We observed that the order of switch event notification was changed occasionally.
In the result, TopologyManager received the notification of an edge addition before the notification of Node & NodeConnector which is a part of the edge.
Therefore, TopologyManager ignored the notification of the edge addition.

For example, we've found out that this issue occurred in CSIT for VTN.
https://jenkins.opendaylight.org/integration/job/integration-master-csit-karaf-vtn-all/173/artifact/karaf.log

We've observed that TopologyManager received the notification of the edge addition (s3-eth2 <-> s1-eth2) before the notification of Node & NodeConnector (s3, s3-eth2, and s1-eth2) addition so that TopologyManager ignored the edge.
In such a case, dijkstra_implementation and VTN Manager are not notified of edge notification.

2014-10-06 08:58:02,394 | WARN | DOM-OPER-DCL-8 | TopologyManagerImpl | 359 - org.opendaylight.controller.topologymanager - 0.4.2.SNAPSHOT | Ignore edge that contains invalid node connector: (OF|3@OF|00:00:00:00:00:00:00:03->OF|2@OF|00:00:00:00:00:00:00:01)

2014-10-06 08:58:02,398 | WARN | DOM-OPER-DCL-8 | TopologyManagerImpl | 359 - org.opendaylight.controller.topologymanager - 0.4.2.SNAPSHOT | Ignore edge that contains invalid node connector: (OF|2@OF|00:00:00:00:00:00:00:01->OF|3@OF|00:00:00:00:00:00:00:03)

2014-10-06 08:58:02,401 | INFO | DOM-OPER-DCL-6 | VTNManagerImpl | 450 - org.opendaylight.vtn.manager.implementation - 0.2.0.SNAPSHOT | default: addNode: New node OF|00:00:00:00:00:00:00:03

2014-10-06 08:58:02,401 | INFO | DOM-OPER-DCL-6 | VTNManagerImpl | 450 - org.opendaylight.vtn.manager.implementation - 0.2.0.SNAPSHOT | default: addPort: New port: port=OF|2@OF|00:00:00:00:00:00:00:03, prop=PortProperty[name=s3-eth2,cost=1000,enabled]

2014-10-06 08:58:02,416 | INFO | DOM-OPER-DCL-6 | VTNManagerImpl | 450 - org.opendaylight.vtn.manager.implementation - 0.2.0.SNAPSHOT | default: addPort: New port: port=OF|2@OF|00:00:00:00:00:00:00:01, prop=PortProperty[name=s1-eth2,cost=1000,enabled]

sal-compatibility regisiters DataChangeListener for Node, NodeConnector, and Edge, respectively.
Therefore, Node, NodeConnector and Edge events are notified in different threads.
In the result, it is possible that the order of switch event notification is changed.

We guess this is one of causes of the failure of system tests for VTN Manager.
https://bugs.opendaylight.org/show_bug.cgi?id=2158