Selective Topology Distribution¶

Publisher subscriber pattern¶

Selective topology distribution depends on the pub-sub function which is shown in the following diagram. When there is a topology change, for example, a new port is created by a tenant, a publisher (in this case, the Dragonflow Neutron plugin) will publish this event together with detailed info of the new port to a topic. Every topic maintains a list of subscribers. On receiving an event from publisher, the topic then sends the event to every subscriber in the list. For the port creating example, there may be many Dragonflow local controllers which have ports connecting to the same network with the new port. These local controllers care about changes of ports in the network and will subscribe change of this network by registering to the corresponding topic. These controllers will get notified by the topic when this new port is created.

                                      +--------------+
                                  +---> Subscriber A |
                                  |   +--------------+
                                  |
                     +---------+  |
                +----> Topic A +--+   +--------------+
+-----------+   |    +---------+  +--->              |
| Publisher +---+                     | Subscriber B |
+-----------+   |    +---------+  +--->              |
                +----> Topic B +--+   +--------------+
                     +---------+  |
                                  |
                                  |   +--------------+
                                  +---> Subscriber C |
                                      +--------------+

Two ways to distribute topology selectively¶

There are many types of topology change. In large scale data center, number of tenants is also considerable. There will be tons of topics if publish different events to different topics. To simplify the implementation, some degree of convergence has to be taken into account. There are two ways to converge the types of topology change.

Northbound Topology Change¶

When a tenant named tenant1 create a port through Neutron’s northbound api.

• Dragonflow Neutron plugin will publish a event to the tenant’s topic in the pub-sub server.
• The pub-sub server will then check who have subscribed the topic and publish the event to them.
• On receiving the event, local controllers will save the new port’s information and install some flow entries on OVS which is not covered in this spec.

+----------------+ +----------------+ +------------------+  +------------------+
| Neutron plugin | | Sub-pub Server | | Dragonflow local |  | Dragonflow Local |
+-------+--------+ +------+---------+ | Controller 1     |  | Controller2      |
        |                 |           +--------+---------+  +--------+---------+
        |                 |                    |                     |
        |                 |                    |                     |
        | publish(tenant1)|                    |                     |
        +----------------->                    |                     |
        |                 |   publish(tenant1) |                     |
        |                 +-------------------->                     |
        |                 |                    |                     |
        |                 |                    |                     |
        +                 +                    +                     +

In the above diagram, Dragonflow local controller 2 has no VMs belong to tenant1. It will not subscribe tenant1’s topic and therefore will not get notified.

Processing of other northbound topology changes, such as creating, deleting or modifying router, network and port is same as the above example.

Southbound Topology Change¶

When nova starts a VM in a compute node, it will insert a port on the corresponding OVS bridge.

• By monitoring OVSDB, Dragonflow local controller get notified when the new port is added on OVS bridge.
• Dragonflow local controller queries the port’s topology from Dragonflow database and knows which tenant the port belongs to.
• Dragonflow local controller queries local cache to see if it has subscribed the tenant topic.
  • If local controller has already subscribed the tenant’s topic. This means there already are local ports of the same tenant. It will not subscribe the topic again.
  • If local controller hasn’t subscribed the tenant’s topic. This means the new port is the only local port in the compute node belongs to the tenant. Local controller will subscribe the tenant’s topic.

+----------+   +----------------+ +------------------+
| database |   | sub/pub server | | Dragonflow local |
+-----+----+   +------+---------+ | Controller 1     |
      |               |           +--------+---------+
      |               |                    |
      |               |                    |
      |               |                    +----+ new OVS port
      |               |                    |    | online
      |               |                    <----+
      |  Get port's topology form database |
      <------------------------------------+
      |               |                    |
      |               |                    |
      |               | subscribe(tenant1) |
      |               <--------------------+
      |               |                    |
      |               |                    |
      +               +                    +

If nova remove a port from OVS bridge, local controller will check if it’s the tenant’s last port on the compute node. If it is, local controller will unsubscribe he tenant’s topic and will not receive any further event of the tenant’s topology changes.

Dragonflow Local Controller Startup¶

On startup, local controller will get all ports being attached to OVS bridge by querying OVSDB. Once getting all these local ports, local controller will query ports’ topology from Dragonflow database and subscribe the corresponding topics of the ports. This is done for every local port, as described in the previous section.

Dragonflow Local Controller Offline¶

If one local controller exit, for example, killed by administrator for maintenance, connection to the pub-sub server will lose. It’s the pub-sub server’s responsibility to remove the local controller from all topics it has subscribed.