OVN Neutron Worker and Port status handling

When the logical switch port’s VIF is attached or removed to/from the ovn integration bridge, ovn-northd updates the Logical_Switch_Port.up to ‘True’ or ‘False’ accordingly.

In order for the OVN Neutron ML2 driver to update the corresponding neutron port’s status to ‘ACTIVE’ or ‘DOWN’ in the db, it needs to monitor the OVN Northbound db. A neutron worker is created for this purpose.

The implementation of the ovn worker can be found here - ‘networking_ovn.ovsdb.worker.OvnWorker’.

Neutron service will create ‘n’ api workers and ‘m’ rpc workers and 1 ovn worker (all these workers are separate processes).

Api workers and rpc workers will create ovsdb idl client object (‘ovs.db.idl.Idl’) to connect to the OVN_Northbound db. See ‘networking_ovn.ovsdb.impl_idl_ovn.OvsdbNbOvnIdl’ and ‘ovsdbapp.backend.ovs_idl.connection.Connection’ classes for more details.

Ovn worker will create ‘networking_ovn.ovsdb.ovsdb_monitor.OvnIdl’ class object (which inherits from ‘ovs.db.idl.Idl’) to connect to the OVN_Northbound db. On receiving the OVN_Northbound db updates from the ovsdb-server, ‘notify’ function of ‘OVnIdl’ is called by the parent class object.

OvnIdl.notify() function passes the received events to the ovsdb_monitor.OvnDbNotifyHandler class. ovsdb_monitor.OvnDbNotifyHandler checks for any changes in the ‘Logical_Switch_Port.up’ and updates the neutron port’s status accordingly.

If ‘notify_nova_on_port_status_changes’ configuration is set, then neutron would notify nova on port status changes.

ovsdb locks

If there are multiple neutron servers running, then each neutron server will have one ovn worker which listens for the notify events. When the ‘Logical_Switch_Port.up’ is updated by ovn-northd, we do not want all the neutron servers to handle the event and update the neutron port status. In order for only one neutron server to handle the events, ovsdb locks are used.

At start, each neutron server’s ovn worker will try to acquire a lock with id - ‘neutron_ovn_event_lock’. The ovn worker which has acquired the lock will handle the notify events.

In case the neutron server with the lock dies, ovsdb-server will assign the lock to another neutron server in the queue.

More details about the ovsdb locks can be found here [1] and [2]

[1] - https://tools.ietf.org/html/draft-pfaff-ovsdb-proto-04#section-4.1.8 [2] - https://github.com/openvswitch/ovs/blob/branch-2.4/python/ovs/db/idl.py#L67

One thing to note is the ovn worker (with OvnIdl) do not carry out any transactions to the OVN Northbound db.

Since the api and rpc workers are not configured with any locks, using the ovsdb lock on the OVN_Northbound and OVN_Southbound DBs by the ovn workers will not have any side effects to the transactions done by these api and rpc workers.

Handling port status changes when neutron server(s) are down

When neutron server starts, ovn worker would receive a dump of all logical switch ports as events. ‘ovsdb_monitor.OvnDbNotifyHandler’ would sync up if there are any inconsistencies in the port status.

OVN Southbound DB Access

The OVN Neutron ML2 driver has a need to acquire chassis information (hostname and physnets combinations). This is required initially to support routed networks. Thus, the plugin will initiate and maintain a connection to the OVN SB DB during startup.