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277 lines
11 KiB
ReStructuredText
..
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Licensed under the Apache License, Version 2.0 (the "License"); you may
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not use this file except in compliance with the License. You may obtain
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a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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License for the specific language governing permissions and limitations
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under the License.
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Convention for heading levels in Neutron devref:
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======= Heading 0 (reserved for the title in a document)
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------- Heading 1
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~~~~~~~ Heading 2
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+++++++ Heading 3
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''''''' Heading 4
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(Avoid deeper levels because they do not render well.)
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.. note::
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Much of this document discusses upgrade considerations for the Neutron
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reference implementation using Neutron's agents. It's expected that each
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Neutron plugin provides its own documentation that discusses upgrade
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considerations specific to that choice of backend. For example, OVN does
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not use Neutron agents, but does have a local controller that runs on each
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compute node. OVN supports rolling upgrades, but information about how that
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works should be covered in the documentation for networking-ovn, the OVN
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Neutron plugin.
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Upgrade strategy
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================
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There are two general upgrade scenarios supported by Neutron:
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#. All services are shut down, code upgraded, then all services are started again.
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#. Services are upgraded gradually, based on operator service windows.
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The latter is the preferred way to upgrade an OpenStack cloud, since it allows
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for more granularity and less service downtime. This scenario is usually called
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'rolling upgrade'.
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Rolling upgrade
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---------------
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Rolling upgrades imply that during some interval of time there will be services
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of different code versions running and interacting in the same cloud. It puts
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multiple constraints onto the software.
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#. older services should be able to talk with newer services.
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#. older services should not require the database to have older schema
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(otherwise newer services that require the newer schema would not work).
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`More info on rolling upgrades in OpenStack
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<http://governance.openstack.org/reference/tags/assert_supports-rolling-upgrade.html>`_.
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Those requirements are achieved in Neutron by:
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#. If the Neutron backend makes use of Neutron agents, the Neutron server have
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backwards compatibility code to deal with older messaging payloads.
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#. isolating a single service that accesses database (neutron-server).
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To simplify the matter, it's always assumed that the order of service upgrades
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is as following:
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#. first, all neutron-servers are upgraded.
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#. then, if applicable, neutron agents are upgraded.
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This approach allows us to avoid backwards compatibility code on agent side and
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is in line with other OpenStack projects that support rolling upgrades
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(specifically, nova).
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Server upgrade
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~~~~~~~~~~~~~~
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Neutron-server is the very first component that should be upgraded to the new
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code. It's also the only component that relies on new database schema to be
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present, other components communicate with the cloud through AMQP and hence do
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not depend on particular database state.
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Database upgrades are implemented with alembic migration chains.
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Database upgrade is split into two parts:
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#. ``neutron-db-manage upgrade --expand``
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#. ``neutron-db-manage upgrade --contract``
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Each part represents a separate alembic branch.
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The former step can be executed while old neutron-server code is running. The
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latter step requires *all* neutron-server instances to be shut down. Once it's
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complete, neutron-servers can be started again.
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.. note::
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Full shutdown of neutron-server instances can be skipped depending on
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whether there are pending contract scripts not applied to the database::
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$ neutron-db-manage has_offline_migrations
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Command will return a message if there are pending contract scripts.
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:ref:`More info on alembic scripts <alembic_migrations>`.
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Agents upgrade
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~~~~~~~~~~~~~~
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.. note::
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This section does not apply when the cloud does not use AMQP agents to
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provide networking services to instances. In that case, other backend
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specific upgrade instructions may also apply.
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Once neutron-server services are restarted with the new database schema and the
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new code, it's time to upgrade Neutron agents.
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Note that in the meantime, neutron-server should be able to serve AMQP messages
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sent by older versions of agents which are part of the cloud.
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The recommended order of agent upgrade (per node) is:
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#. first, L2 agents (openvswitch, linuxbridge, sr-iov).
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#. then, all other agents (L3, DHCP, Metadata, ...).
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The rationale of the agent upgrade order is that L2 agent is usually
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responsible for wiring ports for other agents to use, so it's better to allow
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it to do its job first and then proceed with other agents that will use the
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already configured ports for their needs.
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Each network/compute node can have its own upgrade schedule that is independent
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of other nodes.
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AMQP considerations
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+++++++++++++++++++
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Since it's always assumed that neutron-server component is upgraded before
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agents, only the former should handle both old and new RPC versions.
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The implication of that is that no code that handles UnsupportedVersion
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oslo.messaging exceptions belongs to agent code.
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Notifications
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'''''''''''''
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For notifications that are issued by neutron-server to listening agents,
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special consideration is needed to support rolling upgrades. In this case, a
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newer controller sends newer payload to older agents.
