CareerCup

I would ask the interviewer a few questions:
1. If a server is already processing a message, can we assign it one more? (i.e. is the server capable of queuing up messages or processing them in parallel?)
2. If No, is it OK to assign fewer messages to a slow server?

I would start with a simple round robin solution like this and improve on that based on the answers to the above:

package com.test;

public class RoundRobinDistribution {
    public static void main(String[] args) {
        String[] servers = new String[] { "S1", "S2", "S3"};
        Distributor distributor = new Distributor(servers);
        for(int i=0; i < 5; i++)
            System.out.println("Message " + i + " to " + distributor.getNextServer());
    }

    private static class Distributor {
        private String[] servers;
        private int index = -1;

        Distributor(String[] servers) {
            if (servers == null || servers.length == 0)
                throw new IllegalArgumentException("Servers list is empty");
            this.servers = servers;
        }

        public String getNextServer() {
            index = (index + 1) % servers.length;
            return servers[index];
        }
    }
}

Hash = MessageID % N (mod N)