Get started

coherence is a network engine, platform and a series of tools to help anyone create a multiplayer game. Our mission is to give any game developer, regardless of how technical they are, the power to make a connected game.

Update an existing project

If you are an existing user and looking to update, you can find our change log.

Tutorial project

The Network Playground is a collection of scenes showing you how to use various features of the coherence Unity SDK. It shows you how to synchronize transforms, physics, persistence, animations, AI navigation and send network commands.

Existing or new Unity project

You can follow our step-by-step guide to learn how to install coherence in Unity, set up your scene, prefabs, interactions, as well as deploy your project to be shared with your friends.

Join the community

Join our community Discord for community chatter and support.

• Join our official Developer Discord channel.

• Contact us at devrel@coherence.io

Rooms

Rooms are best for session-based gameplay where the match between players takes place in a short-lived environment.

Use Case

A good example is a first person shooter multiplayer match. The match takes place between two teams in a single game session, and players enter through a lobby and matchmaking. When the match is concluded, the multiplayer environment the match took place in is closed and players return to a lobby.

This is one example of how Rooms can be used, but it is by no means the only use case. The important distinction between Rooms and Worlds (see below) is that Rooms are relatively short-lived and are meant to be created and closed by the game client through the coherence SDK.

See Rooms API.

Worlds

Worlds, as opposed to Rooms, are long-lived and permanent multiplayer environments provided by coherence. Using the Developer Portal, your project will easily define and manage your World configurations.

See Manage Worlds.

Use Case

A good example of a World is a permanent environment for an Massively Multiplayer Game (MMO). Regardless of the number of players connected, the environment is always available, and players can connect and disconnect at will.

Entities can be permanently saved in the world so that even if there are no active connections, they are persisted when players do connect.

See Worlds API.

Rooms and Worlds Work Together

Your project does not have to choose one-or-the-other. A project in coherence can contain both World and Rooms.

Use Case

A good example of this scenario is again, our MMO. Although players connect to a permanent and persistent World, they may enter a dungeon instance with other players. These dungeon instances can be Rooms.

The primary difference in the configuration and usage of Room and Worlds is that Worlds are managed in the Developer Portal, whereas Rooms are created and managed through the SDK.

coherence is currently in private preview. Some stability and performance issues may still be present and being ironed out. Additionally, more features are planned for the public release.

Current features

General

• Custom UDP transport layer using bit streams with reliability

• Smooth state replication

• Server-side, client-side, distributed authority

• Connected entity support

• Fast authority transfer

• Remote messaging (RPC)

• Persistence

• Multiple examples and showcases

• Verified support for Windows, macOS, Linux, Android, iOS and WebGL

• Support for rooms and worlds

SDK

• Unity SDK with intuitive no-code layer

• Per-field adjustable interpolation and extrapolation

• Input queues

• Easy deployment into the cloud

• SDK source included, no 3rd-party libraries

Optimization and performance

• Per-field compression and quantization

• Per-field sampling frequency adjustable at runtime

• Unlimited per-field levels of detail

• Areas of interest

• Accurate SimulationFrame tracking

Online

• Developer portal with server and service configurator

• Multiple regions (US East, EU Central)

• Player accounts

• Key-value store

• Matchmaking

• Ability to deploy one replication server and one simulation server per environment

• Prometheus and Grafana integration

Coming soon

SDK

• Multi-room simulators

• Input queue UX improvements

• Network Profiler

Online

• Additional regions

• Support for multiple simulators and replicators in a single project

• Dashboard with usage statistics

Mid-term roadmap

• Support for lean pure C# clients and simulators without Unity

• Peer-to-peer (without replication server) with NAT punch-through

• TCP fallback support

• WebSockets support

• MTU detection

• Packet replay

• Ability to deploy multiple simulation servers per environment

• Player analytics

• Developer portal graphs and analytics

• Simulator authentication

• Bare-metal and cloud support

Roadmap

• JavaScript SDK

• Unreal Engine SDK

• Multiple replication servers per game world

• Customer-specific serialization

• User-space load-balancing (SDK framework)

• Game world map with admin interface

• Anti-cheat functionality

• Advanced transaction logs (audit trail)

• Schema versioning (hot updates)

Basic Terms

Replication Server ("Replicator")

A lean and performant server that keeps the state of the world and replicates it efficiently between various simulators and game clients. The Replicator usually runs in the coherence Cloud, but developers can start it locally from the command line or the Unity Editor.

Game Client

A build of the game. To connect to coherence, it will use the coherence SDK.

Simulation Server ("Simulator")

A version of the game client without the graphics ("headless client") optimized and configured to perform server-side simulation of the game world. When we say something is simulated on the server, we mean it is simulated on one or several simulators.

Schema

A text file defining the structure of the world from the network's point of view. The schema is shared between the replicators, simulators and game clients. The world is generally divided in components and archetypes.

Code Generation

The process of generating code specific to the game engine that takes care of network synchronization and other network-specific code. This is done using a CLI tool called Protocol Code Generator that takes the schema file and generates code for various engines (e.g. C# for Unity).

State Replication

The process of making sure the state of the world is eventually the same on the replicator, simulators and game clients, depending on their areas of interest.

State Replication

coherence works by sharing game world data via a Replication Server in the cloud and passing it to the connected clients.

The clients and simulators can define areas of interest (LiveQueries), levels of detail, varying simulation and replication frequencies and other optimization techniques to control how much bandwidth and CPU power is used in different situations.

