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Copyright 2011 Google Inc.
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
See the License for the specific language governing permissions and
limitations under the License.
package memcache
import (
// ServerSelector is the interface that selects a memcache server
// as a function of the item's key.
// All ServerSelector implementations must be safe for concurrent use
// by multiple goroutines.
type ServerSelector interface {
// PickServer returns the server address that a given item
// should be shared onto.
PickServer(key string) (net.Addr, error)
Each(func(net.Addr) error) error
// ServerList is a simple ServerSelector. Its zero value is usable.
type ServerList struct {
mu sync.RWMutex
addrs []net.Addr
// staticAddr caches the Network() and String() values from any net.Addr.
type staticAddr struct {
ntw, str string
func newStaticAddr(a net.Addr) net.Addr {
return &staticAddr{
ntw: a.Network(),
str: a.String(),
func (s *staticAddr) Network() string { return s.ntw }
func (s *staticAddr) String() string { return s.str }
// SetServers changes a ServerList's set of servers at runtime and is
// safe for concurrent use by multiple goroutines.
// Each server is given equal weight. A server is given more weight
// if it's listed multiple times.
// SetServers returns an error if any of the server names fail to
// resolve. No attempt is made to connect to the server. If any error
// is returned, no changes are made to the ServerList.
func (ss *ServerList) SetServers(servers ...string) error {
naddr := make([]net.Addr, len(servers))
for i, server := range servers {
if strings.Contains(server, "/") {
addr, err := net.ResolveUnixAddr("unix", server)
if err != nil {
return err
naddr[i] = newStaticAddr(addr)
} else {
tcpaddr, err := net.ResolveTCPAddr("tcp", server)
if err != nil {
return err
naddr[i] = newStaticAddr(tcpaddr)
ss.addrs = naddr
return nil
// Each iterates over each server calling the given function
func (ss *ServerList) Each(f func(net.Addr) error) error {
for _, a := range ss.addrs {
if err := f(a); nil != err {
return err
return nil
// keyBufPool returns []byte buffers for use by PickServer's call to
// crc32.ChecksumIEEE to avoid allocations. (but doesn't avoid the
// copies, which at least are bounded in size and small)
var keyBufPool = sync.Pool{
New: func() interface{} {
b := make([]byte, 256)
return &b
func (ss *ServerList) PickServer(key string) (net.Addr, error) {
if len(ss.addrs) == 0 {
return nil, ErrNoServers
if len(ss.addrs) == 1 {
return ss.addrs[0], nil
bufp := keyBufPool.Get().(*[]byte)
n := copy(*bufp, key)
cs := crc32.ChecksumIEEE((*bufp)[:n])
return ss.addrs[cs%uint32(len(ss.addrs))], nil