CellsV2 - Move quota tables to the API db
blueprint cells-quota-api-db Change-Id: I7253809253911d64e8637e7b1731889702bcfe3e
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Mark Doffman
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specs/newton/approved/cells-quota-api-db.rst
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specs/newton/approved/cells-quota-api-db.rst
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..
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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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CellsV2 - Move quota tables to API database
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===========================================
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https://blueprints.launchpad.net/nova/+spec/cells-quota-api-db
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As part of the CellsV2 work we are in the process of splitting the current cell
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database. Quotas in nova are global and should apply across cells. Because
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of this their data needs to reside in the API database.
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Problem description
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===================
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Quotas for projects and users need to be enforced across cell boundaries.
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Quotas are also exposed in the API. If quotas remain in the cell
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database then the API would have to me modified to expose cell information.
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Alternatively quota data would have to be replicated across all cells which
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would make it difficult to enforce a global quota.
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Use Cases
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---------
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Operators and users wish to enforce a quota that is applicable across cells.
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Proposed change
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===============
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We propose to move the quota related tables that currently reside in the
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cell database to the API database. These tables are::
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quotas
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quota_classes
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quota_usages
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project_user_quotas
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reservations
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Database models will be created in the API database. These will closely
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match the existing models in the cell database. Some modifications may be
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made to these tables to clean up the existing data model.
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The ``Quota`` object in nova makes use of functions in ``quota.py``. It is
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mostly an RPC facade to the ``quota.py`` API. The ``DbQuotaDriver`` in
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``quota.py`` will be modified to access the API database.
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Wrapper methods will be created for the existing database access. Wrapper
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methods that perform any ``get`` type operation on the ``quotas``,
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``quota_classes``, ``project_user_quotas`` tables will be modified to load
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initially from the API database. If the item is not found then it will be
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searched for in the cell database.
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Because there will often be attempt to access entries in
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``project_user_quotas`` and ``quotas`` that are empty we will need to cache
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a value that indicates the migration is complete. Without this reads of
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empty entries in these tables will be attempted twice. Instead the
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flow will be::
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use_api_database_only = False
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value = get_from_api_database()
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if use_database_only == False:
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if not value and is_cell_database_empty():
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use_api_database_only = True
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else:
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value = get_from_cell_database()
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Migration functions will be created to migrate data from the cell to API
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database. These methods will be added to the nova manage
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``online_data_migrations`` command.
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Migration of flavors has already been completed and for the above tables we
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will generally follow this example. [3]_
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The ``quota_usages`` and ``reservations`` table will not read from both
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databases. No data migration will be made for usages and reservations.
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New database access methods will be created that access these tables in the
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API datbase. The ``DbQuotaDriver`` code will be modified to use these methods.
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When usages or reservations are found to be missing in the API database a
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``sync`` call will be made on the quota to write the current usage to the
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API database.
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Alternatives
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------------
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Without changing the nature of quotas there is likely no alternative for
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the ``DbQuotaDriver`` but to move its tables to the API database. Alternatives
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for quotas in general include 'Quotas Reimagined' [1]_ and the 'Delimiter' [2]_
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project. The former specification would likely still make use of some of the
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existing tables that would require migration. A separate quotas library or
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service would create an entirely new quota data store.
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Data model impact
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-----------------
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The data model impact will be large as many new tables will be created in
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the API database. The models will not be detailed here as they are
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essentially unchanged from the cell database.
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REST API impact
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---------------
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None - Cells implementation should not be exposed in the API.
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Security impact
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---------------
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None, moving the Quota data to the API databse keeps the implementation
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as close as possible to the current model. It should be no more or less
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difficult than now to break quota enforcement.
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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 performance impact of the changes will be negligible. However, the
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performance of the ``DbQuotaDriver`` will also not be improved for the
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large-scale deployments that CellsV2 is targeting.
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During migration there will be a larger performance impact to quota operations
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and therefore build requests. This comes as database accesses for the
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``quota``, ``project_user_quota``, and ``quota_classes`` tables may require
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two database requests. Also after upgrade the ``quota_usages`` and
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``reservations`` tables will not be migrated. This will require a ``sync`` call
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until these tables are populated.
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This performance impact will be short term and will last until data has been
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migrated.
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Other deployer impact
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---------------------
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Deployers will have to be aware of the data migration when upgrading. They
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will have to know about ``nova-manage`` commands to migrate data to the API
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database.
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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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<mjdoffma@us.ibm.com>
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Other contributors:
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None
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Work Items
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----------
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* Create database models and migrations for quota tables in the API database.
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* Create databases models and migrations for quota usages and reservations.
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* Create database access and wrapper methods for API datbase.
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* Modify the ``DbQuotaDriver`` to use the new database access methods.
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* Ensure that the ``DbQuotaDriver`` re-syncs quotas when usages are not
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found in the API datbase.
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* Create migration methods to move data to the API database and add this
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method to ``online_data_migrations``.
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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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* Add unit and functional tests for new database models.
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* Add new unit tests for database access wrapper methods.
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* Add new functional tests for data migration.
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* Modify unit tests for the quota driver.
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* Enhance existing functional tests for quotas.
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Documentation Impact
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====================
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Operator documentation may need to me modified to include details of
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upgrading and migrating data using ``nova-manage`` command.
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References
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==========
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.. [1] https://review.openstack.org/#/c/182445/
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.. [2] https://review.openstack.org/#/c/284454/
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.. [3] https://blueprints.launchpad.net/nova/+spec/flavor-cell-api
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History
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=======
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.. list-table:: Revisions
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:header-rows: 1
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* - Release Name
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- Description
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* - Newton
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- Introduced
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