Merge "Enforce unique instance uuid in data model"
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specs/juno/enforce-unique-instance-uuid-in-db.rst
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specs/juno/enforce-unique-instance-uuid-in-db.rst
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This work is licensed under a Creative Commons Attribution 3.0 Unported
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License.
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http://creativecommons.org/licenses/by/3.0/legalcode
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==========================================
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Enforce unique instance uuid in data model
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==========================================
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https://blueprints.launchpad.net/nova/+spec/enforce-unique-instance-uuid-in-db
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The instances.uuid column in the data model is not unique but by definition a
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UUID should be unique, and given how it's used within nova and across other
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openstack services like glance, neutron, ceilometer, etc, it should be unique.
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Furthermore, there are Foreign Keys created against instances.uuid so it should
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be unique.
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Problem description
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===================
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* Uniqueness for instances.uuid is not enforced in the data model.
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* There are foreign keys created on the instances.uuid column so it should be
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unique.
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Proposed change
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===============
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Add a database migration that checks for existing records where the
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instances.uuid or related instance_uuid column is NULL and if found, fails the
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migration until those are deleted.
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A tool will be provided to scan the database for these records and list them,
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then prompt the user to delete them. A --force option could also be provided
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in the tool to ignore any prompts and just delete the records if found.
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The new migration would be blocked until the records are deleted. Once there
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are no records left, the migration will make those columns non-nullable via
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SQLAlchemy and create a UniqueConstraint on the instances.uuid column.
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Note that the fixed_ips table is the exception here since it can, by design,
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contain NULL instance_uuid records to indicate deallocated and disassociated
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fixed IPs.
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Alternatives
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------------
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Do nothing and leave the nova objects layer to enforce unique instance uuid
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entries, but this does not help with the foreign key issue in the data model.
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Data model impact
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-----------------
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#. NULL instances.uuid/instance_uuid records must be deleted, except in table
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fixed_ips as described above.
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#. The instances.uuid column will be made non-nullable.
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#. A UniqueConstraint will be created on the instances.uuid column.
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REST API impact
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---------------
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None.
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Security impact
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---------------
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None.
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Notifications impact
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--------------------
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None.
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Other end user impact
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---------------------
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None.
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Performance Impact
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------------------
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The only performance impact on existing deployments is in the migration
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script changes which would be tested with turbo-hipster.
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Other deployer impact
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---------------------
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The main impacts to deployers are:
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#. The biggest impact is the new migration. Migrations are potentially slow and
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require the controller service to be down when run.
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#. The hope is that existing deployments are not carrying records where
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instances.uuid or instance_uuid are None so the NULL queries in the new
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migration script would not yield large result sets. However, the impact to
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the deployer here is that they would be forced to manually prune those
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records before the migration can continue. Note that it's expected that
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those cases are exceptional and they are only the result of an inconsistent
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database. So finding these records is not expected, but if it is a problem
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the migration will fail clearly with instructions on how to fix the problem.
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Developer impact
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----------------
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None.
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Implementation
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==============
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Assignee(s)
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-----------
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Primary assignee:
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mriedem@us.ibm.com
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Work Items
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----------
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#. Add a new migration to make instances.uuid non-nullable and put a unique
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constraint on that column.
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#. Write a tool to check for null instance_uuid records within the database
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for operators to use before the actual migration.
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See the WIP patch for details: https://review.openstack.org/#/c/97946/
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Dependencies
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============
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None.
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Testing
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=======
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* Unit tests for the new database migration will be added to stub a database
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with NULL instance_uuid records to make sure the migration fails when those
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records are found and then test that when they are removed, the migration
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completes successfully and the unique constraint is created. Similarly the
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downgrade path will be unit tested.
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* Unit tests will also be written for the scan tool used to run outside of the
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actual database migrations. This will mock out the backend database but will
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be used to test the CLI and logic.
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* It is expected that turbo-hipster will cover scale testing the new migration
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for MySQL.
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Documentation Impact
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====================
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None.
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References
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==========
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* Work in progress nova patch: https://review.openstack.org/#/c/97946/
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* Mailing list thread on making instances.uuid non-nullable:
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http://lists.openstack.org/pipermail/openstack-dev/2014-March/029467.html
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