PHP: Function to shard/distribute (consistent hashing) ?

Question

Hi everybody,

I have thought a bit about making a somewhat lightweight consistent-hashing-like PHP function to shard uploaded files between different servers.

Obviously, rand() would work to distribute the files among the servers somewhat evenly, but when requesting the files, no one will know which file lies on what server...

I know that there's some extensive libraries out there to create consistent-hashing, but I wonder how these works and how I can do to roll out my own, very lightweight one?

Note: I do not take into account that servers will be removed, but instead more ones added to the pool further on.

Thanks!

Update:

Here's a quick line of psuedocode:

$config['shards'] = array('192.168.1.1, 192.168.1.2');

function shard ($filename) {

    $servers = $config['shards'];

    // do lookup in some magic way to decide which server to return.

    return $appropriateserver;
}


echo shard('filename.jpg'); // returns the appropriate server to distribute the file.

Answer 1

Well, one thing you could do would be to use crc32...

$crc = crc32($mykey);
$serverNo = $crc % count($servers);

It should be fairly consistent (meaning evenly balanced), and 100% reproducible...