Merge "Introduce etcd for service coordination"
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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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Incorporate ETCD as service coordination
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========================================
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https://storyboard.openstack.org/#!/story/2001842
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This spec is part of the ironic-inspector HA work. To further split the
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inspector service, this spec proposes to introduce etcd as the base service
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for the coordination between ironic-inspector API and conductor services.
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Problem description
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===================
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Based on the previous work, the single process ironic-inspector is logically
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splitted into two services both running under ``oslo.service``, namely
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``ironic_inspector`` and ``ironic-inspector-conductor``.
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Currently the functional test uses fake messaging driver which only works
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within a single process, To split two services into two processes, we need to
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address it before we can split two services into respective executables.
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we can either add rabbitmq support for functional test environment or
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introduce another messaging mechanism like ``json-rpc``. Since etcd is already
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an OpenStack base service, the community has a preference on the later one.
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Even when services are splitted, we are facing the challenge of service
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coordination, for multiple inspector conductor services, we need a way to
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prevent racing of concurrent operation on the same node, or choosing which
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inspector conductor should the request be delivered to.
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Proposed change
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===============
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etcd [#]_ is a distributed key value store that provides a reliable way to
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store data across a cluster of machines. As etcd is already a base service
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for the OpenStack platform [#]_, the spec proposes to add ``python-etcd3``
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and ``tooz`` [#]_ as project requirements for the service coordination.
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``tooz`` provides several feature encapsulations like group management,
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locking, etc. Group management is only implemented for ETCD API v3, thus
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``python-etcd3`` is required.
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All proposed work is implemented via ``tooz`` interfaces. Each service will
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create a coordinator and keep heartbeating, the example workflow for
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ironic-inspector API service is listed below:
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#. Create a coordinator with hostname
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#. Create a predefined group, bypass if the group already exists. By default,
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the group name is ``ironic-inspector-service-group``.
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#. Upon each API request, the API service will query members from the group,
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randomly pick one conductor and deliver the request to it through rpc
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mechanism.
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The example workflow for ironic-inspector conductor service is listed below:
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#. Create a coordinator with hostname
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#. Join the predefined group as a member, the group name is
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``ironic-inspector-service-group`` by default. If the group does not exist,
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it will be created.
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#. Leave group when service is shutdown.
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The spec will add two console scripts to support executing API and conductor
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services separately, namely ``ironic-inspector-api`` and
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``ironic-inspector-conductor``.
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To keep backwards compatibility, ``ironic-inspector`` continues to serve the
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single process mode, and sticks to use the fake messaging driver. It will be
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removed when gets obsolete.
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Adds a configuration option ``[DEFAULT]rpc_transport`` to specify the rpc
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backend, values can be ``fake`` (the default) or ``oslo``. This option will be
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used to determine current execution mode for three console scripts mentioned
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above. ``ironic-inspector`` only runs when the rpc transport is ``fake``, while
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``ironic-inspector-api`` and ``ironic-inspector-conductor`` only run when it's
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not ``fake``. We expect to add ``json-rpc`` in the future.
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There is no distributed locking support for ironic-inspector, this spec will
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introduce an abstract lock layer, and implement locking support based on tooz.
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After the spec is implemented, there will be two kind of locks: internal and
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etcd. The using of different locking types is decided internally and not
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exposed to end users at the moment. ironic-inspector running as a single
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process will adopt semaphore based internal locking, otherwise etcd locking
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will be used.
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Alternatives
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------------
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Though it's totally workable to utilize database as the coordination source
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just like ironic, it would be much lighter if implemented with tooz.
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tooz also supports multiple backends, which brings more possibilities in
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deployement.
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Data model impact
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-----------------
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None.
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HTTP API impact
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---------------
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None.
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Client (CLI) impact
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-------------------
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None.
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Ironic python agent impact
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--------------------------
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None.
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Ironic impact
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-------------
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None.
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Performance and scalability impact
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----------------------------------
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There should be no obvious performance and scalability impact before services
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are actually splitted.
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Security impact
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---------------
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None.
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Deployer impact
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---------------
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A new configuration section ``etcd`` with following options will be added to
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support etcd operation:
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* ``host`` and ``port``: specify the etcd service endpoint.
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* ``ca_cert``, ``cert_key`` and ``cert_cert``: specify SSL related
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authentication.
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* ``timeout``: connection timeout per request.
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* ``user`` and ``password``: the username and password if etcd authentication
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is required.
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* ``group_path``: the name of service group used to coordinate inspector
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services, it can be a key path, a key prefix or both. By default, the value
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will be ``/openstack/ironic-inspector/service-group``.
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* ``lock_prefix``: a string prefix for a lock name, for example, locking a node
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``fake-node-uuid`` with prefix ``ironic-inspector`` will have a lock name of
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``ironic-inspector.fake-node-uuid`` passed to tooz.
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The configuration option ``[DEFAULT]rpc_transport`` defaults to ``fake`` which
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has no impact on the single process ``ironic-inspector``.
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New options introduced in this spec only needs to be configured when ironic
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inspector service is running distributedly.
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Developer impact
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----------------
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None.
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Upgrades and Backwards Compatibility
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------------------------------------
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The single process ``ironic-inspector`` service is unchanged. However,
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for installations adopt distributed ironic-inspector services, the etcd v3
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will be a mandatory requirement, and necessary configuration options will
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be required.
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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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kaifeng - kaifeng.w@gmail.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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Implement proposed work.
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Dependencies
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============
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``python-etcd3`` and ``tooz`` are required library support for ironic-inspector
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running as separated services.
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There should be a etcd v3 service running in the same cloud.
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Testing
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=======
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Will be covered by unittest and bifrost.
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References
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==========
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.. [#] https://coreos.com/etcd/
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.. [#] https://governance.openstack.org/tc/reference/base-services.html#current-list-of-base-services
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.. [#] https://docs.openstack.org/tooz/latest/user/index.html
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