By: David Pankhurst

David Pankhurst — Thu, 03 Apr 2008 06:42:22 +0000

…of course, the above assumes all data is equal, and any server pick is as good as another. If you need consistency (say like the original article, where a specific key goes to the same server each time) then you don’t want mt_rand() – the recommendation of md6(keyvalue) works well until # of servers change.

By: David Pankhurst

David Pankhurst — Thu, 03 Apr 2008 05:30:37 +0000

This is actually an old trick for randomly picking from uneven lists:
1-Create an array, with each entry representing the server capacity.
2-Now pick a random number from this total.
3-Find the server that represents that position in the list.
Eg:
$server=array(10,50,20,7);
$x=mt_rand(0,array_sum($server)-1);
$i=-1;
while ($x-$server[++$i]>=0) $x-=$server[$i];
// or more cryptic but faster:
// while ( ($x-=$server[++$i]) >= 0 ) {;}

The server to use is ‘$i’.

You can get fancier, but that will split based on ‘how much’ each can handle of the whole.

Comments on: Consistent Hashing

By: David Pankhurst

By: David Pankhurst