关于Memcached一致性hash的探究

参考文章

http://blog.chinaunix.net/uid-20498361-id-4303232.html

http://blog.csdn.net/kongqz/article/details/6695417

https://my.oschina.net/goal/blog/203593?p=1

一直认为一致性hash是个很神奇的东东，技术大牛的想法果然不同凡响。

下列代码并没有作优化处理，仅供参考理解ketama算法，当然也有助于自己理解记忆。如果有错误的地方欢迎指出

 function addServer($hostName = array()){
     $serverList = array();
     foreach($hostName as $val) {
         for($i = 0;$i<40;$i++) {
             $digest = md5($val.'_'.$i, true);
             for ($h = 0; $h < 4; $h++) {
                 $hash = (ord($digest[3 + $h * 4]) << 24) | (ord($digest[2 + $h * 4]) << 16)
                     | (ord($digest[1 + $h * 4]) << 8) | ord($digest[$h * 4]);
                 $serverList[$val][$i*4+$h] = $hash;
             }
         }
     }
     return $serverList;
 }
 //md5的原因是怕用户输入前缀一致  hash一样  分布不均匀？？
 function getKeyHash($str){
     $digest = md5($str,1);
     return (ord($digest[3])<<24) | (ord($digest[2])<<16) | (ord($digest[3])<<8) | (ord($digest[0]));
 }

 function findKey($str){
     $hostName = array(
         '10.100.20.1',
         '10.100.20.2',
         '10.100.20.3',
         '10.100.20.4',
         '10.100.20.5',
         '10.100.20.6',
     );
     $serverList  = addServer($hostName);
     $serverHash = array();
     foreach($serverList as $server){
         $serverHash = array_merge($serverHash,$server);
     }
     sort($serverHash);
     $i = 0;
     $len = count($serverHash);
     $keyHash = getKeyHash($str);
     if($keyHash>$serverHash[$len-1]){
         foreach($serverList as $key=>$val){
             if(in_array($serverHash[$len-1],$val)){
                 return $key;
             }
         }
         return 0;
     }
     while($i<$len){
         $curr = $serverHash[$i];
         $next = $serverHash[++$i];
         if($next>=$keyHash && $keyHash>=$curr){
             foreach($serverList as $key=>$val){
                 if(in_array($serverHash[$i],$val)){
                     return $key;
                 }
             }
         }
     }
     return false;
 }
 $result = array();
 for($i=1;$i<1000;$i++){
     $tmp = findKey('key'.$i);
     if(isset($result[$tmp])){
         $result[$tmp]++;
     }else{
         $result[$tmp] = 0;
     }
 }
 print_r($result);

上面输出结果如下，所以说该算法分布还是相当均匀的。

[root@silence suanfa]# php ketama.php
Array
(
[10.100.20.6] => 149
[10.100.20.5] => 162
[10.100.20.4] => 171
[10.100.20.2] => 146
[10.100.20.3] => 196
[10.100.20.1] => 169
)

查找键值所在服务器可以采用下列折半的方式

 $rangeArr =
 array (
     0 => 19,
     1 => 99,
     2 => 47,
     3 => 49,
     4 => 74,
     5 => 77,
     6 => 45,
     7 => 95,
     8 => 87,
     9 => 15,
     10 => 14,
     11 => 29,
     12 => 57,
     13 => 3,
     14 => 8,
     15 => 38,
     16 => 28,
     17 => 67,
     18 => 55,
     19 => 43,
     20 => 5,
     21 => 25,
     22 => 48,
     23 => 32,
     24 => 58,
     25 => 90,
     26 => 27,
     27 => 50,
     28 => 6,
     29 => 63,
     30 => 65,
     31 => 81,
     32 => 51,
     33 => 52,
     34 => 66,
     35 => 30,
     36 => 56,
     37 => 54,
     38 => 86,
     39 => 84,
     40 => 83,
     41 => 64,
     42 => 42,
     43 => 26,
     44 => 46,
     45 => 79,
     46 => 69,
     47 => 98,
     48 => 24,
     49 => 37,
 );
 sort($rangeArr);
 function findPoint($num,$rangeArr){
     $len = count($rangeArr);
     if($num> $rangeArr[$len-1] || $num<$rangeArr[0]){
         return $num.'in'.$rangeArr[0];
     }
     if($len == 2){
         return $num.'in'.$rangeArr[1];
     }
     if($len == 1){
         return $num.'in'.$rangeArr[0];
     }
     $half  =(int)floor($len/2);
     if($num<$rangeArr[$half]){
         $tmp = array_slice($rangeArr,0,$half+1);
         return findPoint($num,$tmp);
     }else{
         $tmp = array_slice($rangeArr,$half,$len-$half);
         return findPoint($num,$tmp);
     }
 }
 echo findPoint(4,$rangeArr);exit;