Merge "Implement the oslo_db enginefacade proposal"
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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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Use the enginefacade from oslo_db
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==================================
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https://blueprints.launchpad.net/cinder/+spec/use-oslodb-enginefacade
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Implement the new oslo.db enginefacade interface described here:
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https://blueprints.launchpad.net/oslo.db/+spec/make-enginefacade-a-facade
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Problem description
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===================
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The linked oslo.db spec contains the details of the proposal, including its
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general advantages to all projects. In summary, we transparently track database
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transactions using the cinder.RequestContext object. This means that if there
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is already a transaction in progress we will use it by default, only creating a
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separate transaction if explicitly requested.
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Use Cases
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=========
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These changes will only affect developers.
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* Allow a class of database races to be fixed
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Cinder currently only exposes db transactions in cinder/db/sqlalchemy/api.py,
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which means that every db api call is in its own transaction. Although this
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will remain the same initially, the new interface allows a caller to extend a
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transaction across several db api calls if they wish. This will enable callers
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who need these to be atomic to achieve this, which includes the save operation
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on several Cinder objects.
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* Reduce connection load on the database
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Many database api calls currently create several separate database connections,
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which increases load on the database. By reducing these to a single connection,
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load on the db will be decreased.
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* Improve atomicity of API calls
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By ensuring that database api calls use a single transaction, we fix a class of
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bug where failure can leave a partial result.
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* Make greater use of slave databases for read-only transactions
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The new api marks sections of code as either readers or writers, and enforces
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this separation. This allows us to automatically use a slave database
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connection for all read-only transactions. It is currently only used when
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explicitly requested in code.
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Proposed change
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===============
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* Decorate the cinder.RequestContext class
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cinder.RequestContext is annotated with the
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@enginefacade.transaction_context_provider decorator. This adds several code
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hooks which provide access to the transaction context via the
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cinder.RequestContext object, including attributes 'session', 'connection',
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'transaction'.
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* Ensure that all database apis use context session
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Use context session in cinder.db.sqlalchemy.api.model_query().
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* Update database apis incrementally
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Individual calls will be annotated as either readers or writers. Existing
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transaction management will be replaced.
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Database apis will be updated in batches, by function. For example, Volume
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apis, Snapshot apis, Quota apis. Calls into apis which have not been upgraded
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yet will continue to explicitly pass the session or connection object.
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Alternatives
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------------
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Alternatives were examined during the design of the oslo.db code. The goal
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of this change is to implement a solution which is common across OpenStack
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projects.
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Data model impact
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-----------------
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None
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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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By reducing connection load on the database, the change is expected to provide
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a small performance improvement. However, the primary purpose is correctness.
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Other deployer impact
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---------------------
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None
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Developer impact
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----------------
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The initial phase of this work will be to implement the new engine facade in
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cinder/db/sqlalchemy/api.py only. Callers will not have to consider transaction
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context if they do not currently do so, as it will be created and destroyed
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automatically.
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This change will allow developers to explicitly extend database transaction
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context to cover several database calls. This allows the caller to make
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multiple database changes atomically.
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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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javeme
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Work Items
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----------
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* Enable use of the new api in Cinder
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* Migrate api bundles along functional lines:
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* Volume,
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* VolumeMetadata, VolumeAdminMetadata, VolumeGlanceMetadata
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* VolumeAttachment
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* VolumeTypes, VolumeTypeProjects, VolumeTypeExtraSpecs
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* Quota, QuotaClass, QuotaUsage, Reservation
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* Snapshot, SnapshotMetadata
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* Backup
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* QualityOfServiceSpecs
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* Encryption
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* Transfer
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* DriverInitiatorData
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* ImageVolumeCacheEntry
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* ConsistencyGroup, Cgsnapshot
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* Service
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Dependencies
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============
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A version of oslo.db including the new enginefacade api:
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https://review.openstack.org/#/c/138215/
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Testing
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=======
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This change is intended to have no immediate functional impact. The current
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tests should continue to pass.
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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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* Implement the oslo.db enginefacade interface described here:
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https://blueprints.launchpad.net/oslo.db/+spec/make-enginefacade-a-facade
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* Use the new oslo_db enginefacade in Nova:
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https://blueprints.launchpad.net/nova/+spec/new-oslodb-enginefacade
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