I saw an error in neutron-server.log:

2018-10-18 16:23:39.258 30 ERROR neutron.plugins.ml2.managers [req-61134bd9-1f45-4aad-9148-d64dba96b1d0 - - - - -] Mechanism driver 'l2population' failed in update_port_postcommit: AttributeError: 'Context' object has no attribute 'begin'
2018-10-18 16:23:39.258 30 ERROR neutron.plugins.ml2.managers Traceback (most recent call last):
2018-10-18 16:23:39.258 30 ERROR neutron.plugins.ml2.managers   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/plugins/ml2/managers.py", line 426, in _call_on_drivers
2018-10-18 16:23:39.258 30 ERROR neutron.plugins.ml2.managers     getattr(driver.obj, method_name)(context)
2018-10-18 16:23:39.258 30 ERROR neutron.plugins.ml2.managers   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/l2pop/mech_driver.py", line 183, in update_port_postcommit
2018-10-18 16:23:39.258 30 ERROR neutron.plugins.ml2.managers     self.update_port_up(context)
2018-10-18 16:23:39.258 30 ERROR neutron.plugins.ml2.managers   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/l2pop/mech_driver.py", line 295, in update_port_up
2018-10-18 16:23:39.258 30 ERROR neutron.plugins.ml2.managers     if is_first_port or self.agent_restarted(context):
2018-10-18 16:23:39.258 30 ERROR neutron.plugins.ml2.managers   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/l2pop/mech_driver.py", line 246, in agent_restarted
2018-10-18 16:23:39.258 30 ERROR neutron.plugins.ml2.managers     agent = l2pop_db.get_agent_by_host(port_context, agent_host)
2018-10-18 16:23:39.258 30 ERROR neutron.plugins.ml2.managers   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/plugins/ml2/drivers/l2pop/db.py", line 60, in get_agent_by_host
2018-10-18 16:23:39.258 30 ERROR neutron.plugins.ml2.managers     with session.begin(subtransactions=True):
2018-10-18 16:23:39.258 30 ERROR neutron.plugins.ml2.managers AttributeError: 'Context' object has no attribute 'begin'
2018-10-18 16:23:39.258 30 ERROR neutron.plugins.ml2.managers 
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager [req-61134bd9-1f45-4aad-9148-d64dba96b1d0 - - - - -] Error during notification for neutron.plugins.ml2.plugin.Ml2Plugin._port_provisioned--9223372036854067064 port, provisioning_complete: MechanismDriverError
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager Traceback (most recent call last):
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron_lib/callbacks/manager.py", line 197, in _notify_loop
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     callback(resource, event, trigger, **kwargs)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/plugins/ml2/plugin.py", line 260, in _port_provisioned
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     self.update_port_status(context, port_id, const.PORT_STATUS_ACTIVE)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/plugins/ml2/plugin.py", line 1696, in update_port_status
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     context, {port_id: status}, host)[port_id]
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/common/utils.py", line 674, in inner
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     return f(self, context, *args, **kwargs)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/db/api.py", line 163, in wrapped
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     return method(*args, **kwargs)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/db/api.py", line 93, in wrapped
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     setattr(e, '_RETRY_EXCEEDED', True)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     self.force_reraise()
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     six.reraise(self.type_, self.value, self.tb)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/db/api.py", line 89, in wrapped
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     return f(*args, **kwargs)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/oslo_db/api.py", line 154, in wrapper
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     ectxt.value = e.inner_exc
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     self.force_reraise()
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     six.reraise(self.type_, self.value, self.tb)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/oslo_db/api.py", line 142, in wrapper
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     return f(*args, **kwargs)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/db/api.py", line 128, in wrapped
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     LOG.debug("Retry wrapper got retriable exception: %s", e)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     self.force_reraise()
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     six.reraise(self.type_, self.value, self.tb)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/db/api.py", line 124, in wrapped
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     return f(*dup_args, **dup_kwargs)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/plugins/ml2/plugin.py", line 1711, in update_port_statuses
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     context, port_dbs_by_id[port_id], status, host)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/plugins/ml2/plugin.py", line 1723, in _safe_update_individual_port_db_status
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     ectx.reraise = bool(db.get_port(context, port_id))
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/oslo_utils/excutils.py", line 220, in __exit__
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     self.force_reraise()
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/oslo_utils/excutils.py", line 196, in force_reraise
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     six.reraise(self.type_, self.value, self.tb)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/plugins/ml2/plugin.py", line 1719, in _safe_update_individual_port_db_status
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     context, port, status, host)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/plugins/ml2/plugin.py", line 1787, in _update_individual_port_db_status
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     self.mechanism_manager.update_port_postcommit(mech_context)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/plugins/ml2/managers.py", line 694, in update_port_postcommit
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     continue_on_failure=True)
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager   File "/var/lib/kolla/venv/local/lib/python2.7/site-packages/neutron/plugins/ml2/managers.py", line 444, in _call_on_drivers
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager     errors=errors
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager MechanismDriverError
2018-10-18 16:23:39.294 30 ERROR neutron_lib.callbacks.manager