Implement external scheduler call

cinder/opts.py

@@ -57,6 +57,7 @@
 from cinder.message import api as cinder_message_api
 from cinder import quota as cinder_quota
 from cinder.scheduler import driver as cinder_scheduler_driver
+from cinder.scheduler import external as cinder_scheduler_external
 from cinder.scheduler.filters import shard_filter as \
     cinder_scheduler_filters_shardfilter
 from cinder.scheduler import host_manager as cinder_scheduler_hostmanager
@@ -274,6 +275,7 @@ def list_opts():
                 cinder_message_api.messages_opts,
                 cinder_quota.quota_opts,
                 cinder_scheduler_driver.scheduler_driver_opts,
+                cinder_scheduler_external.scheduler_external_opts,
                 cinder_scheduler_hostmanager.host_manager_opts,
                 cinder_scheduler_manager.scheduler_manager_opts,
                 [cinder_scheduler_scheduleroptions.

cinder/scheduler/external.py

@@ -0,0 +1,141 @@
+# 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.
+"""
+This module provides functionality to interact with an external scheduler API
+to reorder and filter hosts based on additional criteria.
+
+The external scheduler API is expected to take a list of weighed hosts and
+their weights, along with the request specification, and return a reordered
+and filtered list of host names.
+"""
+import jsonschema
+from oslo_config import cfg
+from oslo_log import log as logging
+import requests
+
+
+scheduler_external_opts = [
+    cfg.StrOpt(
+        "external_scheduler_api_url",
+        default="",
+        help="""
+The API URL of the external scheduler.
+
+If this URL is provided, Cinder will call an external service after filters
+and weighers have been applied. This service can reorder and filter the
+list of hosts before Cinder attempts to place the volume.
+
+If not provided, this step will be skipped.
+"""),
+    cfg.IntOpt(
+        "external_scheduler_timeout",
+        default=5,
+        min=1,
+        help="""
+The timeout in seconds for the external scheduler.
+
+If external_scheduler_api_url is configured, Cinder will call and wait for the
+external scheduler to respond for this long. If the external scheduler does not
+respond within this time, the request will be aborted. In this case, the
+scheduler will continue with the original host selection and weights.
+"""),
+]
+
+CONF = cfg.CONF
+CONF.register_opts(scheduler_external_opts)
+
+LOG = logging.getLogger(__name__)
+
+# The expected response schema from the external scheduler api.
+# The response should contain a list of ordered host names.
+RESPONSE_SCHEMA = {
+    "type": "object",
+    "properties": {
+        "hosts": {
+            "type": "array",
+            "items": {
+                "type": "string"
+            }
+        }
+    },
+    "required": ["hosts"],
+    "additionalProperties": False,
+}
+
+
+def call_external_scheduler_api(context, weighed_hosts, spec_dict):
+    """Reorder and filter hosts using an external scheduler service.
+
+    :param context: The RequestContext object containing request id, user, etc.
+    :param weighed_hosts: List of w. hosts to send to the external scheduler.
+    :param spec_dict: The RequestSpec object with the share specification.
+    """
+    if not weighed_hosts:
+        return weighed_hosts
+    if not (url := CONF.external_scheduler_api_url):
+        LOG.debug("External scheduler API is not enabled.")
+        return weighed_hosts
+    timeout = CONF.external_scheduler_timeout
+    # We shouldn't pass and log the auth token. Thus, we delete it here.
+    ctx_dict = context.to_dict()
+    if "auth_token" in ctx_dict:
+        del ctx_dict["auth_token"]
+    json_data = {
+        "spec": spec_dict,
+        # Also serialize the request context, which contains the global request
+        # id and other information helpful for logging and request tracing.
+        "context": ctx_dict,
+        # Only provide basic information for the hosts for now.
+        # The external scheduler is expected to fetch statistics
+        # about the hosts separately, so we don't need to pass
+        # them here.
+        "hosts": [
+            {
+                "host": h.obj.host,
+            } for h in weighed_hosts
+        ],
+        # Also pass previous weights from the Cinder weigher pipeline.
+        # The external scheduler api is expected to take these weights
+        # into account if provided.
+        "weights": {h.obj.host: h.weight for h in weighed_hosts},
+    }
+    LOG.debug("Calling external scheduler API with %s", json_data)
+    try:
+        response = requests.post(url, json=json_data, timeout=timeout)
+        response.raise_for_status()
+        # If the JSON parsing fails, this will also raise a RequestException.
+        response_json = response.json()
+    except requests.RequestException as e:
+        LOG.error("Failed to call external scheduler API: %s", e)
+        return weighed_hosts
+
+    # The external scheduler api is expected to return a json with
+    # a sorted list of host names. Note that no weights are returned.
+    try:
+        jsonschema.validate(response_json, RESPONSE_SCHEMA)
+    except jsonschema.ValidationError as e:
+        LOG.error("External scheduler response is invalid: %s", e)
+        return weighed_hosts
+
+    # The list of host names can also be empty. In this case, we trust
+    # the external scheduler decision and return an empty list.
+    if not (host_names := response_json["hosts"]):
+        # If this case happens often, it may indicate an issue.
+        LOG.warning("External scheduler filtered out all hosts.")
+
+    # Reorder the weighed hosts based on the list of host names returned
+    # by the external scheduler api.
+    weighed_hosts_dict = {h.obj.host: h for h in weighed_hosts}
+    return [weighed_hosts_dict[h] for h in host_names]

cinder/scheduler/filter_scheduler.py

@@ -33,6 +33,7 @@
 from cinder.i18n import _
 from cinder import objects
 from cinder.scheduler import driver
+from cinder.scheduler.external import call_external_scheduler_api
 from cinder.scheduler.host_manager import BackendState
 from cinder.scheduler import scheduler_options
 from cinder.scheduler.weights import WeighedHost
@@ -405,6 +406,13 @@ def _get_weighted_candidates(
         # backend for the job.
         weighed_backends = self.host_manager.get_weighed_backends(
             backends, filter_properties)
+
+        # Call an external service that can modify `weighed_hosts` once more.
+        # This service may filter out some hosts, or it may re-order them.
+        # Note: the result can also be empty.
+        weighed_backends = call_external_scheduler_api(
+            context, weighed_backends, request_spec)
+
         return weighed_backends
 
     def _get_weighted_candidates_generic_group(