The game world can be run using multiple simulators that split up simulation functions or areas of the world accordingly.

Fast authority transfer and remote commands allow different authority models, including client authority, server authority, distributed authority and combinations like client prediction with input queues.

The platform handles scaling, synchronization, persistence and load balancing automatically.

Setup Video Tutorial

It's quick and easy to set up a networked scene from scratch using the coherence SDK. This example will show you the basic steps to sync up some moving characters.

Add these components to your scene to prepare it for network synchronization.

1. Add MonoBridge

coherence -> Scene Setup -> Create MonoBridge

This object takes care of connected GameObject lifetimes and allows us to develop using traditional MonoBehaviour scripts.

2. Add LiveQuery - Hierarchy

coherence -> Scene Setup -> Create LiveQuery

3. Add the Sample UI

coherence -> Scene Setup -> Add Sample UI

Creates a Canvas (and Event System if not already present in the scene) with a sample UI that helps you connect to a local or remote replication server. You can create your own connection dialog, this one is just a quick way to get started.

Now we can build the project and try out network replication locally.

This example will show you how to launch a local and connect multiple instances.

Running a Replication Server Locally

You can run a local replication server from the coherence menu by clicking:

coherence -> Server -> Run Local Worlds Server.

This will open a new terminal window with the replication server and world created in it.

Build and Connect

Now it's time to make a standalone build and test network replication.

Open the Build Settings window (File -> Build Settings). Click on Add Open Scenes to add the current scene to the build. Click Build and Run.

Select a folder (e.g. builds) and click OK.

When the build is done, start another instance of the executable (or run the project in the Game Window in Unity).

Click Connect on both clients. Now try focusing one and using WSAD keys. You will see the box move on the other side as well.

Connect across the local network

If you want to connect to the local replication server from another local device (such as another PC, Mac, Mobile or VR device), you can find your IPv4 address and use that as your server address in the Connect dialog. These devices need to be connected to the same network.

You can find your IPv4 address by going to your command line tool and type ipconfig . Remember to include the port number, for example 192.168.1.185:32001.

Cloud Deploy Video Tutorial

Now that we have tested our project locally, it's time to upload it to the cloud and share it with our friends and colleagues. To be able to do that, we need to create a free account with coherence.

Sign up or log in

Create an account or log into an existing one.

Create an organization

Create an organization.

Create project

Under the organization, create a new project.

Get portal token

Copy the token to your clipboard.

Copy Portal token into Unity

Open Unity and navigate to the Project Settings. Open the coherence tab.

Paste the token into the Portal Token section.

Fetch the Runtime key

Once you have pasted the portal token successfully, you need to fetch the runtime token as well.

Baking Video Tutorial

Overview

Out of the box, coherence will use C# Reflection to sync all the data at runtime. This is a great way to get started but it is very costly performance-wise.

coherence offers an automatic way of doing that called baking.

Bake schemas

Click on coherence -> Schema and Baking -> Bake Schemas.

This will go through all CoherenceSync components in the project and generate a schema file based on the selected variables, commands and other settings. It will also take into account any LODs added.

For every prefab with a CoherenceSync object, the baking process will generate a bespoke C# file in the coherence/baked folder in the project.

Adding that file to the prefab will make that prefab use bespoke generated code instead of C# reflection.

Activate baked schema on prefab

Once the Schema has been baked, you will be able to switch to baked mode in the CoherenceSync inspector.

The name of the baked script will be CoherenceSync[prefabName].

Modifying bound data, safe mode and the watchdog

When you bind to your script's fields and bake, coherence generates specific code that accesses your code directly, without using reflection. This means, whenever you change your scripts, you might break compilation.

For example, if you have a Health.cs script which exposes a public float health; field, and you mark health as a binding in the Bindings window, the baked script will access your component via type name, and your field via field name.

Your baked script might now reference your component:

This means that if you decide to change your component name (Health) or any of your bound field names (health), Unity script recompilation will fail. In this example, we will be deprecating health and adding health2 in its place.

Our watchdog is able to understand when this happens, and offer you a solution right away.

It will suggest you to bake in safe mode, and then diagnose the state of your prefabs. After a few seconds of script recompilation, you'll be presented with the diagnosis window.

In this window, you can easily spot bindings in your prefabs that are no longer valid. In our example, health is no longer valid since we've moved it elsewhere (or deleted it).

Click on the hand pointing button to open the bindings window, and take a look at your script:

Now, we can manually rebind our data: unbind health and bind health2. Once we do, we can now safely bake again.

Game Engine Support

coherence currently supports Unity. For custom engine integration, please contact our developer relations team. For updates regarding Unreal Engine support, please check the Unreal Engine support page.

Unity Requirements

• Unity 2020.3.0f1 or later.

• A Windows, Linux or macOS system.

Persistent vs. session-based entities

When we connect to a game world with a game client, the traditional approach is that all entities originating on our client are session-based. This means that when the client disconnects, they will disappear from the network world for all players.

A persistent object, however, will remain on the replication server even when the client or simulator that created or last simulated it, is gone.

This allows us to create a living world where player actions leave lasting effects.

