378453e93d
Related-Bug: #2087953 Change-Id: I7282fca9f739c2ae1f3c3f9c57977f3f129118d3 Signed-off-by: Rodolfo Alonso Hernandez <ralonsoh@redhat.com>
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190 lines
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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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==============================
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Eventlet Deprecation Reference
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==============================
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This document contains the information related to the ``eventlet`` library
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deprecation. Each section describes how each module has been migrated, the
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caveats, the pending technical debt and the missing parts.
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OVN Agent
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---------
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Launch process
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~~~~~~~~~~~~~~
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The execution of the OVN agent has been replaced. Instead of using
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``oslo_services.launch``, that is still using eventlet, the agent creates
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a ``threading.Event`` instance and holds the main thread execution by waiting
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for this event.
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.. note::
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Once the ``oslo_services`` library removes the usage of
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eventlet, the previous implementation will be restored. The
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``oslo_services.service.ProcessLauncher`` service launcher implements a
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signal handler.
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Metadata proxy
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~~~~~~~~~~~~~~
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The ``UnixDomainWSGIServer`` class has been replaced with a new implementation.
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This implementation does not rely on ``neutron.api.wsgi.Server`` nor
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``eventlet.wsgi.server``. It inherits from the built-in library class
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``socketserver.StreamRequestHandler``.
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.. note::
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This implementation doesn't use ``oslo_services`` to spawn the
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processes or the local threads depending on the ``metadata_workers``
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configuration variable. Right now only the embedded form (local thread)
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is implemented (``metadata_workers=0``, the default value). Future
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implementations will enable again this configuration variable.
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OVN metadata agent
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------------------
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Metadata proxy
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~~~~~~~~~~~~~~
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The OVN metadata agent uses the same implementation as the OVN agent. The same
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limitations apply.
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Metadata agent
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--------------
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The Metadata agent uses the same implementation as the OVN agent and the same
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limitations apply. The ``MetadataProxyHandler`` class is now instantiated every
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time a new request is done; after the call, the instance is destroyed. The
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cache used to store the previous RPC calls results is no longer relevant and
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has been removed. In order to implement an RPC cache, it should be implemented
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outside the mentioned class.
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L3 agent
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--------
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The L3 agent now uses the ``oslo_service.backend.BackendType.THREADING``
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backend, that doesn't import eventlet. The HA flavor replaces the
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``UnixDomainWSGIServer`` with the ``UnixDomainWSGIThreadServer``. This new
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Unix socket WSGI server is based on ``socketserver.ThreadingUnixStreamServer``
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and doesn't use ``eventlet``.
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Several functional and fullstack tests have been skipped until ``eventlet``
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has been completely removed from the repository and the test frameworks. The
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WSGI server cannot be spawned in an ``eventlet`` patched environment. The
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thread that waits for new messages is a blocking function. In a kernel threads
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environment, where the threads are preemptive, it is not needed to manually
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yield the Python GIL; on the contrary, in an ``eventlet`` environment, the
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threads must yield the executor to the next one.
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Neutron API
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-----------
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The Neutron API currently can be executed only with the uWSGI module; the
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eventlet executor has been deprecated, although the code has not been removed
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from the repository yet. It is now mandatory to define the configuration
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variable ``start-time`` in the uWSGI configuration file, using the magic
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variable [1]_ "%t" that provides the *unix time (in seconds, gathered at
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instance startup)*.
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.. code::
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[uwsgi]
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start-time = %t
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The Neutron API consists of the following executables:
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* The API server: is a multiprocess worker; each process is created by the
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``uWSGI`` server.
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* The periodic worker: a mult process worker that spawns several threads to
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execute the periodic workers.
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* The RPC worker: a multiprocess process worker that attends the requests from
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the RPC clients, for example the Neutron agents.
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* The ML2/OVN maintenance worker: single process worker, needed by the ML2/OVN
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mechanism driver.
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.. note::
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Right now, the API server, the OVN maintenance task and the periodic workers
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taks are running without eventlet.
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ML2/OVN
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~~~~~~~
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The mechanism driver ML2/OVN requires a synchronization method between all
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nodes (controllers) and workers. The OVN database events will be received by
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all workers in all nodes; however, only one worker should process this event.
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The ``HashRingManager``, locally instantiated in each worker, is in charge of
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hashing the event received and decide what worker will process the event.
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The ``HashRingManager`` uses the information stored in the Neutron database to
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determine how many workers are alive at this time. Each worker will register
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itself in the Neutron database, creating a register in the table
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``ovn_hash_ring``. The UUID of each register is created using a deterministic
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method that depends on (1) the hash ring group (always "mechanism_driver" for
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the API workers), (2) the host name and (3) the worker ID. If the worker is
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restarted, this method will provide the same register UUID and the previous
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register (if present in the database) will be overwritten.
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OVN maintenance task
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~~~~~~~~~~~~~~~~~~~~
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The ``OvnDbSynchronizer`` class now uses a ``threading.Thread`` to spawn the
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``do_sync`` function. This is used by the Northbound and Southbound
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synchronizer classes (``OvnNbSynchronizer``, ``OvnSbSynchronizer``).
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.. note::
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The ``stop`` method needs to be refactored, along with the function
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``do_sync`` implemented in each child class. The ``stop`` method needs to
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support a fast exit mechanism to stop as fast as possible the running
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synchronization.
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Removals and deprecations
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-------------------------
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The ``api.wsgi.Server`` class, based on ``eventlet``, used in the Neutron API
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and the Metadata server to handle multiple WSGI sockets, is removed from the
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repository.
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References
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----------
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.. [1] https://uwsgi-docs.readthedocs.io/en/latest/Configuration.html#magic-variables
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