func (c *BootstrapCommand) startMongo(addrs []network.Address, agentConfig agent.Config) error {
	logger.Debugf("starting mongo")

	info, ok := agentConfig.MongoInfo()
	if !ok {
		return fmt.Errorf("no state info available")
	}
	// When bootstrapping, we need to allow enough time for mongo
	// to start as there's no retry loop in place.
	// 5 minutes should suffice.
	mongoDialOpts := mongo.DialOpts{Timeout: 5 * time.Minute}
	dialInfo, err := mongo.DialInfo(info.Info, mongoDialOpts)
	if err != nil {
		return err
	}
	servingInfo, ok := agentConfig.StateServingInfo()
	if !ok {
		return fmt.Errorf("agent config has no state serving info")
	}
	// Use localhost to dial the mongo server, because it's running in
	// auth mode and will refuse to perform any operations unless
	// we dial that address.
	dialInfo.Addrs = []string{
		net.JoinHostPort("127.0.0.1", fmt.Sprint(servingInfo.StatePort)),
	}

	logger.Debugf("calling ensureMongoServer")
	ensureServerParams, err := cmdutil.NewEnsureServerParams(agentConfig)
	if err != nil {
		return err
	}
	err = cmdutil.EnsureMongoServer(ensureServerParams)
	if err != nil {
		return err
	}

	peerAddr := mongo.SelectPeerAddress(addrs)
	if peerAddr == "" {
		return fmt.Errorf("no appropriate peer address found in %q", addrs)
	}
	peerHostPort := net.JoinHostPort(peerAddr, fmt.Sprint(servingInfo.StatePort))

	if err := initiateMongoServer(peergrouper.InitiateMongoParams{
		DialInfo:       dialInfo,
		MemberHostPort: peerHostPort,
	}); err != nil {
		return err
	}
	logger.Infof("started mongo")
	return nil
}

func (s *MongoSuite) TestSelectPeerAddress(c *gc.C) {
	addresses := []network.Address{{
		Value: "10.0.0.1",
		Type:  network.IPv4Address,
		Scope: network.ScopeCloudLocal,
	}, {
		Value: "8.8.8.8",
		Type:  network.IPv4Address,
		Scope: network.ScopePublic,
	}}

	address := mongo.SelectPeerAddress(addresses)
	c.Assert(address, gc.Equals, "10.0.0.1")
}

// ensureMongoServer ensures that mongo is installed and running,
// and ready for opening a state connection.
func (a *MachineAgent) ensureMongoServer(agentConfig agent.Config) (err error) {
	a.mongoInitMutex.Lock()
	defer a.mongoInitMutex.Unlock()
	if a.mongoInitialized {
		logger.Debugf("mongo is already initialized")
		return nil
	}
	defer func() {
		if err == nil {
			a.mongoInitialized = true
		}
	}()

	servingInfo, ok := agentConfig.StateServingInfo()
	if !ok {
		return fmt.Errorf("state worker was started with no state serving info")
	}

	// When upgrading from a pre-HA-capable environment,
	// we must add machine-0 to the admin database and
	// initiate its replicaset.
	//
	// TODO(axw) remove this when we no longer need
	// to upgrade from pre-HA-capable environments.
	var shouldInitiateMongoServer bool
	var addrs []network.Address
	if isPreHAVersion(a.previousAgentVersion) {
		_, err := a.ensureMongoAdminUser(agentConfig)
		if err != nil {
			return err
		}
		if servingInfo.SharedSecret == "" {
			servingInfo.SharedSecret, err = mongo.GenerateSharedSecret()
			if err != nil {
				return err
			}
			if err = a.ChangeConfig(func(config agent.ConfigSetter) error {
				config.SetStateServingInfo(servingInfo)
				return nil
			}); err != nil {
				return err
			}
			agentConfig = a.CurrentConfig()
		}
		// Note: we set Direct=true in the mongo options because it's
		// possible that we've previously upgraded the mongo server's
		// configuration to form a replicaset, but failed to initiate it.
		st, m, err := openState(agentConfig, mongo.DialOpts{Direct: true})
		if err != nil {
			return err
		}
		ssi := paramsStateServingInfoToStateStateServingInfo(servingInfo)
		if err := st.SetStateServingInfo(ssi); err != nil {
			st.Close()
			return fmt.Errorf("cannot set state serving info: %v", err)
		}
		st.Close()
		addrs = m.Addresses()
		shouldInitiateMongoServer = true
	}

