Merge "Do not log WARN messages about lack of L3 agents for DVR routers"

neutron/db/l3_dvrscheduler_db.py

@@ -49,7 +49,45 @@ class CentralizedSnatL3AgentBinding(model_base.BASEV2):
 
 
 class L3_DVRsch_db_mixin(l3agent_sch_db.L3AgentSchedulerDbMixin):
-    """Mixin class for L3 DVR scheduler."""
+    """Mixin class for L3 DVR scheduler.
+
+    DVR currently supports the following use cases:
+
+     - East/West (E/W) traffic between VMs: this is handled in a
+       distributed manner across Compute Nodes without a centralized element.
+       This includes E/W traffic between VMs on the same Compute Node.
+     - North/South traffic for Floating IPs (FIP N/S): this is supported on the
+       distributed routers on Compute Nodes without any centralized element.
+     - North/South traffic for SNAT (SNAT N/S): this is supported via a
+       centralized element that handles the SNAT traffic.
+
+    To support these use cases,  DVR routers rely on an L3 agent that runs on a
+    central node (also known as Network Node or Service Node),  as well as, L3
+    agents that run individually on each Compute Node of an OpenStack cloud.
+
+    Each L3 agent creates namespaces to route traffic according to the use
+    cases outlined above.  The mechanism adopted for creating and managing
+    these namespaces is via (Router,  Agent) binding and Scheduling in general.
+
+    The main difference between distributed routers and centralized ones is
+    that in the distributed case,  multiple bindings will exist,  one for each
+    of the agents participating in the routed topology for the specific router.
+
+    These bindings are created in the following circumstances:
+
+    - A subnet is added to a router via router-interface-add, and that subnet
+      has running VM's deployed in it.  A binding will be created between the
+      router and any L3 agent whose Compute Node is hosting the VM(s).
+    - An external gateway is set to a router via router-gateway-set.  A binding
+      will be created between the router and the L3 agent running centrally
+      on the Network Node.
+
+    Therefore,  any time a router operation occurs (create, update or delete),
+    scheduling will determine whether the router needs to be associated to an
+    L3 agent, just like a regular centralized router, with the difference that,
+    in the distributed case,  the bindings required are established based on
+    the state of the router and the Compute Nodes.
+    """
 
     def dvr_update_router_addvm(self, context, port):
         ips = port['fixed_ips']

neutron/scheduler/l3_agent_scheduler.py

@@ -166,10 +166,9 @@ class L3Scheduler(object):
                 candidates = list(new_l3agentset - old_l3agentset)
             else:
                 candidates = new_l3agents
-            if not candidates:
-                LOG.warn(_('No L3 agents can host the router %s'),
-                         sync_router['id'])
-                return
+                if not candidates:
+                    LOG.warn(_('No L3 agents can host the router %s'),
+                             sync_router['id'])
 
             return candidates