In a virtual world, examples of persistent objects are:

• A door anyone can open, close or lock

• User-generated or user-configured objects left in the world to be found by others

• Game progress objects (e.g. in PvE games)

• Voice or video messages left by users

• NPC's wandering around the world using an AI logic

• Player characters on "auto pilot" that continue affecting the world when the player is offline

• And many, many more

A persistent object with no simulator is called an orphan. Orphans can be configured to be auto-adopted by clients or simulators on a FCFS basis.

In this section, we will learn how to prepare a prefab for network replication.

1. Create a prefab and move it to Resources

Add an asset and create a prefab of it. Make sure the prefab is in a Resources folder in your Unity project.

Here is an example:

1.1 Create a simple Cube in the scene

GameObject -> 3D Object -> Cube

1.2 Convert the Cube into a prefab

Create a Resources folder in your Project. Drag the Cube into the Resources folder to turn it into a prefab.

2. Add CoherenceSync to the prefab

The CoherenceSync component will help you prepare an object for network synchronization during design time. It also exposes an API that allows us to manipulate the object during runtime.

CoherenceSync will query all public variables and methods on any of the attached components, for example Unity components such as Transform, Animator , etc.. This will include any custom scripts such as PlayerInput and even scripts that came with Asset Store packages you may have downloaded.

3. Select variables to replicate

Select which variables you would like to sync across the network. Initially, this will probably be the Transform settings; position, rotation, scale.

Under Configure, click Select fields and methods.

In the Configuration dialog, select position, rotation and scale.

Close the Configuration dialog.

4. Add an input script

This simple input script will use WASD or the Arrow keys to move the prefab around the scene.

Click on Assets -> Create -> C# Script.

Name it Move.cs. Copy-paste the following content into the file.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Move : MonoBehaviour
{
    public float speed = 1f;

    void Update()
    {
        float h = Input.GetAxisRaw("Horizontal");
        float v = Input.GetAxisRaw("Vertical");
    
        var spf = speed * Time.deltaTime;

        transform.position += transform.forward * (v * spf);
        transform.position += transform.right * (h * spf);
    }
}

Wait for Unity to compile the file, then add it onto the prefab.

5. Disable Input on replicated object

We have added a Move script to the prefab. This means that if we just run the scene, we will be able to use the keyboard to move the object around.

But what happens on another client where this object is not authoritative, but rather replicated? We will want the position to be replicated over the network, without the keyboard input interfering with it.

Open the Events section in CoherenceSync. Add a new On Network Instantiation handler by clicking on the plus sign next to it.

Pull the Cube prefab into the Runtime Only / None (Object) field.

Now click the dropdown No Function and select Move -> bool enabled.

Leave the Boolean field unchecked.

6. Additional CoherenceSync settings

From the CoherenceSync component you can configure settings for Lifetime (Session-based or Persistent, Authority transfer (Request or Steal), Simulation model (Client Side, Server Side or Server Side with Client Input) and Adoption settings for when local persistent entities are orphaned.

You can find more information in the SDK Fundamentals. There are also some Events that are triggered at different times.

• On Before Networked Instantiation (before the network is instantiated)

• On Networked Instantiation (when the GameObject is instantiated)

• On Networked Destruction (when the GameObject is destroyed)

• On Authority Gained (when authority over the GameObject is transferred to the local client)

• On Authority Lost (when authority over the GameObject is transferred to another client)

• On After Authority Transfer Rejected (when GameObject's Authority transfer was requested and denied).

• On Input Simulator Connected (when client with simulator is ready for Server-side with Client Input)

• On Input Owner Assigned (when InputOwner was changed is ready)

  Extras

  Helper scripts and samples can be found here.

Overview

Commands are network messages sent from one entity to another entity in the networked world. Commands bind to public methods accessible on the GameObject that CoherenceSync sits on.

Design Phase

In the design phase, you can expose public methods the same way you select fields for synchronization: through the Configure window on your CoherenceSync component.

Selected public methods will be exposed as network commands in the baking process.

The button on the right of the method lets you choose the routing mode. Commands with aSend to Authority Only mode can be sent only to the authority of the target entity, while ones with the Send to All Instances can be broadcasted to all clients that have a copy of this entity. The routing is enforced by the Replication Server as a security measure so the outdated or malicious clients don't break the game.

Sending a Command on a Networked Object

To send a command, we call the SendCommand method on the target CoherenceSync object. It takes a number of arguments:

• The type argument (within the <and>) must be the type of the receiving MonoBehaviour. This ensures that the correct method gets called if the receiving GameObject has components that implement methods that share the same name.

• The first argument is the name of the method on the MonoBehaviour that we want to call. It is good practice to use the C# nameof expression when referring to the method name, since it prevents accidentally misspelling it, or forgetting to update the string if the method changes name.

• The second argument is an enum that specifies the MessageTarget of the command. The possible values are:

  • MessageTarget.All – this will send the command to each client that has an instance of this entity.

  • MessageTarget.AuthorityOnly – this will send the command only to the client that has authority over the entity.

  Mind that the target must be compatible with the routing mode set in the bindings, i.e. Send to authority will allow only for the MessageTarget.AuthorityOnly while Send to all instances allows for both values.

• The rest of the arguments (if any) vary depending on the command itself. We must supply as many parameters as are defined in the target method and the schema.

Here's an example of how to send a command:

var target = enemy.GetComponent<CoherenceSync>();
                    
target.SendCommand<Character>(nameof(SendDamage), 
                MessageTarget.All,
                damageValue, staminaBlockCost,
                staminaRecoveryDelay, ignoreDefense, 
                hitReaction, hitPosition, 
                ConnectionId, isPlayer);

Receiving a Command

We don't have to do anything special to receive the command. The system will simply call the corresponding method on the target network entity.

