func (p *ClientPool) Get(id proto.NodeID) (cli Client, err error) {
	list, _ := p.loadFreeList(id)
	return list.get()
}
Get returns existing freelist to the node, if not exist try best to create one.
package rpc
import (
	"net/rpc"
	"strings"
	"sync"
	"github.com/pkg/errors"
	"github.com/SQLess/SQLess/conf"
	"github.com/SQLess/SQLess/proto"
)
type pooledClient struct {
	*rpc.Client
	sync.Mutex
	freelist *freelist
	lastErr  error
}
func (c *pooledClient) SetLastErr(err error) {
	c.Lock()
	defer c.Unlock()
	if err != nil {
		c.lastErr = err
	}
}
func (c *pooledClient) Call(serviceMethod string, args interface{}, reply interface{}) error {
	err := c.Client.Call(serviceMethod, args, reply)
	if err != nil {  
		c.SetLastErr(err)
	}
	return err
}
func (c *pooledClient) Close() error {
	c.Lock()
	if c.freelist != nil && c.lastErr == nil {
		err := c.freelist.put(c.Client)  
		c.freelist = nil
		c.Unlock()
		return err
	}
	c.Unlock()
	return c.Client.Close()
}
type freelist struct {
	sync.RWMutex
	target proto.NodeID
	freeCh chan *rpc.Client
}
func (l *freelist) close() error {
	l.Lock()
	defer l.Unlock()
	close(l.freeCh)
	var errmsgs []string
	for s := range l.freeCh {
		if err := s.Close(); err != nil {
			errmsgs = append(errmsgs, err.Error())
		}
	}
	l.freeCh = nil  
	if len(errmsgs) > 0 {
		return errors.Wrapf(errors.New(strings.Join(errmsgs, ", ")), "close free list %s", l.target)
	}
	return nil
}
func (l *freelist) put(cli *rpc.Client) (err error) {
	l.RLock()  
	defer l.RUnlock()
	select {
	case l.freeCh <- cli:
	default:
		err = cli.Close()
	}
	return
}
func (l *freelist) getFree() (cli *rpc.Client, ok bool) {
	l.RLock()
	defer l.RUnlock()
	select {
	case cli, ok = <-l.freeCh:
	default:
	}
	return
}
func (l *freelist) get() (cli Client, err error) {
	var (
		raw *rpc.Client
		ok  bool
	)
	if raw, ok = l.getFree(); !ok {
		if raw, err = l.newClient(); err != nil {
			return
		}
	}
	return &pooledClient{
		Client:   raw,  
		freelist: l,
	}, nil
}
func (l *freelist) len() int {
	l.RLock()
	defer l.RUnlock()
	return len(l.freeCh)
}
func (l *freelist) newClient() (*rpc.Client, error) {
	conn, err := Dial(l.target)
	if err != nil {
		return nil, errors.Wrap(err, "dialing new connection failed")
	}
	return NewClient(conn), err
}
type ClientPool struct {
	nodeFreeLists sync.Map  
}
func (p *ClientPool) loadFreeList(id proto.NodeID) (list *freelist, ok bool) {
	var v interface{}
	if v, ok = p.nodeFreeLists.Load(id); ok {
		list = v.(*freelist)
		return
	}
	v, ok = p.nodeFreeLists.LoadOrStore(id, &freelist{
		target: id,
		freeCh: make(chan *rpc.Client, conf.MaxRPCPoolPhysicalConnection),
	})
	list = v.(*freelist)
	return
}