Round-robin container attribution on service network_mode

compose/service.py

@@ -621,11 +621,12 @@ def _get_container_create_options(
 
         container_options['host_config'] = self._get_container_host_config(
             override_options,
-            one_off=one_off)
+            one_off=one_off,
+            number=number)
 
         return container_options
 
-    def _get_container_host_config(self, override_options, one_off=False):
+    def _get_container_host_config(self, override_options, one_off=False, number=1):
         options = dict(self.options, **override_options)
 
         logging_dict = options.get('logging', None)
@@ -637,7 +638,7 @@ def _get_container_host_config(self, override_options, one_off=False):
             binds=options.get('binds'),
             volumes_from=self._get_volumes_from(),
             privileged=options.get('privileged', False),
-            network_mode=self.network_mode.mode,
+            network_mode=self.network_mode.for_number(number),
             devices=options.get('devices'),
             dns=options.get('dns'),
             dns_search=options.get('dns_search'),
@@ -790,6 +791,9 @@ def id(self):
 
     mode = id
 
+    def for_number(self, number):
+        return self.mode
+
 
 class ContainerNetworkMode(object):
     """A network mode that uses a container's network stack."""
@@ -807,6 +811,9 @@ def id(self):
     def mode(self):
         return 'container:' + self.container.id
 
+    def for_number(self, number):
+        return self.mode
+
 
 class ServiceNetworkMode(object):
     """A network mode that uses a service's network stack."""
@@ -822,9 +829,13 @@ def id(self):
 
     @property
     def mode(self):
+        return self.for_number(1)
+
+    def for_number(self, number):
         containers = self.service.containers()
         if containers:
-            return 'container:' + containers[0].id
+            target_container = (number - 1) % len(containers)
+            return 'container:' + containers[target_container].id
 
         log.warn("Service %s is trying to use reuse the network stack "
                  "of another service that is not running." % (self.id))

tests/unit/service_test.py

@@ -631,6 +631,24 @@ def test_network_mode_service_no_containers(self):
         self.assertEqual(network_mode.mode, None)
         self.assertEqual(network_mode.service_name, service_name)
 
+    def test_network_mode_service_multiple_containers(self):
+        service_name = 'web'
+        mock_client = mock.create_autospec(docker.Client)
+        mock_client.containers.return_value = [
+            {'Id': 'a', 'Name': 'a', 'Image': 'abcd'},
+            {'Id': 'b', 'Name': 'b', 'Image': 'abcd'},
+            {'Id': 'c', 'Name': 'c', 'Image': 'abcd'},
+        ]
+
+        service = Service(name=service_name, client=mock_client)
+        network_mode = ServiceNetworkMode(service)
+
+        self.assertEqual(network_mode.mode, 'container:a')
+        self.assertEqual(network_mode.for_number(1), 'container:a')
+        self.assertEqual(network_mode.for_number(2), 'container:b')
+        self.assertEqual(network_mode.for_number(3), 'container:c')
+        self.assertEqual(network_mode.for_number(4), 'container:a')
+
 
 def build_mount(destination, source, mode='rw'):
     return {'Source': source, 'Destination': destination, 'Mode': mode}