If the target is a locally simulated entity, SendCommand will recognize that and not send a network command, but instead simply call the method directly.

Sending a Command to multiple entities

Sometimes you want to inform a bunch of different entities about a change. For example, an explosion impact on a few players. To do so, we have to go through the instances we want to notify, and send commands to each of them.

using Coherence;
using Coherence.Toolkit;
using UnityEngine;

public class MyCommands : MonoBehaviour
{
    public void MulticastCommand()
    {
        var self = GetComponent<CoherenceSync>();
        var listeners = FindObjectsOfType<CoherenceSync>(); 
        
        foreach (var listener in listeners)
        {
            if (!listener || listener == self)
            {
                continue;
            }
            
            listener.SendCommand<MyCommands>(nameof(ReceiveCommand), MessageTarget.AuthorityOnly);
        }
    }

// bind to this method via the Bindings window
    public void ReceiveCommand()
    {
        Debug.Log("Received!");
    }
}

In this example, a command will get sent to each network entity under the authority of this client. To make it only affect entities within certain criteria, you need to filter to which CoherenceSync you send the command to on your own.

Supported types in Commands

When a prefab is not using a baked script there are some restrictions for what types can be sent in a single command:

• 4 integers

• 4 floats

• 4 bools

• 4 entity references

• 1 string

• 1 byte array

If you ever need to send commands containing any other type (like a vector) or a higher quantity (like two strings) you must mark the prefab to use baked code. This will generate a custom command that can handle all the supported primitive types of the coherence SDK.

Now we can finally deploy our schema and replication server into the cloud.

Upload schemas

In the Project Settings tab for coherence, click on Upload.

The status should change from "Unknown" to "In Sync".

Your project schema is now deployed with the correct version of the replication server already running in the cloud. You will be able to see this in your cloud dashboard status.

Run project

The Connect Dialog uses the runtime key to fetch all the regions available for your project. This depends on the project configuration (e.g. the regions that you have selected for your project in the portal).

You can now build the project again and send the build to your friends for testing.

You will be able to play over the internet without worrying about firewalls and local network connections.

Install Video Tutorial

Unity Package Manager

1. Add a scoped registry

First, open Project Settings.

Under Package Manager, add a new Scoped Registry with the following fields:

• Name: coherence

• URL: https://registry.npmjs.org

• Scope(s): io.coherence.sdk

• Enable Preview / Pre-release Packages: checked

• Show Dependencies: checked

Click Apply.

2. Find and install the coherence package

Now open the Package Manager.

Click Packages and My Registries.

Under coherence, click Install.

Alternative Method: Edit manifest.json manually

If you want to install coherence manually, go to the folder of your project and open the file /Packages/manifest.json.

Copy paste the lines surrounded by comments that look like this:/* comment */.

{
  "dependencies": {
    "com.unity.collab-proxy": "1.3.9",
    "com.unity.ide.rider": "2.0.7",
    "com.unity.ide.visualstudio": "2.0.7",
    "com.unity.ide.vscode": "1.2.3",
    "com.unity.test-framework": "1.1.24",
    "com.unity.textmeshpro": "3.0.1",
    "com.unity.timeline": "1.4.6",
    "com.unity.ugui": "1.0.0",

    /*** ADD THIS START ***/
    "io.coherence.sdk": "0.7.0",
    /*** ADD THIS END ***/
        
    "com.unity.modules.ai": "1.0.0",
    "com.unity.modules.androidjni": "1.0.0",
    "com.unity.modules.animation": "1.0.0",
    "com.unity.modules.assetbundle": "1.0.0",
    "com.unity.modules.audio": "1.0.0",
    "com.unity.modules.cloth": "1.0.0",
    "com.unity.modules.director": "1.0.0",
    "com.unity.modules.imageconversion": "1.0.0",
    "com.unity.modules.imgui": "1.0.0",
    "com.unity.modules.jsonserialize": "1.0.0",
    "com.unity.modules.particlesystem": "1.0.0",
    "com.unity.modules.physics": "1.0.0",
    "com.unity.modules.physics2d": "1.0.0",
    "com.unity.modules.screencapture": "1.0.0",
    "com.unity.modules.terrain": "1.0.0",
    "com.unity.modules.terrainphysics": "1.0.0",
    "com.unity.modules.tilemap": "1.0.0",
    "com.unity.modules.ui": "1.0.0",
    "com.unity.modules.uielements": "1.0.0",
    "com.unity.modules.umbra": "1.0.0",
    "com.unity.modules.unityanalytics": "1.0.0",
    "com.unity.modules.unitywebrequest": "1.0.0",
    "com.unity.modules.unitywebrequestassetbundle": "1.0.0",
    "com.unity.modules.unitywebrequestaudio": "1.0.0",
    "com.unity.modules.unitywebrequesttexture": "1.0.0",
    "com.unity.modules.unitywebrequestwww": "1.0.0",
    "com.unity.modules.vehicles": "1.0.0",
    "com.unity.modules.video": "1.0.0",
    "com.unity.modules.vr": "1.0.0",
    "com.unity.modules.wind": "1.0.0",
    "com.unity.modules.xr": "1.0.0"
  }, /* add this comma if not already present */
  
  /*** ADD THIS SECTION START ***/
  "scopedRegistries": [
    {
      "name": "coherence",
      "url": "https://registry.npmjs.org",
      "scopes": [
        "io.coherence.sdk"
      ]
    }
  ]
  /*** ADD THIS SECTION END ***/
  
}

When you install the coherence Unity Package, all the services for the SDK are installed in the background as well.

