied, one after the other.

This does not violate any order constraints as the application procedure
of (n+1)th role is transparent to the nth role

Author: Sumit Jamgade

crowbar_framework/app/models/service_object.rb

@@ -919,6 +919,35 @@ def self.proposal_to_role(proposal, bc_name)
     RoleObject.new role
   end
 
+  #  we can speed-up the application of (n+1)th role if both(n,n+1)
+  #  roles are being applied on the same node. 
+  #  
+  #  eg. In our 2node deployment ceilometer{server,central} are always
+  #  applied on the same node, given that they have different priorities,
+  #  they are to be applied, one after the other.
+  #
+  #  In other words: it's actually reducing the number of times chef-client
+  #  is run rather than speeding up execution of any single run, by
+  #  merging batches together
+  #
+  #  a batch is [roles, nodes]
+  def mergebatches(batches)
+    merged_batches = []
+    unless bathces.empty?
+      current_batch = batches[0]
+      batches[1..-1].each do |next_batch|
+        if next_batch[1] == current_batch[1] && !current_batch[0].nil?
+          current_batch[0] << next_batch[0]
+          next
+        end
+        merged_batches << current_batch
+        current_batch = next_batch
+      end
+      merged_batches << current_batch
+    end
+    return merged_batches
+  end
+
   #
   # After validation, this is where the role is applied to the system The old
   # instance (if one exists) is compared with the new instance.  roles are
@@ -1171,6 +1200,8 @@ def apply_role(role, inst, in_queue, bootstrap = false)
 
       batches << [roles, nodes_in_batch] unless nodes_in_batch.empty?
     end
+
+    batches = mergebatches(batches)
     Rails.logger.debug "batches: #{batches.inspect}"
 
     # Cache attributes that are useful later on