Identifying Entities Across Worlds (Networking)

Background

Creating multiplayer games is tough with artemis-odb. It’s hard to synchronize two systems. I’ve read an issue #103 of @DaanVanYperen who discussed creating a co-op multiplayer game example, but wasn’t able to find said example.

In that thread, @junkdog mentioned ‘slave-worlds’ and ‘nested-worlds’ but I couldn’t find any documentation with those keywords and didn’t really know what he meant.

Global Entity Identification

I hope to identify entities globally (across worlds) using the following simple structure:

Server -> “master world” assigns entityIDs Client -> “slave world” uses IdEntityManager (similar to UuidEntityManager) This way, entities are identified using the master world’s entityID assignment.

Can anyone foresee any complications in this setup? Would you agree that this is simpler than having a custom EntityManager & World?

[Wulf]

Okay so this was what I went with, and it turned out just fine:

On the server, there was a World object, which created entities by itself. On the client, there was a World object, which created entities by itself and also assigned the entity another ID - a “server id” using an IdEntityManager.

Here’s the IdEntityManager that I used on the client which helped me attach another id to an entity. It’s just a manager (which are now systems), so add it and use it as you would any other System!

import java.util.HashMap;
import java.util.Map;

import com.artemis.Entity;
import com.artemis.Manager;
import com.artemis.utils.Bag;

/**
 * Helps manage entities in a slave-world (i.e. a client-side world).
 */
public class IdEntityManager extends Manager {
    private final Map<Integer, Entity> idToEntity;
    private final Bag<Integer> entityToId;

    public IdEntityManager() {
        this.idToEntity = new HashMap<Integer, Entity>();
        this.entityToId = new Bag<Integer>();
    }

    @Override
    public void deleted(Entity e) {
        Integer id = entityToId.safeGet(e.getId());
        if (id == null) {
            return;
        }

        Entity oldEntity = idToEntity.get(id);
        if(oldEntity != null && oldEntity.equals(e)) {
            idToEntity.remove(id);
        }

        entityToId.set(e.getId(), null);
    }

    public Entity getEntity(int id) {
        return idToEntity.get(id);
    }

    /**
     * @param e Entity to get ID for
     * @return MAY RETURN NULL
     */
    public int getId(Entity e) {
        int id = entityToId.safeGet(e.getId());

        return id;
    }

    public void setId(Entity e, int newId) {
        Integer oldId = entityToId.safeGet(e.getId());
        if (oldId != null)
            idToEntity.remove(oldId);

        idToEntity.put(newId, e);
        entityToId.set(e.getId(), newId);
    }
}

Hope this helps.

[Wulf]

My reasoning for not using UuidEntityManager was that it was too expensive (in terms of bandwidth over a network) to send a whole Java UUID (something like d5628473-e293-4288-8db4-80fc1e5a349e).

This may be feasible for your situation, but my application required sending many packets per second, so I was very bitter about ever bit 😉