Make native threading the default concurrency mode

This change makes native threading the default concurrency mode for all
Watcher services (API, Decision Engine, and Applier), deprecating the
Eventlet mode.

Key changes:
 - Invert the OS_WATCHER_DISABLE_EVENTLET_PATCHING logic so threading
   is enabled by default; users must explicitly set the variable to
   'false' to use Eventlet mode
 - Rename CI jobs and tox environments from *-threading to *-eventlet
   since threading is now the default

Assisted-By: Claude (opus-4-5)
Change-Id: I3c2c7782542bce64f0232f9f79ed07b48fbcfa12
Signed-off-by: Douglas Viroel <viroel@gmail.com>

.zuul.yaml

@@ -330,31 +330,31 @@
           CEILOMETER_PIPELINE_INTERVAL: 15
 
 - job:
-    name: watcher-tempest-prometheus-threading
+    name: watcher-tempest-prometheus-eventlet
     parent: watcher-tempest-prometheus
     description: |
       Watcher multinode devstack tempest job with Prometheus as datasource and
-      threading mode enabled in specific services.
-    vars: &threading_vars
+      eventlet mode enabled in specific services.
+    vars: &eventlet_vars
       devstack_localrc:
-        'SYSTEMD_ENV_VARS["watcher-api"]': OS_WATCHER_DISABLE_EVENTLET_PATCHING=true
-        'SYSTEMD_ENV_VARS["watcher-decision-engine"]': OS_WATCHER_DISABLE_EVENTLET_PATCHING=true
-        'SYSTEMD_ENV_VARS["watcher-applier"]': OS_WATCHER_DISABLE_EVENTLET_PATCHING=true
+        'SYSTEMD_ENV_VARS["watcher-api"]': OS_WATCHER_DISABLE_EVENTLET_PATCHING=false
+        'SYSTEMD_ENV_VARS["watcher-decision-engine"]': OS_WATCHER_DISABLE_EVENTLET_PATCHING=false
+        'SYSTEMD_ENV_VARS["watcher-applier"]': OS_WATCHER_DISABLE_EVENTLET_PATCHING=false
       devstack_local_conf:
         post-config:
           $WATCHER_CONF:
             DEFAULT:
               print_thread_pool_stats: true
     group-vars:
-      subnode: *threading_vars
+      subnode: *eventlet_vars
 
 - job:
-    name: openstack-tox-py312-threading
-    parent: openstack-tox-py312
+    name: openstack-tox-py313-eventlet
+    parent: openstack-tox-py313
     description: |
-      Run tox with the py3-threading environment.
+      Run tox with the py3-eventlet environment.
     vars:
-      tox_envlist: py3-threading
+      tox_envlist: py3-eventlet
 
 - job:
     name: watcher-tempest-aetos-realdata
@@ -383,7 +383,7 @@
       - release-notes-jobs-python3
     check:
       jobs:
-        - openstack-tox-py312-threading
+        - openstack-tox-py313-eventlet
         - watcher-grenade
         - watcher-grenade-skip-level-always
         - watcher-tempest-api-ipv6-only
@@ -392,11 +392,11 @@
             files: &watcherclient_functional_files
               - ^watcher/api/*
         - watcher-tempest-prometheus
-        - watcher-tempest-prometheus-threading
+        - watcher-tempest-prometheus-eventlet
         - watcher-tempest-aetos
     gate:
       jobs:
-        - openstack-tox-py312-threading
+        - openstack-tox-py313-eventlet
         - watcher-grenade
         - watcher-grenade-skip-level-always
         - watcher-tempest-api-ipv6-only
@@ -404,7 +404,7 @@
         - python-watcherclient-functional:
             files: *watcherclient_functional_files
         - watcher-tempest-prometheus
-        - watcher-tempest-prometheus-threading
+        - watcher-tempest-prometheus-eventlet
         - watcher-tempest-aetos
     experimental:
       jobs:

doc/source/contributor/concurrency.rst

@@ -55,20 +55,22 @@ types of concurrency used in various services of Watcher.
 Concurrency modes
 #################
 