	// ensureMongoServer installs/upgrades the upstart config as necessary.
	ensureServerParams, err := newEnsureServerParams(agentConfig)
	if err != nil {
		return err
	}
	if err := ensureMongoServer(ensureServerParams); err != nil {
		return err
	}
	if !shouldInitiateMongoServer {
		return nil
	}

	// Initiate the replicaset for upgraded environments.
	//
	// TODO(axw) remove this when we no longer need
	// to upgrade from pre-HA-capable environments.
	stateInfo, ok := agentConfig.MongoInfo()
	if !ok {
		return fmt.Errorf("state worker was started with no state serving info")
	}
	dialInfo, err := mongo.DialInfo(stateInfo.Info, mongo.DefaultDialOpts())
	if err != nil {
		return err
	}
	peerAddr := mongo.SelectPeerAddress(addrs)
	if peerAddr == "" {
		return fmt.Errorf("no appropriate peer address found in %q", addrs)
	}
	if err := maybeInitiateMongoServer(peergrouper.InitiateMongoParams{
		DialInfo:       dialInfo,
		MemberHostPort: net.JoinHostPort(peerAddr, fmt.Sprint(servingInfo.StatePort)),
		// TODO(dfc) InitiateMongoParams should take a Tag
		User:     stateInfo.Tag.String(),
		Password: stateInfo.Password,
	}); err != nil {
		return err
	}
//.........这里部分代码省略.........

// ensureMongoServer ensures that mongo is installed and running,
// and ready for opening a state connection.
func (a *MachineAgent) ensureMongoServer(agentConfig agent.Config) error {
	servingInfo, ok := agentConfig.StateServingInfo()
	if !ok {
		return fmt.Errorf("state worker was started with no state serving info")
	}
	namespace := agentConfig.Value(agent.Namespace)

	// When upgrading from a pre-HA-capable environment,
	// we must add machine-0 to the admin database and
	// initiate its replicaset.
	//
	// TODO(axw) remove this when we no longer need
	// to upgrade from pre-HA-capable environments.
	var shouldInitiateMongoServer bool
	var addrs []network.Address
	if isPreHAVersion(agentConfig.UpgradedToVersion()) {
		_, err := a.ensureMongoAdminUser(agentConfig)
		if err != nil {
			return err
		}
		if servingInfo.SharedSecret == "" {
			servingInfo.SharedSecret, err = mongo.GenerateSharedSecret()
			if err != nil {
				return err
			}
			if err = a.ChangeConfig(func(config agent.ConfigSetter) {
				config.SetStateServingInfo(servingInfo)
			}); err != nil {
				return err
			}
			agentConfig = a.CurrentConfig()
		}
		st, m, err := openState(agentConfig)
		if err != nil {
			return err
		}
		if err := st.SetStateServingInfo(servingInfo); err != nil {
			st.Close()
			return fmt.Errorf("cannot set state serving info: %v", err)
		}
		st.Close()
		addrs = m.Addresses()
		shouldInitiateMongoServer = true
	}

	// ensureMongoServer installs/upgrades the upstart config as necessary.
	if err := ensureMongoServer(
		agentConfig.DataDir(),
		namespace,
		servingInfo,
	); err != nil {
		return err
	}
	if !shouldInitiateMongoServer {
		return nil
	}

	// Initiate the replicaset for upgraded environments.
	//
	// TODO(axw) remove this when we no longer need
	// to upgrade from pre-HA-capable environments.
	stateInfo, ok := agentConfig.StateInfo()
	if !ok {
		return fmt.Errorf("state worker was started with no state serving info")
	}
	dialInfo, err := mongo.DialInfo(stateInfo.Info, mongo.DefaultDialOpts())
	if err != nil {
		return err
	}
	peerAddr := mongo.SelectPeerAddress(addrs)
	if peerAddr == "" {
		return fmt.Errorf("no appropriate peer address found in %q", addrs)
	}
	return maybeInitiateMongoServer(peergrouper.InitiateMongoParams{
		DialInfo:       dialInfo,
		MemberHostPort: net.JoinHostPort(peerAddr, fmt.Sprint(servingInfo.StatePort)),
		User:           stateInfo.Tag,
		Password:       stateInfo.Password,
	})
}