[Bug 1843582] Re: ovn: additional systemd units requested

[Frode Nordahl]

Another note about the proposed systemd service file referenced in
comment #1.  As they are laid out in the charm atm. they give some
unintended consequences, i.e if you restart ``ovn-northd`` also the
databases will be restarted.  That would need to be sorted before we add
them to the packages.

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Title:
  ovn: additional systemd units requested

Status in openvswitch package in Ubuntu:
  New

Bug description:
  I am working on Juju Charms for deploying OVN.

  At present the OVN components are distributed in the following binary
  packages:

   - ``ovn-common``

     - CLI tools, man pages, scripts

   - ``ovn-docker``

     - Docker components

   - ``ovn-host``

     - Host components, the local controller to install on each
  participant in a OVN network

   - ``ovn-central``

     - ovn-northd daemon, OVSDB database schemas for the Northbound and
  Southbound databases

   - ``ovn-controller-vtep``

     - Controller to integrate with hardware VTEP-enabled switches

  The ``ovn-central`` package contains a SysV init script that reads
  some parameters from ``/etc/default/ovn-central`` and calls out to the
  ``/usr/share/openvswitch/scripts/ovn-ctl`` script from the ``ovn-
  common`` package.

  The default mode of operation for this init script is to start ovsdb-
  server processes for both the Northbound and Southbound databases and
  then start the ``ovn-northd`` daemon.

  While working on the design of the charm [0] it has been identified
  [1][2] that the Southbound DB has different scaling requirements than
  the other components, and its load will be relative to the number of
  transport nodes (controllers) in the system [3].

  As a consequence the charm will aim to support separate placement of
  the Southbound DB.  To accomplish this I would need individual service
  control for the ``ovn-northd`` daemon, the ``ovsdb-server`` for
  Northound DB and the ``ovsdb-server`` for Southbound DB.

  
  You get to pull most levers by controlling command line arguments through the ``OVN_CTL_OPTS`` variable in ``/etc/default/ovn-central``, including starting the ``ovn-northd`` daemon by itself.

  But you do not get to individual service control of the Northbound-
  and Southbound- databases.

  
  What I propose is that we add systemd units for ``nb_ovsdb`` and ``sb_ovsdb`` which pulls in variables from /etc/default/<unit name> in similar fashion as the existing ``ovn-central`` script and then calls out to ``/usr/share/openvswitch/scripts/ovn-ctl`` with ``start_nb_ovsdb`` and ``start_sb_ovsdb`` parameters respectively.

  I guess a generic approach would be to support this for all the start
  and stop targets the ``ovn-ctl`` script supports, but I only have use
  for individual control of ``ovn-northd``, ``nb_ovsdb`` and
  ``sb_ovsdb`` at this point.

  
  A argument for not just using the ``ovn-ctl`` script directly from the charm is that there are well established workflows in our charm frameworks for service control through init scripts, and it would be useful to be able to use that directly.

  
  Side note: the OVN project have recently moved into its own set of repositories [4] and we may need to consider creating a separate source package for it at some point.

  
  0: https://specs.openstack.org/openstack/charm-specs/specs/train/approved/charm-openstack-ovn.html
  1: https://review.opendev.org/#/c/680415/
  2: https://pbs.twimg.com/media/EDr3IPXXsAAg2Vb.jpg
  3: http://www.openvswitch.org/support/dist-docs/ovn-architecture.7.html
  4: https://github.com/ovn-org/

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