How is RPC execution order determined on remote clients receiving RPC?

[zachstronaut]

Description

How is RPC execution order determined on the clients receiving RPCs?

I have a host with one client.

I have a scene with a FruitRpcTest object and a Banana object. And I have an Apple prefab.

There are three scripts:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using Unity.Netcode;

public class FruitRpcTest : NetworkBehaviour
{
    public NetworkObject applePrefab;
    public Banana bananaObject;

    [ContextMenu("Setup Test")]
    void SetupTest()
    {
        if (NetworkManager.ConnectedClientsIds.Count != 2)
        {
            Debug.LogError("Client was not connected.");
            return;
        }

        var temp = GameObject.Instantiate(applePrefab);
        NetworkObject networkObj = temp.GetComponent<NetworkObject>();
        networkObj.SpawnWithOwnership(1);

        bananaObject.NetworkObject.ChangeOwnership(1);
    }

    [ContextMenu("Run Rpcs")]
    void RunRpcs()
    {
        var apple = GameObject.FindObjectOfType<Apple>();
        apple.banana = bananaObject;

        Debug.LogError("Send AppleRpc");
        apple.AppleRpc();
    }
}

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using Unity.Netcode;

public class Apple : NetworkBehaviour
{
    public Banana banana { get; set; }

    [Rpc(SendTo.Everyone, RequireOwnership = true)]
    public void AppleRpc()
    {
        Debug.LogError("Recv AppleRpc");
        
        if (IsOwner)
        {
            Debug.LogError("Send BananaRpc");
            banana.BananaRpc();
        }
    }
}

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using Unity.Netcode;

public class Banana : NetworkBehaviour
{
    [Rpc(SendTo.NotMe, RequireOwnership = true)]
    public void BananaRpc()
    {
        Debug.LogError("Recv BananaRpc");
    }
}

SetupTest() is run on the host and then RunRpcs() is run on the client.

Client log output is:

Send AppleRpc
Recv AppleRpc
Send BananaRpc

Host log output is:

Recv BananaRpc
Recv AppleRpc

Is this reverse receive order by chance or by design?

Netcode Version 2.6.0

[EmandM]

Rpcs are invoked locally before they are sent to other clients, so if further Rpcs are sent from within a local RPC, they will be sent before the outer RPC is sent.

That's a good catch as something to add to our docs.

[zachstronaut]

@EmandM using DeferLocal = true on the AppleRpc seems to restore the call order.

[EmandM]

Yeah, that'd track. DeferLocal will then delay the RPC call until after the messages have been sent.

[NoelStephensUnity]

@zachstronaut

Sounds like this isn't an issue? Could the documentation clarity be improved?

• AppleRpc (invoked locally and immediately)
  • Invokes BananaRpc (does not invoke locally)
    • BananaRpc message queued. (1st)
  • AppleRpc message queued. (2nd)

Message order seen by receiving clients: [BannaRpc message][AppleRpc message]

If you defer the invocation to simulate it acting more like a message being received (it defers to the next frame), then:

• AppleRpc ( local invoke deferred to next frame)
  • AppleRpc message queued. (1st)
• (next frame)
  • Invokes AppleRpc locally
    • BananaRpc message queued. (2nd)

Message order seen by receiving clients: [AppleRpc message][BananaRpc message]