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Hi Neutrinos,

Presently, In the OVN ML2 driver we have 2 ways to sync neutron DB and OVN
DB
 - At neutron-server startup, OVN ML2 driver syncs the neutron DB and OVN
DB if sync mode is set to repair.
 - Admin can run the "neutron-ovn-db-sync-util" to sync the DBs.

Recently, in the v2 of networking-odl ML2 driver (Please see (1) below
which has more details). (ODL folks please correct me if I am wrong here)

  - a journal thread is created which does the CRUD operations of neutron
resources asynchronously (i.e it sends the REST APIs to the ODL controller).
  - a maintenance thread is created which does some cleanup periodically
and at startup does full sync if it detects ODL controller cold reboot.


Few question I have
 - can OVN ML2 driver take same or similar approach. Are there any
advantages in taking this approach ? One advantage is neutron resources can
be created/updated/deleted even if the OVN ML2 driver has lost connection
to the ovsdb-server. The journal thread would eventually sync these
resources in the OVN DB. I would like to know the communities thoughts on
this.

 - Are there are other ML2 drivers which might have to handle the DB sync's
(cases where the other controllers also maintain their own DBs) and how
they are handling it ?

 - Can a common approach be taken to sync the neutron DB and controller DBs
?


-----------------------------------------------------------------------------------------------------------

(1)
Sync threads created by networking-odl ML2 driver
--------------------------------------------------
ODL ML2 driver creates 2 threads (threading.Thread module) at init
 - Journal thread
 - Maintenance thread

Journal thread
----------------
The journal module creates a new journal table by name
“opendaylightjournal”  -
https://github.com/openstack/networking-odl/blob/master/networking_odl/db/models.py#L23

Journal thread will be in loop waiting for the sync event from the ODL ML2
driver.

 - ODL ML2 driver resource (network, subnet, port) precommit functions when
called by the ML2 plugin adds an entry in the “opendaylightjournal” table
with the resource data and sets the journal operation state for this entry
to “PENDING”.
 - The corresponding resource postcommit function of the ODL ML2 plugin
when called, sets the sync event flag.
 - A timer is also created which sets the sync event flag when it expires
(the default value is 10 seconds).
 - Journal thread wakes up, looks into the “opendaylightjournal” table with
the entries with state “pending” and runs the CRUD operation on those
resources in the ODL DB. Once done, it sets the state to “completed”.

Maintenance thread
------------------
Maintenance thread does 3 operations
 - JournalCleanup - Delete completed rows from journal table
“opendaylightjournal”.
 - CleanupProcessing - Mark orphaned processing rows to pending.
 - Full sync - Re-sync when detecting an ODL "cold reboot”.



Thanks
Numan
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