ovsdb: New ovsdb 'relay' service model.

New database service model 'relay' that is needed to scale out
read-mostly database access, e.g. ovn-controller connections to
OVN_Southbound.

In this service model ovsdb-server connects to existing OVSDB
server and maintains in-memory copy of the database.  It serves
read-only transactions and monitor requests by its own, but
forwards write transactions to the relay source.

Key differences from the active-backup replication:
- support for "write" transactions (next commit).
- no on-disk storage. (probably, faster operation)
- support for multiple remotes (connect to the clustered db).
- doesn't try to keep connection as long as possible, but
  faster reconnects to other remotes to avoid missing updates.
- No need to know the complete database schema beforehand,
  only the schema name.
- can be used along with other standalone and clustered databases
  by the same ovsdb-server process. (doesn't turn the whole
  jsonrpc server to read-only mode)
- supports modern version of monitors (monitor_cond_since),
  because based on ovsdb-cs.
- could be chained, i.e. multiple relays could be connected
  one to another in a row or in a tree-like form.
- doesn't increase availability.
- cannot be converted to other service models or become a main
  active server.

Some performance test results can be found here:
  https://mail.openvswitch.org/pipermail/ovs-dev/2021-July/385825.html

Acked-by: Mark D. Gray <mark.d.gray@redhat.com>
Acked-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/ovsdb/ovsdb.h b/ovsdb/ovsdb.h
index 72e127c84..16bd5f5ec 100644
--- a/ovsdb/ovsdb.h
+++ b/ovsdb/ovsdb.h
@@ -91,6 +91,9 @@ struct ovsdb {
     bool need_txn_history;     /* Need to maintain history of transactions. */
     unsigned int n_txn_history; /* Current number of history transactions. */
     struct ovs_list txn_history; /* Contains "struct ovsdb_txn_history_node. */
+
+    /* Relay mode. */
+    bool is_relay;
 };
 
 struct ovsdb *ovsdb_create(struct ovsdb_schema *, struct ovsdb_storage *);