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you can read this wiki to get a general knowledge: https://wiki.openstack.org/wiki/Slave_usage

the sql connection/slave_connection config opt is used by oslo sqlalchemy session module[0], not neutron directly, for now, I didn't find code in neutron using slave db feature. (If I'm wrong, please let me know, thanks)

some code may use slave db to reduce the master's load, for i.e. nova periodic task[1][2], they are read operations and can accept a little lag, but operator should ensure the lag is not too high.

[0] https://github.com/openstack/oslo-incubator/blob/master/openstack/common/db/sqlalchemy/session.py [1] https://github.com/openstack/nova/blob/master/nova/db/api.py (#631, for i.e.) [2] https://blueprints.launchpad.net/nova/+spec/periodic-tasks-to-db-slave