Rough implementation of mixed filter-weigher for feedback

nova/scheduler/external.py

@@ -0,0 +1,43 @@
+# Copyright (c) 2025 SAP SE
+# All Rights Reserved.
+#
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+"""
+
+"""
+import requests
+from oslo_log import log as logging
+
+LOG = logging.getLogger(__name__)
+
+
+def call_external_scheduler_api(host_objs, spec_obj, url):
+    """Reorder and filter hosts using an external scheduler service."""
+    if not url or not host_objs:
+        return host_objs
+    # TODO: Check utils.request_is_rebuild(spec) here?
+    json_data = {
+        "hosts": [ o.obj_to_primitive() for o in host_objs ],
+        "request_spec": spec_obj.obj_to_primitive(),
+    }
+    try:
+        response = requests.post(url, json=json_data, timeout=10)
+        response.raise_for_status()
+    except requests.RequestException as e:
+        LOG.error("Failed to call external scheduler API: %s", e)
+        return host_objs
+    response_json = response.json()
+    # We expect the service to return an ordered list
+    # of host names. This list can also be empty.
+    host_names = response_json.get("hosts", [])
+    return [ o for o in host_objs if o.host in host_names ]

nova/scheduler/manager.py

@@ -38,6 +38,7 @@
 from nova import quota
 from nova import rpc
 from nova.scheduler.client import report
+from nova.scheduler.external import call_external_scheduler_api
 from nova.scheduler import host_manager
 from nova.scheduler import request_filter
 from nova.scheduler import utils
@@ -633,6 +634,12 @@ def _get_sorted_hosts(self, spec_obj, host_states, index):
         # Strip off the WeighedHost wrapper class...
         weighed_hosts = [h.obj for h in weighed_hosts]
 
+        # Call an external service that can modify `weighed_hosts` once more.
+        # This service may filter out some hosts, or it may re-order them.
+        if url := CONF.filter_scheduler.external_scheduler_api_url:
+            weighed_hosts = call_external_scheduler_api(
+                weighed_hosts, spec_obj, url)
+
         # We randomize the first element in the returned list to alleviate
         # congestion where the same host is consistently selected among
         # numerous potential hosts for similar request specs.