-Evenlet has been the main concurrency library within the OpenStack community
+Eventlet has been the main concurrency library within the OpenStack community
 for the last 10 years since the removal of twisted. Over the last few years,
 the maintenance of eventlet has decreased and the efforts to remove the GIL
 from Python (PEP 703), have fundamentally changed how concurrency works, making
-eventlet no longer viable. While transitioning to a new native thread
-solution, Watcher services will be supporting both modes, with the usage of
-native threading mode initially classified as ``experimental``.
+eventlet no longer viable.
 
-It is possible to enable the new native threading mode by setting the following
-environment variable in the corresponding service configuration:
+Starting from the 2026.2 release, Watcher services use native threading mode
+by default. The eventlet mode is still supported but is now deprecated and
+will be removed in a future release.
+
+To re-enable the legacy eventlet mode, set the following environment variable
+in the corresponding service configuration:
 
 .. code:: bash
 
-   OS_WATCHER_DISABLE_EVENTLET_PATCHING=true
+   OS_WATCHER_DISABLE_EVENTLET_PATCHING=false
 
 Decision engine concurrency
 ***************************

releasenotes/notes/deprecate_eventlet_mode-1f8ecbc2e09ba6f1.yaml

@@ -0,0 +1,13 @@
+---
+upgrade:
+  - |
+    All Watcher services (API, Decision Engine, and Applier) now run in
+    ``native threading`` mode by default. Previously, Eventlet was the default
+    concurrency library. For more information, please check `eventlet removal
+    <https://wiki.openstack.org/wiki/Eventlet-removal>`__ documentation.
+deprecations:
+  - |
+    The Eventlet concurrency mode is now deprecated and will be removed in a
+    future release. To temporarily re-enable Eventlet mode, set the environment
+    variable ``OS_WATCHER_DISABLE_EVENTLET_PATCHING=false`` in the service
+    configuration. Users are encouraged to migrate to native threading mode.

tox.ini

@@ -37,9 +37,9 @@ passenv =
   # debugging issue with fixtures and mocks.
   PYTHONOPTIMIZE
 
-[testenv:py3-threading]
+[testenv:py3-eventlet]
 setenv =
-  OS_WATCHER_DISABLE_EVENTLET_PATCHING=true
+  OS_WATCHER_DISABLE_EVENTLET_PATCHING=false
 commands =
   rm -f .testrepository/times.dbm
   find . -type f -name "*.py[c|o]" -delete

watcher/common/oslo_service_helper.py

@@ -24,10 +24,13 @@ LOG = log.getLogger(__name__)
 def init_oslo_service_backend():
     if eventlet_helper.is_patched():
         backend.init_backend(backend.BackendType.EVENTLET)
-        LOG.warning("Service is starting with Eventlet based service backend.")
+        LOG.warning(
+            "Service is starting with Eventlet based service backend. "
+            "Running in eventlet mode is deprecated and it will "
+            "be removed in future releases."
+        )
     else:
         backend.init_backend(backend.BackendType.THREADING)
         LOG.warning(
-            "Service is starting with Threading based service backend. "
-            "This is an experimental feature, do not use it in production."
+            "Service is starting with Threading based service backend."
         )

watcher/eventlet.py

@@ -34,9 +34,13 @@ def _monkey_patch():
 
 
 def _is_patching_enabled():
-    if os.environ.get(
-        'OS_WATCHER_DISABLE_EVENTLET_PATCHING', ''
-    ).lower() not in ('1', 'true', 'yes', 'y'):
+    # NOTE(dviroel): The default is now to always disable eventlet patching
+    if os.environ.get('OS_WATCHER_DISABLE_EVENTLET_PATCHING', '').lower() in (
+        '0',
+        'false',
+        'no',
+        'n',
+    ):
         return True
     return False