neutron/doc/source/eventlet_deprecation/index.rst

Eventlet Deprecation Reference

This document contains the information related to the eventlet library deprecation. Each section describes how each module has been migrated, the caveats, the pending technical debt and the missing parts.

OVN Agent

Launch process

The execution of the OVN agent has been replaced. Instead of using oslo_services.launch, that is still using eventlet, the agent creates a threading.Event instance and holds the main thread execution by waiting for this event.

Note

Once the oslo_services library removes the usage of eventlet, the previous implementation will be restored. The oslo_services.service.ProcessLauncher service launcher implements a signal handler.

Metadata proxy

The UnixDomainWSGIServer class has been replaced with a new implementation. This implementation does not rely on neutron.api.wsgi.Server nor eventlet.wsgi.server. It inherits from the built-in library class socketserver.StreamRequestHandler.

Note

This implementation doesn't use oslo_services to spawn the processes or the local threads depending on the metadata_workers configuration variable. Right now only the embedded form (local thread) is implemented (metadata_workers=0, the default value). Future implementations will enable again this configuration variable.

OVN metadata agent

Metadata proxy

The OVN metadata agent uses the same implementation as the OVN agent. The same limitations apply.

Metadata agent

The Metadata agent uses the same implementation as the OVN agent and the same limitations apply. The MetadataProxyHandler class is now instantiated every time a new request is done; after the call, the instance is destroyed. The cache used to store the previous RPC calls results is no longer relevant and has been removed. In order to implement an RPC cache, it should be implemented outside the mentioned class.

L3 agent

The L3 agent now uses the oslo_service.backend.BackendType.THREADING backend, that doesn't import eventlet. The HA flavor replaces the UnixDomainWSGIServer with the UnixDomainWSGIThreadServer. This new Unix socket WSGI server is based on socketserver.ThreadingUnixStreamServer and doesn't use eventlet.

Several functional and fullstack tests have been skipped until eventlet has been completely removed from the repository and the test frameworks. The WSGI server cannot be spawned in an eventlet patched environment. The thread that waits for new messages is a blocking function. In a kernel threads environment, where the threads are preemptive, it is not needed to manually yield the Python GIL; on the contrary, in an eventlet environment, the threads must yield the executor to the next one.

Neutron API

The Neutron API currently can be executed only with the uWSGI module; the eventlet executor has been deprecated, although the code has not been removed from the repository yet. It is now mandatory to define the configuration variable start-time in the uWSGI configuration file, using the magic variable¹ "%t" that provides the unix time (in seconds, gathered at instance startup).

[uwsgi]
start-time = %t

The Neutron API consists of the following executables:

• The API server: is a multiprocess worker; each process is created by the uWSGI server.
• The periodic worker: a mult process worker that spawns several threads to execute the periodic workers.
• The RPC worker: a multiprocess process worker that attends the requests from the RPC clients, for example the Neutron agents.
• The ML2/OVN maintenance worker: single process worker, needed by the ML2/OVN mechanism driver.

Note

Right now, the API server, the OVN maintenance task and the periodic workers taks are running without eventlet.

ML2/OVN

The mechanism driver ML2/OVN requires a synchronization method between all nodes (controllers) and workers. The OVN database events will be received by all workers in all nodes; however, only one worker should process this event. The HashRingManager, locally instantiated in each worker, is in charge of hashing the event received and decide what worker will process the event.

The HashRingManager uses the information stored in the Neutron database to determine how many workers are alive at this time. Each worker will register itself in the Neutron database, creating a register in the table ovn_hash_ring. The UUID of each register is created using a deterministic method that depends on (1) the hash ring group (always "mechanism_driver" for the API workers), (2) the host name and (3) the worker ID. If the worker is restarted, this method will provide the same register UUID and the previous register (if present in the database) will be overwritten.

OVN maintenance task

The OvnDbSynchronizer class now uses a threading.Thread to spawn the do_sync function. This is used by the Northbound and Southbound synchronizer classes (OvnNbSynchronizer, OvnSbSynchronizer).

Note

The stop method needs to be refactored, along with the function do_sync implemented in each child class. The stop method needs to support a fast exit mechanism to stop as fast as possible the running synchronization.

Removals and deprecations

The api.wsgi.Server class, based on eventlet, used in the Neutron API and the Metadata server to handle multiple WSGI sockets, is removed from the repository.

References

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1. https://uwsgi-docs.readthedocs.io/en/latest/Configuration.html#magic-variables↩︎