SAP: Introduce an external scheduler filter

This allows us to rely on an external http service to do the filtering,
on a potentially more complex rule-set.
Instead of filtering each host one-by-one, we want to filter them in
a single requests and therefor override the `BaseFilter.filter_all`

nova/conf/scheduler.py

@@ -947,6 +947,13 @@
 Related options:
 
 * ``[DEFAULT] bigvm_mb``
+"""),
+
+    # External scheduler filter.
+    cfg.StrOpt("external_scheduler_api_url",
+        default="",
+        help="""
+The API URL of the external scheduler. If not provided, the filter is skipped.
 """)
 ]
 

nova/scheduler/filters/external_scheduler_filter.py

@@ -0,0 +1,78 @@
+# Copyright 2024 SAP SE or an SAP affiliate company.
+#
+#    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
+
+import nova.conf
+from nova.scheduler import filters
+from oslo_log import log as logging
+
+LOG = logging.getLogger(__name__)
+CONF = nova.conf.CONF
+
+
+class ExternalSchedulerFilter(filters.BaseHostFilter):
+    """Filter using an out of tree scheduler via a REST API."""
+
+    RUN_ON_REBUILD = False
+
+    def __init__(self):
+        super().__init__()
+        self.api_url = CONF.filter_scheduler.external_scheduler_api_url
+
+    def filter_all(self, filter_obj_list, spec_obj):
+        """Yield objects that pass the filter."""
+        if not self.api_url:
+            LOG.debug(
+                "external_scheduler_api_url not configured. skipping filter"
+            )
+            return filter_obj_list
+
+        hosts_data = []
+        for obj in filter_obj_list:
+            hosts_data.append({
+                "name": obj.host,
+                "status": obj.status,
+                "vcpus_total": obj.vcpus_total,
+                "vcpus_used": obj.vcpus_used,
+                "memory_mb": obj.memory_mb,
+                "memory_mb_used": obj.memory_mb_used
+            })
+
+        spec_data = {
+            "vcpus": spec_obj.vcpus,
+            "memory_mb": spec_obj.memory_mb,
+        }
+
+        json_data = {
+            "hosts": hosts_data,
+            "request_spec": spec_data
+        }
+
+        try:
+            response = requests.post(
+                self.api_url, json=json_data, timeout=10
+            )
+            response.raise_for_status()
+            response_json = response.json()
+            valid_hosts = response_json.get('valid_hosts', [])
+            for obj in filter_obj_list:
+                if obj.host in valid_hosts:
+                    yield obj
+
+        except requests.RequestException as e:
+            # Log an error if the request fails
+            LOG.error("Failed to query the external API: %s", e)
+            return filter_obj_list

nova/tests/unit/scheduler/filters/test_external_scheduler_filter.py

@@ -0,0 +1,73 @@
+# Copyright 2024 SAP SE or an SAP affiliate company.
+#
+#    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.
+
+# Tests for external scheduler filter.
+
+from unittest.mock import MagicMock
+from unittest.mock import patch
+from unittest.mock import sentinel
+
+import nova.conf
+from nova import objects
+from nova.scheduler.filters import external_scheduler_filter
+from nova import test
+from nova.tests.unit.scheduler import fakes
+
+
+CONF = nova.conf.CONF
+
+
+class ExternalSchedulerFilterTestCase(test.NoDBTestCase):
+
+    def setUp(self):
+        super(test.NoDBTestCase, self).setUp()
+        CONF.set_override('external_scheduler_api_url',
+                          'http://127.0.0.1:1234', 'filter_scheduler')
+
+    @patch('requests.post')
+    def test_filter_all(self, mock_post):
+        mock_response = MagicMock()
+        mock_response.status_code = 200
+        mock_response.json.return_value = {
+            'valid_hosts': ['host1', 'host3']
+        }
+        mock_post.return_value = mock_response
+
+        host_attributes = {
+            "status": "up",
+            "vcpus_total": 100,
+            "vcpus_used": 10,
+            "memory_mb": 1000,
+            "memory_mb_used": 10,
+        }
+        all_hosts = [
+            fakes.FakeHostState('host1', 'node1', host_attributes),
+            fakes.FakeHostState('host2', 'node2', host_attributes),
+            fakes.FakeHostState('host3', 'node3', host_attributes)
+        ]
+        instance_request_spec = objects.RequestSpec(
+            context=sentinel.ctx,
+            flavor=objects.Flavor(
+                name="small",
+                vcpus=4,
+                memory_mb=1024
+            ),
+        )
+
+        f = external_scheduler_filter.ExternalSchedulerFilter()
+        valid_hosts = f.filter_all(all_hosts, instance_request_spec)
+        self.assertEqual(
+            ['host1', 'host3'],
+            [h.host for h in valid_hosts]
+        )