Sharded load balancing with nginx and perl

sharded-balancing-nginx.txt

    # Balancing over 4 nodes by hashing URI (consistently with md5) 
    # onto 16 shards. 

    upstream x0 { server 192.168.0.2;  server 192.168.0.3 backup; }
    upstream x1 { server 192.168.0.2;  server 192.168.0.4 backup; }
    upstream x2 { server 192.168.0.2;  server 192.168.0.5 backup; }
    upstream x3 { server 192.168.0.2;  server 192.168.0.3 backup; }

    upstream x4 { server 192.168.0.3;  server 192.168.0.4 backup; }
    upstream x5 { server 192.168.0.3;  server 192.168.0.5 backup; }
    upstream x6 { server 192.168.0.3;  server 192.168.0.2 backup; }
    upstream x7 { server 192.168.0.3;  server 192.168.0.4 backup; }

    upstream x8 { server 192.168.0.4;  server 192.168.0.5 backup; }
    upstream x9 { server 192.168.0.4;  server 192.168.0.2 backup; }
    upstream xa { server 192.168.0.4;  server 192.168.0.3 backup; }
    upstream xb { server 192.168.0.4;  server 192.168.0.5 backup; }

    upstream xc { server 192.168.0.5;  server 192.168.0.2 backup; }
    upstream xd { server 192.168.0.5;  server 192.168.0.3 backup; }
    upstream xe { server 192.168.0.5;  server 192.168.0.4 backup; }
    upstream xf { server 192.168.0.5;  server 192.168.0.2 backup; }

    perl_set $x 'sub {
        use Digest::MD5 qw(md5_hex);

        my $r = shift;
        my $uri = $r->uri;
        my $shard = substr md5_hex($uri), 0, 1;

        "x$shard"
    }';

    server {
        location / {
            proxy_pass http://$x;
        }
    }