You will then see this package in the Package Manager under "My Registries".

The coherence SDK has some dependencies on other Unity packages you can see in the image above. If you're already using these in your project, you might have to adjust their version number (Unity will tell you about this).

When you successfully install the coherence SDK you'll get this quickstart winodow pop-up and you'll be good to go.

Overview

The state of network entities that are currently not simulated locally (either because they are being simulated on another game client or on a simulator) cannot be affected directly.

Network commands can help us affect state indirectly, but for anything more involved, an authority transfer might be necessary.

Types of Authority Transfer

In the design phase, CoherenceSync objects can be configured to handle authority transfer in different ways:

• Request. Authority transfer may be requested, but it may be rejected by the receiving party (i.e. when Approve requests is false).

• Steal. Authority will always be given to the requesting party on a FCFS ("first come first serve") basis.

• Not transferable. Authority cannot be transferred.

Note also that you need to set up Auto-adopt Orphan if you wan't orphans to be adopted automatically by nearest player.

If you uncheck the Approve Requests, you can use the unity event OnAuthorityRequest to respond to the request.

The request can be approved or rejected in the callback.

public void OnRequest(ushort client)
{
    var coherenceSync = target.GetComponent<CoherenceSync>();
    // do some extra checks
    // transfer authority
    coherenceSync.TransferAuthority(client);
    
    // or reject authority request
    // coherenceSync.RejectAuthorityRequest(client);
}

Requesting Authority in Code

Requesting authority is very straight-forward.

var coherenceSync = target.GetComponent<CoherenceSync>();
coherenceSync.RequestAuthority();

As the transfer is asynchronous, we have to subscribe to one or more Unity Events in CoherenceSync to learn the result.

The request will first go to the replication server and be passed onto the receiving simulator or game client, so it may take a few frames to get a response.

// There Unity Events in CoherenceSync help us understand 
// what happened with authority requests and act accordingly.

// called when the CoherenceSync entity becomes the simulation authority
public UnityEvent OnAuthorityGained;

// called when the CoherenceSync entity loses simulatino authority
public UnityEvent OnAuthorityLost;

// called when a request to assume authority over the CoherenceSync entity
// is rejected
public UnityEvent OnAuthorityTransferRejected;

These events are also exposed to the Unity inspector in the Events on authority transfer section of the CoherenceSync behaviour.

coherence allows you to upload and share the builds of your games to your team, friends or adoring fans via an eay access play link.

Right now we support desktop (PC, Mac, Linux) and also WebGL, where you can host and instantly play your multiplayer game and share it around the world.

Build the game

Build your game to a local folder on your desktop as you would normally.

In the coherence menu in Unity select "Share -> Build Upload"

Upload to coherence

in this window you can select which platform the build is for an you also need to browse to the local path folder.

Click "Upload" or "Begin Upload" and coherence will confirm that it's okay to compress and upload the build to the webdashboard.

Share your build

Now that build has been updated (signified by the green tick), you can share it by enabing and share the public URL. Anyone with this link can access the build.

WebGL

If you uploaded a WebGL build then you can play it instantly from that public link for instant play.

Unity Animator's parameters are bindable out of the box, with the exception of triggers.

Triggers

Now, bind to PlayJumpAnimator.

Overview

Client messages are a form of that are used for client-to-client communication rather than client-to-entity. To achieve this type of communication a special connection entity is automatically created by the for each connected client. Those entities are subject to a different rule set than standard entities. Connection entities:

• Can't be created or destroyed by the client - they are always replication server-driven

• Are global - they are replicated across clients regardless of the in-simulation distance or radius

Client messages shine whenever there's a need to communicate something to all the connected players. Usage examples:

• Global chat

• Game state changes: game started, game ended, map changed

• Server announcements

• Server-wide leaderboard

• Server-wide events

Enabling client messages

The globality of client messages doesn't fit all game types - for example, it rarely makes sense to keep every client informed about the presence of all players on the server in an MMORPG (think World Of Warcraft). Due to this reason, client messages are turned off by default. To enable client messages:

1. Turn on the global query in the (the Global Query On toggle)

Global query enables querying for entities containing a global component, including a connection entity.

2. Create a class that inherits from the CoherenceClientConnection class

3. Create a client connection prefab

4. Binding of the method to CoherenceSync

Next, we have to create a binding. Navigate to CoherenceSync component on your prefab and press Select, in appeared window open Methods tab and check OnChatMessage:​

5. Link the connection prefab to the MonoBridge

For the system to know which object to create for every new client connection, we have to link our prefab to the Coherence MonoBridge. Simply drag the prefab to the Client Connection Prefab field in the MonoBridge inspector:​

Connection management

Once we have everything set up, whenever a new client joins an instance of the connection prefab will be created. The instantiated object resembles a client connection and will be kept alive for as long as that client remains connected to the replication server.

