Custom Side Channels

You can create your own side channel in C# and Python and use it to communicate custom data structures between the two. This can be useful for situations in which the data to be sent is too complex or structured for the built-in FloatPropertiesChannel, or is not related to any specific agent, and therefore inappropriate as an agent observation.

Overview

In order to use a side channel, it must be implemented as both Unity and Python classes.

Unity side

The side channel will have to implement the SideChannel abstract class and the following method.

OnMessageReceived(byte[] data) : You must implement this method to specify what the side channel will be doing with the data received from Python. The data is a byte[] argument.

The side channel must also assign a ChannelId property in the constructor. The ChannelId is a Guid (or UUID in Python) used to uniquely identify a side channel. This Guid must be the same on C# and Python. There can only be one side channel of a certain id during communication.

To send a byte array from C# to Python, call the base.QueueMessageToSend(data) method inside the side channel. The data argument must be a byte[].

To register a side channel on the Unity side, call Academy.Instance.RegisterSideChannel with the side channel as only argument.

Python side

The side channel will have to implement the SideChannel abstract class. You must implement :

on_message_received(self, data: bytes) -> None : You must implement this method to specify what the side channel will be doing with the data received from Unity. The data is a byte[] argument.

The side channel must also assign a channel_id property in the constructor. The channel_id is a UUID (referred in C# as Guid) used to uniquely identify a side channel. This number must be the same on C# and Python. There can only be one side channel of a certain id during communication.

To assign the channel_id call the abstract class constructor with the appropriate channel_id as follows:

super().__init__(my_channel_id)

To send a byte array from Python to C#, call the super().queue_message_to_send(bytes_data) method inside the side channel. The bytes_data argument must be a bytes object.

To register a side channel on the Python side, pass the side channel as argument when creating the UnityEnvironment object. One of the arguments of the constructor (side_channels) is a list of side channels.

Example implementation

Below is a simple implementation of a side channel that will exchange ascii encoded strings between a Unity environment and Python.

Example Unity C# code

The first step is to create the StringLogSideChannel class within the Unity project. Here is an implementation of a StringLogSideChannel that will listen for messages from python and print them to the Unity debug log, as well as send error messages from Unity to python.

using UnityEngine;
using MLAgents;
using MLAgents.SideChannels;
using System.Text;
using System;

public class StringLogSideChannel : SideChannel
{
    public StringLogSideChannel()
    {
        ChannelId = new Guid("621f0a70-4f87-11ea-a6bf-784f4387d1f7");
    }

    public override void OnMessageReceived(byte[] data)
    {
        var receivedString = Encoding.ASCII.GetString(data);
        Debug.Log("From Python : " + receivedString);
    }

    public void SendDebugStatementToPython(string logString, string stackTrace, LogType type)
    {
        if (type == LogType.Error)
        {
            var stringToSend = type.ToString() + ": " + logString + "\n" + stackTrace;
            var encodedString = Encoding.ASCII.GetBytes(stringToSend);
            base.QueueMessageToSend(encodedString);
        }
    }
}

Once we have defined our custom side channel class, we need to ensure that it is instantiated and registered. This can typically be done wherever the logic of the side channel makes sense to be associated, for example on a MonoBehaviour object that might need to access data from the side channel. Here we show a simple MonoBehaviour object which instantiates and registeres the new side channel. If you have not done it already, make sure that the MonoBehaviour which registers the side channel is attached to a gameobject which will be live in your Unity scene.

using UnityEngine;
using MLAgents;


public class RegisterStringLogSideChannel : MonoBehaviour
{

    StringLogSideChannel stringChannel;
    public void Awake()
    {
        // We create the Side Channel
        stringChannel = new StringLogSideChannel();

        // When a Debug.Log message is created, we send it to the stringChannel
        Application.logMessageReceived += stringChannel.SendDebugStatementToPython;

        // The channel must be registered with the Academy
        Academy.Instance.RegisterSideChannel(stringChannel);
    }

    public void OnDestroy()
    {
        // De-register the Debug.Log callback
        Application.logMessageReceived -= stringChannel.SendDebugStatementToPython;
        if (Academy.IsInitialized){
            Academy.Instance.UnregisterSideChannel(stringChannel);
        }
    }

    public void Update()
    {
        // Optional : If the space bar is pressed, raise an error !
        if (Input.GetKeyDown(KeyCode.Space))
        {
            Debug.LogError("This is a fake error. Space bar was pressed in Unity.");
        }
    }
}

Example Python code

Now that we have created the necessary Unity C# classes, we can create their Python counterparts.

from mlagents_envs.environment import UnityEnvironment
from mlagents_envs.side_channel.side_channel import SideChannel
import numpy as np
import uuid


# Create the StringLogChannel class
class StringLogChannel(SideChannel):

    def __init__(self) -> None:
        super().__init__(uuid.UUID("621f0a70-4f87-11ea-a6bf-784f4387d1f7"))

    def on_message_received(self, data: bytes) -> None:
        """
        Note :We must implement this method of the SideChannel interface to
        receive messages from Unity
        """
        # We simply print the data received interpreted as ascii
        print(data.decode("ascii"))

    def send_string(self, data: str) -> None:
        # Convert the string to ascii
        bytes_data = data.encode("ascii")
        # We call this method to queue the data we want to send
        super().queue_message_to_send(bytes_data)

We can then instantiate the new side channel, launch a UnityEnvironment with that side channel active, and send a series of messages to the Unity environment from Python using it.

# Create the channel
string_log = StringLogChannel()

# We start the communication with the Unity Editor and pass the string_log side channel as input
env = UnityEnvironment(base_port=UnityEnvironment.DEFAULT_EDITOR_PORT, side_channels=[string_log])
env.reset()
string_log.send_string("The environment was reset")

group_name = env.get_agent_groups()[0]  # Get the first group_name
for i in range(1000):
    step_data = env.get_step_result(group_name)
    n_agents = step_data.n_agents()  # Get the number of agents
    # We send data to Unity : A string with the number of Agent at each
    string_log.send_string(
        "Step " + str(i) + " occurred with " + str(n_agents) + " agents."
    )
    env.step()  # Move the simulation forward

env.close()

Now, if you run this script and press Play the Unity Editor when prompted, the console in the Unity Editor will display a message at every Python step. Additionally, if you press the Space Bar in the Unity Engine, a message will appear in the terminal.

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