select isolated node inside of queue based on capacity

awx/main/models/unified_jobs.py

@@ -961,9 +961,7 @@ class UnifiedJob(PolymorphicModel, PasswordFieldsModel, CommonModelNameNotUnique
         args = [self.pk]
         if ig.controller_id:
             if self.supports_isolation():  # case of jobs and ad hoc commands
-                # TODO: dock capacity from controller instance, use capacity to select isolated node
-                import random
-                isolated_instance = random.choice(ig.instances.all())
+                isolated_instance = ig.instances.order_by('-capacity').first()
                 args.append(isolated_instance.hostname)
             else:  # proj & inv updates, system jobs run on controller
                 queue = ig.controller.name

awx/main/scheduler/__init__.py

@@ -406,6 +406,7 @@ class TaskManager():
         for rampart_group in self.graph:
             self.graph[rampart_group]['capacity_used'] = 0
         for t in tasks:
+            # TODO: dock capacity for isolated job management tasks running in queue
             for group_actual in InstanceGroup.objects.filter(instances__hostname=t.execution_node).values_list('name'):
                 if group_actual[0] in self.graph:
                     self.graph[group_actual[0]]['capacity_used'] += t.task_impact

awx/main/tests/functional/models/test_unified_job.py

@@ -1,10 +1,12 @@
 import pytest
+import mock
 
 # Django
 from django.contrib.contenttypes.models import ContentType
 
 # AWX
 from awx.main.models import UnifiedJobTemplate, Job, JobTemplate, WorkflowJobTemplate, Project
+from awx.main.models.ha import InstanceGroup
 
 
 @pytest.mark.django_db
@@ -65,3 +67,36 @@ class TestCreateUnifiedJob:
         assert second_job.inventory == job_with_links.inventory
         assert second_job.limit == 'my_server'
         assert net_credential in second_job.extra_credentials.all()
+
+
+@pytest.mark.django_db
+class TestIsolatedRuns:
+
+    def test_low_capacity_isolated_instance_selected(self):
+        ig = InstanceGroup.objects.create(name='tower')
+        iso_ig = InstanceGroup.objects.create(name='thepentagon', controller=ig)
+        iso_ig.instances.create(hostname='iso1', capacity=50)
+        i2 = iso_ig.instances.create(hostname='iso2', capacity=200)
+        job = Job.objects.create(
+            instance_group=iso_ig
+        )
+
+        mock_async = mock.MagicMock()
+        success_callback = mock.MagicMock()
+        error_callback = mock.MagicMock()
+
+        class MockTaskClass:
+            apply_async = mock_async
+
+        with mock.patch.object(job, '_get_task_class') as task_class:
+            task_class.return_value = MockTaskClass
+            job.start_celery_task([], error_callback, success_callback, 'thepentagon')
+        mock_async.assert_called_with([job.id, 'iso2'], [], link_error=error_callback, link=success_callback, queue='thepentagon')
+
+        i2.capacity = 20
+        i2.save()
+
+        with mock.patch.object(job, '_get_task_class') as task_class:
+            task_class.return_value = MockTaskClass
+            job.start_celery_task([], error_callback, success_callback, 'thepentagon')
+        mock_async.assert_called_with([job.id, 'iso1'], [], link_error=error_callback, link=success_callback, queue='thepentagon')