Merge "Promote backend after failover (cheesecake)"

specs/newton/cheesecake-promote-backend.rst

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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
+
+=====================================================
+Promote Secondary Backend After Failover (Cheesecake)
+=====================================================
+
+https://blueprints.launchpad.net/cinder/+spec/replication-backend-promotion
+
+Problem description
+===================
+
+After failing backend A over to backend B, there is not a mechanism in
+Cinder to promote backend B to the master backend in order to then replicate
+to a backend C.
+
+Current Workflow
+----------------
+1. Setup Replication
+2. Failure Occurs
+3. Fail over
+4. Promote Secondary Backend
+  a. Freeze backend to prevent manage operations
+  b. Stop cinder volume service
+  c. Update cinder.conf to have backend A replaced with B and B with C
+  d. *Hack db to set backend to no longer be in 'failed-over' state*
+    * This is the step this spec is concerned with
+    * Example:
+      ::
+
+        update services set disabled=0,
+                            disabled_reason=NULL,
+                            replication_status='enabled',
+                            active_backend_id=NULL
+          where id=3;
+  e. Start cinder volume service
+  f. Unfreeze backend
+
+Use Cases
+=========
+There was a fire in my data center and my primary backend (A) was destroyed.
+Luckily, I was replicating that backend to backend (B). After failing over
+to backend B and repairing the data center, we installed backend C to be a
+new replication target for B.
+
+Proposed change
+===============
+
+Add the following commands to reset the active backend for a host.
+::
+
+    cinder reset-active-backend <backend-name>
+    PUT /os-services/reset_active_backend {"host": <backend-name>}
+
+Equivalent to:
+::
+
+    update services set disabled=0,
+                        disabled_reason=NULL,
+                        replication_status='disabled',
+                        active_backend_id=NULL
+       where host='<backend-name>;
+
+The following to re-enable replication from backend B -> C.
+::
+
+    cinder replication-enable <backend-name>
+    PUT /os-services/replication_enable {"host": <backend-name>}
+
+Equivalent to:
+::
+
+    update services set replication_status='enabled'
+       where host='<backend-name>;
+
+Alternatives
+------------
+
+Instead of 'admin' APIs, we add cinder-manage commands.
+::
+
+    cinder-manage reset-active-backend <backend-host>
+    cinder-manage replication-enable <backend-name>
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+None
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  ?
+
+Work Items
+----------
+
+* Implement cinder reset-active-backend API
+* Implement cinder replication-enable API
+* Document post-fail-over recovery in Admin guide
+
+Dependencies
+============
+None
+
+Testing
+=======
+
+None
+
+Documentation Impact
+====================
+
+Documentation in the Admin guide for how to perform a backend promotion.
+
+References
+==========
+
+None