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specs/juno/gbp-extension-drivers.rst
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specs/juno/gbp-extension-drivers.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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GBP Extension Drivers
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==========================================
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Launchpad blueprint:
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https://blueprints.launchpad.net/group-based-policy/+spec/gbp-extension-drivers
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The existing group-policy-mapping extension makes assumptions that are
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not valid for all GBP policy drivers. Also, certain policy drivers may
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need to extend the GBP API to enable capabilities not common to all
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policy drivers. To address both these issues, this blueprint adds an
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extension driver framework to GBP, and re-factors the
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group-policy-mapping extension as an optional driver within this
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framework.
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Problem description
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===================
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Currently, both the group-policy and group-policy-mapping extensions
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are hardwired into the GBP plugin implementation. The group-policy
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extension defines the core GBP API resources, while the
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group-policy-mapping extension adds attributes to these resources
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exposing a specific mapping from the GBP resources to Neutron
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resources. Since GBP policy drivers are likely to vary in whether and
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how they map GBP resources to Neutron resources, the set of attributes
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defined in the group-policy-mapping extension are not likely to all be
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meaningful for all policy drivers. Additional mapping attributes may
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be also needed.
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Also, there is currently no way to extend GBP resource APIs with
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attributes enabling capabilities that are not common to all policy
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drivers, without building those attributes into the GBP service plugin
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itself.
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Proposed change
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===============
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First, the extension driver framework from ML2 will be ported to GBP,
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allowing GBP resources to be extended with additional attributes. An
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ExtensionDriver abstract base class will be added to the
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group_policy_driver_api module with the following methods and
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properties:
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* initialize(self) - Perform driver initialization.
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* extension_alias(self) - Supported extension alias.
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* process_create_<resource>(self, session, data, result) - Process
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extended attributes for <resource> creation.
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* process_update_<resource>(self, session, data, result) - Process
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extended attributes for <resource> update.
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* extend_<resource>_dict(self, session, result) - Add extended
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attributes to <resource> dictionary.
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See the ML2 extension driver specification and code review listed
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below in the references for more details.
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Then GBP's current mapping extension will be re-factored as a driver
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within this framework, allowing the GBP plugin to be configured with
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or without this extension, or with additional or alternative resource
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mapping extensions.
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Either as part of this blueprint or as a follow-on blueprint, the
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existing group-policy-mapping extension may be split into a
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group-policy-base-mapping extension that defines only the mapping from
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policy target to port, and a separate extension that defines the
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remainder of the mapping exposed by the resource_mapping policy
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driver.
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Alternatives
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------------
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The alternatives considered include:
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* Don't change anything in the GBP service plugin. Individual policy
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drivers can chose whether to allow each group-policy-mapping
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attribute to be set during create and update operations or to raise
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an exception when the attribute value is supplied. If they don't use
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or want to expose a specific group-policy-mapping attribute, they
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can avoid setting any value for it in the DB. No additional GBP API
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extensions are possible.
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* As above, but remove attributes from the group-policy-mapping
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extension that are not applicable to most or all policy
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drivers. Most likely, only the mapping for policy target to Neutron
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port would remain.
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Data model impact
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-----------------
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Adding the extension driver framework has no direct data model
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impact. Extension drivers implemented within the framework each have
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their own data models.
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Re-factoring the existing group-policy-mapping extension as an
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extension driver should not by itself require structural changes to
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the data model currently defined in group_policy_mapping_db.py. But
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making the mapping attributes optional may require using separate
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tables instead of extending the existing group-policy tables as is
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currently done. Since we don't yet support upgrades between GBP
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releases, existing DB migrations will be modified as necessary.
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REST API impact
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---------------
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The group-policy-mapping extension is no longer built into the GBP
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service plugin, and therefore may not always be available as part of
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its REST API. This blueprint itself does not require any specific REST
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API changes.
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Security impact
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---------------
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This blueprint has no security impact.
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Notifications impact
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--------------------
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This blueprint has no impact on notifications.
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Other end user impact
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---------------------
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The python client will need to handle the possibility that different
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extensions are available based on which extension drivers are
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configured.
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Performance Impact
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------------------
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This blueprint should have minimal impact on performance. The GBP
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service plugin will make calls on the registered extension drivers
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during all REST operations. The group-policy-mapping extension
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attributes that are now implemented within the same tables as the GBP
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resources will be moved to separate tables.
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Other deployer impact
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---------------------
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A new extension_drivers configuration variable will need to be set to
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enable any extensions required driver(s) specified in the
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policy_drivers configuration variable.
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Developer impact
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----------------
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Developers of policy drivers will now be able to define any needed
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extensions to the GBP API by defining extension drivers.
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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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rkukura
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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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* Port extension driver API and unit tests from ML2 to GBP, with
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support for extending the policy target, policy target group, l2
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policy, and l3 policy resources.
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* Re-factor the group-policy-mapping extension as an extension driver.
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* Possibly add support for extending additional resource.
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* Possibly split the group-policy-mapping extension into two separate
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extensions implemented by corresponding drivers.
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Dependencies
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============
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* This blueprint enables
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https://blueprints.launchpad.net/openstack/?searchtext=mapping-extension-refactor.
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Testing
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=======
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New unit tests will be added for the extension driver framework
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itself, and existing unit tests for the mapping will be updated to
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configure the required extension driver(s).
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Documentation Impact
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====================
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Eventual GBP documentation will need to address configuring extension
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drivers and the fact that different policy drivers may require
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different API extensions.
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References
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==========
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* ML2 extension driver blueprint:
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https://blueprints.launchpad.net/neutron/+spec/extensions-in-ml2
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* ML2 extension driver specification:
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http://git.openstack.org/cgit/openstack/neutron-specs/tree/specs/juno/neutron-ml2-mechanismdriver-extensions.rst
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* ML2 extension driver code review: https://review.openstack.org/#/c/89211/
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