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Until we have proper RPC version pinning feature to enforce older payload
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format during upgrade (as it's implemented in other projects like nova), we
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leave our agents resistant against unknown arguments sent as part of server
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notifications. This is achieved by consistently capturing those unknown
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arguments with keyword arguments and ignoring them on agent side; and by not
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enforcing newer RPC entry point versions on server side.
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This approach is not ideal, because it makes RPC API less strict. That's why
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other approaches should be considered for notifications in the future.
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:ref:`More information about RPC versioning <rpc_versioning>`.
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Interface signature
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'''''''''''''''''''
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An RPC interface is defined by its name, version, and (named) arguments that
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it accepts. There are no strict guarantees that arguments will have expected
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types or meaning, as long as they are serializable.
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Message content versioning
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''''''''''''''''''''''''''
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To provide better compatibility guarantees for rolling upgrades, RPC interfaces
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could also define specific format for arguments they accept. In OpenStack
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world, it's usually implemented using oslo.versionedobjects library, and
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relying on the library to define serialized form for arguments that are passed
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through AMQP wire.
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Note that Neutron has *not* adopted oslo.versionedobjects library for its RPC
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interfaces yet (except for QoS feature).
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:ref:`More information about RPC callbacks used for QoS <rpc_callbacks>`.
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Networking backends
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~~~~~~~~~~~~~~~~~~~
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Backend software upgrade should not result in any data plane disruptions.
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Meaning, e.g. Open vSwitch L2 agent should not reset flows or rewire ports;
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Neutron L3 agent should not delete namespaces left by older version of the
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agent; Neutron DHCP agent should not require immediate DHCP lease renewal; etc.
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The same considerations apply to setups that do not rely on agents. Meaning,
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f.e. OpenDaylight or OVN controller should not break data plane connectivity
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during its upgrade process.
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Upgrade testing
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---------------
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`Grenade <https://github.com/openstack-dev/grenade>`_ is the OpenStack project
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that is designed to validate upgrade scenarios.
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Currently, only offline (non-rolling) upgrade scenario is validated in Neutron
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gate. The upgrade scenario follows the following steps:
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#. the 'old' cloud is set up using latest stable release code
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#. all services are stopped
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#. code is updated to the patch under review
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#. new database migration scripts are applied, if needed
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#. all services are started
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#. the 'new' cloud is validated with a subset of tempest tests
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The scenario validates that no configuration option names are changed in one
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cycle. More generally, it validates that the 'new' cloud is capable of running
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using the 'old' configuration files. It also validates that database migration
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scripts can be executed.
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The scenario does *not* validate AMQP versioning compatibility.
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Other projects (for example Nova) have so called 'partial' grenade jobs where
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some services are left running using the old version of code. Such a job would
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be needed in Neutron gate to validate rolling upgrades for the project. Till
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that time, it's all up to reviewers to catch compatibility issues in patches on
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review.
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Another hole in testing belongs to split migration script branches. It's
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assumed that an 'old' cloud can successfully run after 'expand' migration
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scripts from the 'new' cloud are applied to its database; but it's not
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validated in gate.
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.. _upgrade_review_guidelines:
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Review guidelines
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-----------------
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There are several upgrade related gotchas that should be tracked by reviewers.
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First things first, a general advice to reviewers: make sure new code does not
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violate requirements set by `global OpenStack deprecation policy
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<http://governance.openstack.org/reference/tags/assert_follows-standard-deprecation.html>`_.
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Now to specifics:
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#. Configuration options:
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* options should not be dropped from the tree without waiting for
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deprecation period (currently it's one development cycle long) and a
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deprecation message issued if the deprecated option is used.
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* option values should not change their meaning between releases.
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#. Data plane:
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* agent restart should not result in data plane disruption (no Open vSwitch
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ports reset; no network namespaces deleted; no device names changed).
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#. RPC versioning:
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* no RPC version major number should be bumped before all agents had a
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chance to upgrade (meaning, at least one release cycle is needed before
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compatibility code to handle old clients is stripped from the tree).
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* no compatibility code should be added to agent side of AMQP interfaces.
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* server code should be able to handle all previous versions of agents,
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unless the major version of an interface is bumped.
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* no RPC interface arguments should change their meaning, or names.
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* new arguments added to RPC interfaces should not be mandatory. It means
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that server should be able to handle old requests, without the new
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argument specified. Also, if the argument is not passed, the old behaviour
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before the addition of the argument should be retained.
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* minimal client version must not be bumped for server initiated
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notification changes for at least one cycle.
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#. Database migrations:
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* migration code should be split into two branches (contract, expand) as
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needed. No code that is unsafe to execute while neutron-server is running
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should be added to expand branch.
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* if possible, contract migrations should be minimized or avoided to reduce
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the time when API endpoints must be down during database upgrade.
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