openstack trove代码的主从切换promote slave to master的源代码分析：

    def promote_to_replica_source(self, context, instance_id):
        # TODO(atomic77) Promote and eject need to be able to handle the case
        # where a datastore like Postgresql needs to treat the slave to be
        # promoted differently from the old master and the slaves which will
        # be simply reassigned to a new master. See:
        # https://bugs.launchpad.net/trove/+bug/1553339

        def _promote_to_replica_source(old_master, master_candidate,
                                       replica_models):
            # First, we transition from the old master to new as quickly as
            # possible to minimize the scope of unrecoverable error

            # NOTE(zhaochao): we cannot reattach the old master to the new
            # one immediately after the new master is up, because for MariaDB
            # the other replicas are still connecting to the old master, and
            # during reattaching the old master as a slave, new GTID may be
            # created and synced to the replicas. After that, when attaching
            # the replicas to the new master, 'START SLAVE' will fail by
            # 'fatal error 1236' if the binlog of the replica diverged from
            # the new master. So the proper order should be:
            # -1. make the old master read only (and detach floating ips)
            # -2. make sure the new master is up-to-date
            # -3. detach the new master from the old one
            # -4. enable the new master (and attach floating ips)
            # -5. attach the other replicas to the new master
            # -6. attach the old master to the new one
            #     (and attach floating ips)
            # -7. demote the old master
            # What we changed here is the order of the 6th step, previously
            # this step took place right after step 4, which causes failures
            # with MariaDB replications.
            old_master.make_read_only(True)
            latest_txn_id = old_master.get_latest_txn_id()
            master_candidate.wait_for_txn(latest_txn_id)
            master_candidate.detach_replica(old_master, for_failover=True)
            master_candidate.enable_as_master()
            master_candidate.make_read_only(False)

            # At this point, should something go wrong, there
            # should be a working master with some number of working slaves,
            # and possibly some number of "orphaned" slaves

            exception_replicas = []
            error_messages = ""
            for replica in replica_models:
                try:
                    if replica.id != master_candidate.id:
                        replica.detach_replica(old_master, for_failover=True)
                        replica.attach_replica(master_candidate)
                except exception.TroveError as ex:
                    log_fmt = ("Unable to migrate replica %(slave)s from "
                               "old replica source %(old_master)s to "
                               "new source %(new_master)s on promote.")
                    exc_fmt = _("Unable to migrate replica %(slave)s from "
                                "old replica source %(old_master)s to "
                                "new source %(new_master)s on promote.")
                    msg_content = {
                        "slave": replica.id,
                        "old_master": old_master.id,
                        "new_master": master_candidate.id}
                    LOG.error(log_fmt, msg_content)

                    exception_replicas.append(replica)
                    error_messages += "%s (%s)\n" % (
                        exc_fmt % msg_content, ex)

            # dealing with the old master after all the other replicas
            # has been migrated.
            old_master.attach_replica(master_candidate)
            try:
                old_master.demote_replication_master()
            except Exception as ex:
                log_fmt = "Exception demoting old replica source %s."
                exc_fmt = _("Exception demoting old replica source %s.")
                LOG.error(log_fmt, old_master.id)
                exception_replicas.append(old_master)
                error_messages += "%s (%s)\n" % (
                    exc_fmt % old_master.id, ex)

            self._set_task_status([old_master] + replica_models,
                                  InstanceTasks.NONE)

            if exception_replicas:
                self._set_task_status(exception_replicas,
                                      InstanceTasks.PROMOTION_ERROR)
                msg = (_("promote-to-replica-source %(id)s: The following "
                         "replicas may not have been switched: %(replicas)s:"
                         "\n%(err)s") %
                       {"id": master_candidate.id,
                        "replicas": [repl.id for repl in exception_replicas],
                        "err": error_messages})
                raise ReplicationSlaveAttachError(msg)

            LOG.info('Finished to promote %s as master.', instance_id)

        with EndNotification(context):
            LOG.info('Promoting %s as replication master', instance_id)

            master_candidate = BuiltInstanceTasks.load(context, instance_id)
            old_master = BuiltInstanceTasks.load(context,
                                                 master_candidate.slave_of_id)

            replicas = []
            for replica_dbinfo in old_master.slaves:
                if replica_dbinfo.id == instance_id:
                    replica = master_candidate
                else:
                    replica = BuiltInstanceTasks.load(context,
                                                      replica_dbinfo.id)
                replicas.append(replica)

            try:
                _promote_to_replica_source(old_master, master_candidate,
                                           replicas)
            except ReplicationSlaveAttachError:
                raise
            except Exception:
                self._set_task_status([old_master] + replicas,
                                      InstanceTasks.PROMOTION_ERROR)
                raise

切换流程如下:
主从切换适合：一主多从的结构，而且绑定了floating_ip的情况。

  1. 根据要切换的slave_id获取实例作为新的maste；
  2. 将所有slave包括现在的作为新master的slave_id，添加到一个replicas列表

  3. 根据旧的master_id获取旧的master实例
    以下开始进入正式切换步骤：_promote_to_replica_source(old_master, master_candidate,replicas)
    注意: 以下说的公网IP是指floating ip

  4. 设置旧的maste实例为只读
  5. 旧master 实例detach公网ip
  6. 新的master实例detach公网ip
  7. 获取旧master实例的同步位置
  8. 等待新的master实例同步到相同位置
  9. 解除新旧master实例的主从关系
  10. 新的master实例启用为master实例。
  11. 新的maste实例attache公网IP
  12. 新的master设置read_only 为flase,例如mysql: set global read_only = False
    获取获取slave实例，如果slave实例的id不等于新的maste实例的id。
    旧的maste解除主从关系
    attache旧的slave到新的maste实例上
    旧的maste实例和新的maste实例建立主从关系
    旧的maste实例添加公网ip