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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+====================
+Cluster Fast Scaling
+====================
+
+The URL of launchpad blueprint:
+
+https://blueprints.launchpad.net/senlin/+spec/add-attribute-fast-scaling-to-cluster
+
+The major function of senlin is managing clusters, change the capacity of
+cluster use scale out and scale in operation. Generally a single scaling
+operation will cost tens of seconds, even a few minutes in extreme cases.
+It's a long time for actual production environment, so we need to improve
+senlin for fast scaling.
+
+Rather than improve the performance of hardware or optimize code, a better way
+is to create some standby nodes while create a new cluster. When cluster need
+to change the capacity immediately or replace some nodes in 'error' state to
+'active' state nodes, add nodes form standby nodes to cluster, or remove error
+nodes from cluster and add active nodes from standby nodes to cluster.
+
+To make cluster scaling fast, the spec proposes to extend senlin for create
+standby nodes and improve scaling operation.
+
+
+Problem description
+===================
+
+Before real scaling a cluster, senlin need to do many things, the slowest
+process is to create or delete a node.
+
+Use Cases
+---------
+
+If senlin support fast scaling, the follow cases will be possible:
+
+- Change the capacity of cluster immediately, no longer waiting for creating
+or deleting nodes.
+
+- Replace the error nodes from cluster immediately, improve high availability
+for cluster.
+
+- Improve the situation that scaling many times in a short time.
+
+Proposed change
+===============
+
+1. Add a new attribute 'fast_scaling' in metadata to cluster, with the
+attribute set, senlin will create standby nodes when create a new cluster.
+The number of standby nodes could be specify，but sum of standby nodes and
+nodes in cluster should less than max size of the cluster.
+
+2. Revise cluster create and cluster delete operation for support new attr,
+delete standby nodes when delete a cluster.
+
+3. Revise scale out and scale in operation, with the new attribute set, add
+nodes form standby nodes to cluster or remove nodes from cluster to standby
+nodes first.
+
+4. Revise health policy, check the state of standby nodes and support replace
+error nodes to active nodes from standby nodes.
+
+5. Revise deletion policy, delete nodes or remove nodes to standby nodes when
+perform deletion operation.
+
+Alternatives
+------------
+
+Any other ideas of fast scale a cluster.
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+The standby nodes will claimed some resources. We should control the number
+of standby nodes in a reasonable range.
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+chohoor(Hongbin Li) <chohoor@gmail.com>
+
+Work Items
+----------
+
+Depends on the design plan.
+
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+Need unit tests.
+
+
+Documentation Impact
+====================
+
+Documentation about api and operation should be update.
+
+
+References
+==========
+
+None
+
+
+History
+=======
+
+None