Allow DB migration backports to Icehouse

Just as we did at the beginning of the Havana and Icehouse dev cycles,
we need to reserve a range of DB migrations as the first DB change in
Juno. This will allow a range to be used for migration backports to
Icehouse if needed.

This is a straight forward blueprint that has been implemented for the
last two releases (Havana and Icehouse), so we can use this as a trial
run for our new blueprint review process before we open up all other
spec reviews.

Since this is the first Juno spec, re-add the Juno specs directory to
index.rst.

Related to blueprint backportable-db-migrations-juno.

Change-Id: I791c22b54ac8cd533d6fdc03ea93f12d4a907b23

doc/source/index.rst

@@ -18,7 +18,6 @@ Juno approved specs:
    :glob:
    :maxdepth: 1
 
-.. Uncomment this once a spec merges.
    specs/juno/*
 
 ==================

specs/juno/backportable-db-migrations-juno.rst

@@ -0,0 +1,135 @@
+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+==========================================
+Allow DB migration backports to Icehouse
+==========================================
+
+https://blueprints.launchpad.net/nova/+spec/backportable-db-migrations-juno
+
+Just as we did at the beginning of the Havana and Icehouse dev cycles, we need
+to reserve a range of DB migrations as the first DB change in Juno. This will
+allow a range to be used for migration backports to Icehouse if needed.
+
+
+Problem description
+===================
+
+Normally, it is not possible to backport a change that requires a database
+migration due to the linear versioned nature of the migrations.  For the last
+two releases (Havana and Icehouse), we have reserved a set of empty migrations
+as placeholders to allow for migration backports if needed.
+
+
+Proposed change
+===============
+
+The proposed change is to reserve 10 migrations for Icehouse backports.  These
+migrations would be no-ops and would simply result in an increment of the schema
+version.
+
+Alternatives
+------------
+
+When figuring out ways to allow database migrations, alternatives usually
+involve discussion of drastic changes to the way we manage migrations.  For
+example, it could require moving to a new framework.  This proposal works for
+our current use of sqlalchemy-migrate.  This will also be the third release
+we've used this approach, so it's fairly well understood at this point.
+
+Data model impact
+-----------------
+
+There's no changes to the data model as a part of this effort.  It simply gives
+us the ability to backport data model changes to Icehouse.
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+These migrations have minimal cost and can be run against a database without
+taking down Nova services.
+
+Other deployer impact
+---------------------
+
+This set of changes requires doing database migrations.  However, they can be
+done without any Nova downtime.
+
+Developer impact
+----------------
+
+This must be the first set of migrations merged into Juno, or it doesn't work.
+
+Developers must also be very careful when writing migrations that may be
+backported.  They must be idempotent.  For example, if migration 115 is
+backported to 107 in the previous release, someone who has executed the
+backported migration must not suffer any trouble when the migration runs again
+after an upgrade.
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  russellb
+
+Work Items
+----------
+
+* Create 10 placeholder migrations.
+
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+The existing unit tests will cover this.  Both the normal devstack based CI
+systems, as well as the "turbo-hipster" DB CI system will provide functional
+test coverage of these placeholder migrations.
+
+
+Documentation Impact
+====================
+
+None.
+
+
+References
+==========
+
+* https://blueprints.launchpad.net/nova/+spec/backportable-db-migrations-icehouse
+
+* https://blueprints.launchpad.net/nova/+spec/backportable-db-migrations
+
+* http://lists.openstack.org/pipermail/openstack-dev/2013-March/006827.html