Using Photon Engine Interest Groups to Limit Network Traffic in Unity Multiplayer

How Interest Groups Reduce Traffic

Interest Groups let a client tell the Photon server which event groups it wants to receive. The server then forwards only RaiseEvent calls that target those groups, cutting unnecessary UDP packets.

Worked Example: Zone‑Based Subscription

Assume a game world divided into zones numbered 1‑10. Each player subscribes to the group that matches their current zone.

using Photon.Pun;
using UnityEngine;

public class ZoneInterest : MonoBehaviourPunCallbacks
{
    // Change this when the player moves to a new zone
    private byte currentZoneGroup = 1;

    private void Start()
    {
        // Subscribe to the default group (0) and the zone group
        PhotonNetwork.SetInterestGroups(new byte[]{0, currentZoneGroup}, new byte[]{0});
    }

    public void SetZone(byte zoneId)
    {
        if (zoneId == currentZoneGroup) return;
        // Unsubscribe from the old zone group
        PhotonNetwork.SetInterestGroups(new byte[]{0}, new byte[]{currentZoneGroup});
        // Subscribe to the new zone group
        PhotonNetwork.SetInterestGroups(new byte[]{0, zoneId}, new byte[]{0});
        currentZoneGroup = zoneId;
    }

    // Example of sending a zone‑specific update
    public void SendZoneUpdate(string message)
    {
        // RaiseEvent with the current zone group as the target group
        PhotonNetwork.RaiseEvent(
            eventCode: 101,
            customData: "message",
            raiseEventOptions: new RaiseEventOptions { TargetGroup = currentZoneGroup },
            sendOptions: SendOptions.SendReliable);
    }
}

Limits and Considerations

• Photon Server supports a maximum of 255 interest groups (IDs 0‑254 usable for custom groups, ID 0 reserved for default).
• A client may subscribe to many groups, but each additional group adds server‑side bookkeeping; keep the number low.
• Interest group subscriptions are not stored when a player leaves and re‑joins a room; you must restore them after rejoining.

Common Mistakes to Avoid

• Leaving a zone without unsubscribing from its group – the server continues to send irrelevant events.
• Using group ID 0 for custom logic; ID 0 is the default group that all clients receive unless explicitly removed.
• Assuming that setting TargetGroup in RaiseEvent automatically filters receivers; the sender must also have the group in its interest list for the event to be delivered to others.

To verify, enable Photon's NetworkStatisticsTrace in the PhotonServerSettings and watch the log for ReceivedEvents count; only events matching the subscribed group should increase.