3. Neutron OVN client idl

Continue to last topic, this one will try to do some diging on OvsdbOvnIdl, the OVN client idl for server, from two aspects, its __init__ and transaction methods.

IMO, its transaction method is important to dig, since for Neutron ML2 driver, operations to server, is always terminated at transaction [1], like:

with self._nb_idl.transaction(check_error=True) as txn:

3.1. OvsdbOvnIdl superclasses

To figure out what does its initilize method do, we need find out its superclasses first. As previous topic says, OvsdbNbOvnIdl has different superclass than idls like BaseOvnIdl, its inheritation map looks like


and we can find OvsdbNbOvnIdl.__mro__ is: OvsdbNbOvnIdl, OvnNBApiIdlImpl, Backend-1, Backend-2, API-1, API-2. And Backend-2 defines:

  • start and restart connection methods
  • commands to operate db, table, record commonly

API-2 defines:

  • transaction method and other abstract methods about db operation like db_list, db_add.

API-1 defines:

  • abstract methods to operate OVN_Northbound DB, like ovn-nbctl

Backend-1 defines:

  • method to start connection, and create transaction, and is_table_present, is_col_present, idl

OvnNbApiIdlImpl defines:

  • methods defined by API-1.

3.2. OvsdbOvnIdl.__init__

After extract code pieces from its parents, OvsdbNbOvnIdl __init__ looks like:

        self.ovsdb_connection = connection
        self._nested_txn = None

and where self.idl and start_connection is:

    return self.ovsdb_connection.idl


set_probe_interval will reset probe interval for session build on the connection for keepalive purpose, which is done by sending “echo” message. _nested_txn is a transaction object yield by OvsdbNbOvnIdl.transaction method. ovsdb_coonnection.start() will start a thread which handling transaction submitted via [1], more about this will be discussed in idl ovsdb_connection. [2]

3.3. OvsdbNbOvnIdl.transaction

3.3.1. Get transaction

The main entrace for ovn_client to call transaction is [1], and that’s also the place where transaction get created:

    if self._nested_txn:
        yield self._nested_txn
        with self.create_transaction(check_error, log_errors,
                                     **kwargs) as txn:
            self._nested_txn = txn
                yield txn
                self._nested_txn = None

where create_transaction is:

create_transaction(self, check_error=False, log_errors=True)
    return idl_trans.Transaction(
        self, self.ovsdb_connection, self.ovsdb_connection.timeout,
        check_error, log_errors)

So every time [1] is called, if an idl_trans.Transaction object is created already, it will be returned to make sure all operations commands can be submitted into the same transaction to server. idl_trans.Transaction is in ovsdbapp/backend/ovs_idl/transaction.py and inherits from ovsdbapp.api.Transaction. For with statement in [1], its relative methods are defined like:

    self.commands = []
    self.results = Queue.Queue(1)
    self.ovsdb_connection = ovsdb_connection

    return self

    if exc_type is None:
        self.result = self.commit()

add(self, command)
    return command

3.3.2. Transaction commit

txt.add only collect commands, and when transaction exits, commit method is called to put transaction itself into ovsdb_connection’s transaction queue, and wait ovsdb_connection to handle, as [2] mentioned:

        // timeout: 180s by default, from ovsdb_connection.timeout
        // wait an item is available or raise Empty when timeout
        result = self.results.get(timeout=self.timeout)
    except Queue.Empty:
        raise exceptions.TimeoutException(commands=self.commands,
    return result

And idl_trans.Transaction also defines a callback method for ovsdb_connection thread to handle operation commands in transaction:

    // Transaction: ovs.db.idl.Transaction
    // self.api.idl: _nb_idl.ovsdb_connection.idl
    txn = idl.Transaction(self.api.idl)
    for i, command in enumerate(self.commands):
        command.run_idl(txn)    // command execution entrance
    status = txn.commit_block()
    if status == ...
    elif status == txn.SUCCESS:
    return [cmd.result for cmd in self.commands]

as shown, when ovsdb_connection thread start to process transaction, commands in transaction will be processed via them run_idl methods one by one. Once commits are OK, ovsdb_connection thread will put results of transaction into transaction results queue for further process.