* ovn-controller

*** Determine how to split logical pipeline across physical nodes.

    From the original OVN architecture document:

    The pipeline processing is split between the ingress and egress
    transport nodes.  In particular, the logical egress processing may
    occur at either hypervisor.  Processing the logical egress on the
    ingress hypervisor requires more state about the egress vif's
    policies, but reduces traffic on the wire that would eventually be
    dropped.  Whereas, processing on the egress hypervisor can reduce
    broadcast traffic on the wire by doing local replication.  We
    initially plan to process logical egress on the egress hypervisor
    so that less state needs to be replicated.  However, we may change
    this behavior once we gain some experience writing the logical
    flows.

    The split pipeline processing split will influence how tunnel keys
    are encoded.

** ovn-controller parameters and configuration.

*** SSL configuration.

    Can probably get this from Open_vSwitch database.

* ovsdb-server

  ovsdb-server should have adequate features for OVN but it probably
  needs work for scale and possibly for availability as deployments
  grow.  Here are some thoughts.

  Andy Zhou is looking at these issues.

*** Reducing amount of data sent to clients.

    Currently, whenever a row monitored by a client changes,
    ovsdb-server sends the client every monitored column in the row,
    even if only one column changes.  It might be valuable to reduce
    this only to the columns that changes.

    Also, whenever a column changes, ovsdb-server sends the entire
    contents of the column.  It might be valuable, for columns that
    are sets or maps, to send only added or removed values or
    key-values pairs.

    Currently, clients monitor the entire contents of a table.  It
    might make sense to allow clients to monitor only rows that
    satisfy specific criteria, e.g. to allow an ovn-controller to
    receive only Pipeline rows for logical networks on its hypervisor.

*** Reducing redundant data and code within ovsdb-server.

    Currently, ovsdb-server separately composes database update
    information to send to each of its clients.  This is fine for a
    small number of clients, but it wastes time and memory when
    hundreds of clients all want the same updates (as will be in the
    case in OVN).

    (This is somewhat opposed to the idea of letting a client monitor
    only some rows in a table, since that would increase the diversity
    among clients.)

*** Multithreading.

    If it turns out that other changes don't let ovsdb-server scale
    adequately, we can multithread ovsdb-server.  Initially one might
    only break protocol handling into separate threads, leaving the
    actual database work serialized through a lock.

** Increasing availability.

   Database availability might become an issue.  The OVN system
   shouldn't grind to a halt if the database becomes unavailable, but
   it would become impossible to bring VIFs up or down, etc.

   My current thought on how to increase availability is to add
   clustering to ovsdb-server, probably via the Raft consensus
   algorithm.  As an experiment, I wrote an implementation of Raft
   for Open vSwitch that you can clone from:

       https://github.com/blp/ovs-reviews.git raft

** Reducing startup time.

   As-is, if ovsdb-server restarts, every client will fetch a fresh
   copy of the part of the database that it cares about.  With
   hundreds of clients, this could cause heavy CPU load on
   ovsdb-server and use excessive network bandwidth.  It would be
   better to allow incremental updates even across connection loss.
   One way might be to use "Difference Digests" as described in
   Epstein et al., "What's the Difference? Efficient Set
   Reconciliation Without Prior Context".  (I'm not yet aware of
   previous non-academic use of this technique.)