To get notified about the creation and destruction of connections we can use the following events:

To get local connection or connection of any client by the ID use the following MonoBridge methods:

The CoherenceClientConnection class exposes the following properties:

Sending client messages

Client messages are sent using the SendClientMessage method of the CoherencePlayerConnection class:

The first message will be delivered only to the player that owns the created connection. The other message will be delivered to all instances of this connection, that is, every player (including the sender) that knows about this connection will receive this message.

If the connection ID of the message recipient is known we can use the MonoBridge to send a client message:

Overview

coherence input queues are backed by a rolling buffer of inputs transmitted between the clients. This buffer can be used to build a fully deterministic simulation with a client side-prediction, rollback, and input delay. This game networking model is often called the (Good Game Peace Out).

Input delay allows for a smooth, synchronized netplay with almost no negative effect on the user experience. The way it works is input is scheduled to be processed X frames in the future. Consider a fighting game scenario with two players. At frame 10 Player A presses a kick button that is scheduled to be executed at frame 13. This input is immediately sent to Player B. With a decent internet connection, there's a big chance that Player B will receive that input even before his frame 13. Thanks to this the simulation is always in sync and can progress steadily.

Prediction is used to run the simulation forward even in the absence of inputs from other players. Consider the scenario from the previous paragraph - what if Player B doesn't receive the input on time? The answer is very simple - we just assume that the input state hasn't changed and progress with the simulation. As it turns out this assumption is valid most of the time.

Rollback is used to correct the simulation when our predictions turn out wrong. The game keeps historical states of the game for past frames. When an input is received for a past simulation frame the system checks whether it matches the input prediction made at that frame. If it does we don't have to do anything (the simulation is correct up to that point). If it doesn't match, however, we need to restore the simulation state to the last known valid state (last frame which was processed with non-predicted inputs). After restoring the state we re-simulate all frames up to the current one, using the fresh inputs.

Determinism

In a deterministic simulation, given the same set of inputs and a state we are guaranteed to receive the same output. In other words, the simulation is always predictable. Deterministic simulation is a key part of the GGPO model, as well as a lockstep model because it lets us run exactly the same simulation on multiple clients without a need for synchronizing big and complex states.

Implementing a deterministic simulation is a non-trivial task. Even the smallest divergence in simulation can lead to a completely different game outcome. This is usually called a desync. Here's a list of common determinism pitfalls that have to be avoided:

• Using Update to run the simulation (every player might run at a different frame rate)

• Using coroutines, asynchronous code, or system time in a way that affects the simulation (anything time-sensitive is almost guaranteed to be non-deterministic)

• Using Unity physics (it is non-deterministic)

• Using random numbers generator without prior seed synchronization

• Non-symmetrical processing (e.g. processing players by their spawn order which might be different for everyone)

• Relying on a across different platforms, compilations or processor types

Quickstart guide

We'll create a simple, deterministic simulation using provided utility components.

Our simulation will synchronize the movement of multiple clients, using the rollback and prediction in order to cover for the latency.

Player implementation

Start by creating a Player component that derives from the CoherenceClientConnection. Our Player represents a session participant and will be automatically spawned for each player that connects to the server. It will also be responsible for handling inputs using the CoherenceInput component.

Our Player code looks as follows:

The GetMovement and SetMovement will be called by our "central" simulation code. Now that we have our Player defined let's create a prefab for it:

A couple of things to note:

• A Mov axis has been added to the CoherenceInput which will let us sync the movement input state

• In the deterministic simulation, it is our code that is responsible for producing deterministic output on all clients. This means that the automatic transform position syncing is no longer desirable. To turn it off check the CoherenceSync > Simulation and Interpolation > Manual Position Update

• In order for inputs to be processed in a deterministic way, we need to use the fixed simulation frames. Tick the CoherenceInput > Use Fixed Simulation Frames checkbox

Since our player is using the CoherenceClientConnection we must set it as the connection prefab in the CoherenceMonoBridge and enable the global query:

State implementation

Before we move on to the simulation, we need to define our simulation state which is a key part of the rollback system. The simulation state should contain all the information required to "rewind" the simulation in time. For example, in a fighting game that would be a position of all players, their health, and perhaps a combo gauge level. In a shooting game, this could be player positions, their health, ammo, and a map objective progression.

In the example we're building player position is the only state. We need to store it for every player:

Simulation implementation

Simulation code is where all the logic should happen, including applying inputs and moving our Players:

• SetInputs is called by the system when it's time for our local Player to update its input state using the CoherenceInput

• Simulate is called when it's time to simulate a given frame. It is also called during frame re-simulation after misprediction - don't worry though, the complex part is handled by the CoherenceInputSimulation internals - all you need to do in this method is apply inputs from the CoherenceInput to run the simulation

• Rollback is where we need to set the simulation state back to how it was at a given frame. The state is already provided in the state parameter, we just need to apply it

• CreateState is where we create a snapshot of our simulation so it can be used later in case of rollback

• OnClientJoined and OnClientLeft are optional callbacks. We use them here to start and stop the simulation depending on the number of clients

Pause handling

There's a limit to how many frames can be predicted by the clients. This limit is controlled by the CoherenceInput.InputBufferSize. When clients try to predict too many frames into the future (more frames than the size of the buffer) the simulation will issue a pause. This pause affects only the local client. As soon as the client receives enough inputs to run another frame the simulation will resume.

To get notified about the pause use the OnPauseChange(bool isPaused) method from the CoherenceInputSimulation:

This can be used for example to display a pause screen the informs user about a bad internet connection.

