[WIP] allow customers to use python standard threads on rabbitmq driver
Propose changes to introduce 2 executions mode for the oslo.messaging rabbitmq driver: - mode where eventlet is in use during execution - mode where python standards thread is in use during execution The default mode is to use eventlet to don't break existing usages. Users can switch mode at start by using some additional configuration: ``` [oslo_messaging_rabbit] eventlet_mode=off ``` Change-Id: Ia6d873c3fc5dcc6b1c181b983b9d0d2787348163
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Hervé Beraud
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Allow customer to choose to use std threads or greenthreads in rabbitmq driver
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==============================================================================
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https://blueprints.launchpad.net/oslo.messaging/+spec/rabbitmq-driver-threads
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oslo.messaging RabbitMq driver use greenthreads from eventlet. In some use case
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it can be useful to also can use python standard threads. This allow customers
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design that they need exactly in their context. This specification proposes
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changes to adapt the rabbitmq driver to provide mechanismes needed to lets
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customers choice between eventlet greenthreads or python standard threads.
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Problem description
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===================
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The rabbitmq driver is commonly used to healt check the AMQP connections with
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rabbitmq to ensure connections by using the driver heartbeat mechanism.
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The AMQP heartbeat can be used in mod_wsgi. the threading library is
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monkey_patched by eventlet, and it's not compatible with mod_wsgi.
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Indeed, mod_wsgi stops the execution of its embedded interpreter, the
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AMQP heartbeat thread can't be resumed until there's a message to be
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processed in the mod_wsgi queue, which would resume the python
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interpreter and make eventlet resume the thread.
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Allow customers to use standard threads and disabling eventlet will fix
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this kind of issues.
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Proposed change
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===============
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The spec proposes to introduce a new configuration option who lets customers
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choose the threading mode to use. By default we'll keep the eventlet mode
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with the monkey patched threading module, and user by using configuration
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will can choose to turn off the eventlet mode and to use the non monkey
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patched mode.
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The ``eventlet_mode`` config option will be introduced, with the default value
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defined to turned on (``on``).
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User can choose to set this value to ``off`` to turn off the eventlet mode
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and using python standard threads.
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This option will have a default value so doesn't need to update existing
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usages. Also the existing usages of this driver will continue to have the
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same behaviors if nobody changes the configuration explicitely so it's
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transparent for end users.
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Here is a example that describes the introduced option turned off::
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[oslo_messaging_rabbit]
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eventlet_mode = off
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Alternatives
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------------
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Remove eventlet on the rabbitmq driver and prefer to use possible alternatives
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like using futurist.
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Impact on Existing usages
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-------------------------
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No impact on the existing usages since the default behavior will continue
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to use eventlet by default (``eventlet_mode=on``).
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Security impact
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---------------
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There's a slim chance that driver behavior could change if pre-existing
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environment variables are present that happen to match the new option
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name described here.
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Performance Impact
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------------------
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No significant performance impact is expected.
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Configuration Impact
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--------------------
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A new option will be added that can impact users in some unidentified
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scenarios.
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Developer Impact
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----------------
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Developers will have an additional customization option available.
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Testing Impact
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--------------
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Additional unit tests will be required to cover the added functionality.
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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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hberaud
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Other contributors:
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volunteers?
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Milestones
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----------
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..TODO(hberaud): figure this out
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Work Items
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----------
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* Implement the described modes in rabbitmq driver.
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* Update documentation.
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* Update the sample configuration generator to include the variable names.
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* Update the documentation generator to include the variable names.
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Incubation
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==========
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N/A
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Documentation Impact
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====================
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The documentation will need to be updated to indicate the option can be
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overridden with an environment variable and to describe how the described
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modes works.
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Dependencies
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============
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N/A
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References
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==========
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* Eventlet Best Practices:
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`https://review.opendev.org/#/c/154642/`_
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* Related discussion on the ML about mod_wsgi and monkey patch:
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`http://lists.openstack.org/pipermail/openstack-discuss/2019-April/005305.html`_
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.. note::
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This work is licensed under a Creative Commons Attribution 3.0
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Unported License.
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http://creativecommons.org/licenses/by/3.0/legalcode
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