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3.5 KiB
ReStructuredText
157 lines
3.5 KiB
ReStructuredText
..
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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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Enhance Watcher Applier Engine
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===============================
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https://blueprints.launchpad.net/watcher/+spec/enhance-watcher-applier-engine
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`Taskflow`_ is the default workflow engine for Watcher Applier.
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In Watcher the flow pattern for linked actions is Graph flow.
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For parent action A and child action B, this means B depends on A
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and that the execution of B must wait until A finished.
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Taskflow has a callback function to decide at runtime whether B
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should be allowed to execute. The callback function returns
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a boolean True to allow B execution or False to not.
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Currently the callback function always returns True in Watcher Applier.
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Problem description
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===================
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For example, we want to live migrate a VM, if there are two potential
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target nodes that meet the requirements. However, we don't know which
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one will definitely be able to migrate successfully. The existing
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strategy will select one to create a Migrate action and the action may
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fail if there are no enough resource in the destination node.
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A solution is that we can make two linked actions for the two
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potential targets. If first action succeeded, then the second action
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will be ignored. Otherwise continue to execute the second one.
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For this purpose, We need the callback function returns True or False
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depending on the result of previous action A execution.
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Action B should be executed unless action A failed. If action A
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executes success, the execution of action B should be ignored.
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Use Cases
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----------
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As a Watcher user, I want to decide an action whether to be executed
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depending on the result of a previous action.
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Proposed change
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===============
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Now the callback function, named decider, always returns True. We propose
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to add a new config option 'action_execution_rule' which is a dict type.
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Its key field is strategy name and the value is 'ALWAYS' or 'ANY'.
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'ALWAYS' means the callback function returns True as usual.
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'ANY' means the return depends on the result of previous action execution.
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The callback returns True if previous action gets failed, and the engine
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continues to run the next action. If previous action executes success,
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the callback returns False then the next action will be ignored.
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For strategies that aren't in 'action_execution_rule', the callback always
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returns True.
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If exception is throwing out during the action execution, reverting will
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be triggered by taskflow. To continue executing the next action,
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we return False instead of throwing an exception.
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Alternatives
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------------
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None
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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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None
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Other deployer impact
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---------------------
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The default value of 'action_execution_rule' doesn't change
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the behavior of Applier.
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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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licanwei
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Work Items
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----------
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* Add a new config option 'action_execution_rule'
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* Implement the callback function
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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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Unittest for all changes
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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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None
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History
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=======
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None
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.. _Taskflow: https://docs.openstack.org/taskflow/latest/
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