Debugging

The CoherenceInputSimulation has a built-in debugging utility that collects various information about the input simulation on each frame. This data can prove extremely helpful in finding a simulation desync point.

Setup

Since debugging might induce a non-negligible overhead it is turned off by default. To turn it on add, a COHERENCE_INPUT_DEBUG scripting define:

From that point, all the debugging information will be gathered. The debug data is dumped to a JSON file as soon as the client disconnects. The file can be located under a root directory of the executable (in case of Unity Editor the project root directory) under the following name: inputDbg_<ClientId>.json, where <ClientId> is the CoherenceClientConnection.ClientId of the local client.

Data handling behaviour can be overridden by setting the CoherenceInputDebugger.OnDump delegate, where the string parameter is a JSON dump of the data.

The debugger is available as a property in the simulation base class: CoherenceInputSimulation.Debugger. Most of the debugging data is recorded automatically, however, the user is free to append any arbitrary information to a frame debug data, as long as it is JSON serializable. This is done by using the CoherenceInputDebugger.AddEvent method:

Since the simulation can span an indefinite amount of frames it might be wise to limit the number of debug frames kept by the debugging tool (it's unlimited by default). To do this use the CoherenceInputDebugger.FramesToKeep property. For example, setting it to 1000 will instruct the debugger to keep only the latest 1000 frames worth of debugging information in the memory.

Serialization

Since the debugging tool uses JSON as a serialization format, any data that is part of the debug dump must be JSON-serializable. An example of this is the simulation state. The simulation state from the quickstart example is not JSON serializable by default, due to Unity's Vector3 that doesn't serialize well out of the box. To fix this we need to give JSON serializer a hint:

With the JsonProperty attribute, we can control how a given field/property/class will be serialized. In this case, we've instructed the JSON serializer to use the custom UnityVector3Converter for serializing the vectors.

Comparing debug data

To find a problem in the simulation, we can compare the debug dumps from multiple clients. The easiest way to find a divergence point is to search for a frame where the hash differs for one or more of the clients. From there one can inspect the inputs and simulation states from previous frames to find the source of the problem.

Here's the debug data dump example for one frame:

Explanation of the fields:

• Frame - frame of this debug data

• AckFrame - the common acknowledged frame, i.e. the lowest frame for which inputs from all clients have been received and are known to be valid (not mispredicted)

• ReceiveFrame - the common received frame, i.e. the lowest frame for which inputs from all clients have been received

• AckedAt - a frame at which this frame has been acknowledged, i.e. set as known to be valid (not mispredicted)

• MispredictionFrame - a frame that is known to be mispredicted, or -1 if there's no misprediction

• Hash - hash of the simulation state. Available only if the simulation state implements the IHashable interface

• Initial state - the original simulation state at this frame, i.e. a one before rollback and resimulation

• Initial inputs - original inputs at this frame, i.e. ones that were used for the first simulation of this frame

• Updated state - the state of the simulation after rollback and resimulation. Available only in case of rollback and resimulation

• Updated inputs - inputs after being corrected (post misprediction). Available only in case of rollback and resimulation

• Input buffer states - dump of the input buffer states for each client. For details on the fields see the InputBuffer code documentation

• Events - all debug events registered in this frame

State hash

One of the things that are stored as part of the debugging information is the simulation state. In the case of a complex state searching for a difference across multiple clients and hundreds of frames can quickly become tedious. To simplify the problem we can use the hash calculation feature of the input debugger.

Every state simulation class/struct that implements the IHashable interface will have its hash automatically calculated and stored as part of the debugging information. The example of IHashable implementation:

Configuration

Input buffer

There are two main variables which affect the behaviour of the InputBuffer:

• Input buffer size - the size of the buffer determines how far into the future the input system is allowed to predict. The bigger the size, the more frames can be predicted without running into a pause. Note that the further we predict, the more unexpected the rollback can be for the player. The InitialBufferSize value can be set directly in code however it must be done before the Awake of the baked component, which might require a script execution order configuration.

• Input buffer delay - dictates how many frames must pass before applying an input. In other words, it defines how "laggy" the input is. The higher the value, the less likely clients are going to run into prediction (because a "future" input is sent to other clients), but the more unresponsive the game might feel. This value can be changed freely at runtime, even during a simulation (it is however not recommended due to inconsistent input feeling).

The other two options are:

• Disconnect on time reset - if set to 'true' the input system will automatically issue a disconnect on an attempt to resync time with the server. This happens when the client's connection was so unstable that frame-wise it drifted too far away from the server. In order to recover from that situation, the client performs an immediate "jump" to what it thinks is the actual server frame. There's no easy way to recover from such "jump" in the deterministic simulation code so the advised action is to simply disconnect.

• Use fixed simulation frames - if set to 'true' the input system will use the IClient.ClientFixedSimulationFrame frame for simulation - otherwise the IClient.ClientSimulationFrame is used. Setting this to 'true' is recommended for the deterministic simulation.

Fixed frame rate

The fixed network update rate is based on the Fixed Timestep configured through the Unity project settings:

To know the exact fixed frame number that is executing at any given moment use the IClient.ClientFixedSimulationFrame or CoherenceInputSimulation.CurrentSimulationFrame property.

Networked entities can be simulated either on a game client ("client authority") or a simulation server ("server authority).

Client Authority

Client authority is the easiest to set up initially, but it has some drawbacks:

• Higher latency. Because both clients have a non-zero ping to the replication server, the minimum latency for data replication and commands is the combined ping (client 1 to replications server and replication server to client 2).

• Higher exposure to cheating. Because we trust game clients to simulate their own entities, there is a risk that one such client is tampered with and sends out unrealistic data.

In many cases, especially when not working on a competitive PvP game, these are not really issues and are a perfectly fine choice for the game developer.

Client authority does have a few advantages:

• Easier to set up. No client vs. server logic separation in the code, no building and uploading of simulation servers, everything just works out of the box.

• Cheaper. Depending on how optimized the simulator code is, running a simulator in the cloud will in most cases incur more costs than just running a replication server (which is comparatively very lean).

Server Authority

Having one or several simulators taking care of the important world simulation tasks (like AI, player character state, score, health, etc.) is always a good idea for competitive PvP games.

Running a simulator in the cloud next to the replicator (the ping between them being negligible) will also result in lower latency.

The player character can also be simulated on the server, with the client locally predicting its state based on inputs. You can read more about how to achieve that in the section .

Remote Communication

What are Input Queues?

Input queues enable a simulator to take control of the simulation of another client's objects based on the client's inputs.

When To Use Input Queues?

• In situations where you want a centralized simulation of all inputs. Many game genres use input queues and centralized simulation to guarantee the fairness of actions or the stability of physics simulations.

• In situations where clients have low processing power. If the clients don't have sufficient processing power to simulate the world it makes sense to use input queue and just display the replicated results on the clients.

• In situations where determinism is important. RTS and fighting games will use input queues and rollback to process input events in a shared (not centralized), and deterministic, way so that all clients simulate the same conditions and produce the same results.

Setup With and CoherenceInput

Setting up an object to simulate via input queues using CoherenceSync is done in three steps:

1. Preparing the CoherenceSync component on the object prefab

The Simulation Type of the CoherenceSync component is set to Simulation Server With Client Input

2. Declaring Inputs for the simulated object

Each simulated CoherenceSync component is able to define its own, unique set of inputs for simulating that object. An input can be one of:

• Button. A button input is tracked with just a binary on/off state.

• Button Range. A button range input is tracked with a float value from 0 - 1.

• Axis. An axis input is tracked as two floats from 0 - 1 in both the X and Y axis.

To declare the inputs used by the CoherenceSync component, the CoherenceInput component is added to the object. The input is named and the fields are defined.

In this example, the input block is named "Player Movement" and the inputs are WASD and "mouse" for the XY mouse position.

3. Bake the CoherenceSync object

Using CoherenceInput

When a simulator is running it will find objects that are set up using CoherenceInput components and will automatically assume authority and perform simulations. Both the client and simulator need to access the inputs of the CoherenceInput of the replicated object. The client uses the Set* methods and the simulator uses the Get* methods to access the state of the inputs of the object. In all of these methods, the name parameter is the same as the Name field in the CoherenceInput component.

Client-Side Set* Methods

• public void SetButtonState(string name, bool value)

• public void SetButtonRangeState(string name, float value)

• public void SetAxisState(string name, Vector2 value)

Simulator-Side Get* Methods

• public bool GetButtonState(string name)

• public float GetButtonRangeState(string name)

• public Vector2 GetAxisState(string name)

For example, the mouse click position can be passed from the client to the simulator via the "mouse" field in the setup example.

The simulator can access the state of the input to perform simulations on the object which are then reflected back to the client as any replicated object is.

Input Owner

Each object only accepts inputs from one specific client, called the object's Input Owner.

When a client spawns an object it automatically becomes the Input Owner for that object. This way, the object's creator will retain control over the object even after authority has been transferred to the simulator.

If an object is spawned directly by the simulator, you will need to assign the Input Owner manually. Use the SetInputOwner method on the CoherenceInput component to assign or re-assign a client that will take control of the object:

Client-Side Prediction

Generally, using server-side simulation with simulators makes the time from the client providing input to the time the object is updated with that input significantly longer than just client-side simulation because of the time required for the input to be sent to the simulator, processed, and then the updates to the object returned across the network. This can cause visual lag. Using input queues allows the client to perform prediction of the simulation to provide a smooth playing experience.

If the client simulates the inputs on the object as well as applying them to the CoherenceInput component and then blends the authoritative results from the simulator with the locally simulated results, then smoother and more responsive simulation is achieved.

If we don't do any special configuration, entity data is captured at the highest possible frequency and sent to the replication server. This often generates more data than is needed to efficiently replicate the entity's state across the network.

Global simulator frequency

On a simulator, we can limit the framerate globally using Unity's built-in static variable targetFrameRate.

Application.targetFrameRate = 10;

Per-component frequency limitation

Sample rate can also be configured individually for all fields with code.

// Set position sample rate to 10hz
GetComponent<CoherenceSync>().SetSamplingFrequency<WorldPosition>(10);

// Set rotation sample rate to 20hz
GetComponent<CoherenceSync>().SetSamplingFrequency<WorldOrientation>(20);

Persistent objects are stored

All persistent objects remain in the world for the entire lifetime of the replication server and, periodically, the replication server records the state of the world and saves it to physical storage. If the replication server is restarted, then the saved persistent objects are reloaded when the replication server resumes.

Extrapolation or "dead reckoning" uses historical data to predict the future state of a component. The actual network data that arrives later can be used to interpolate or snap the predicted values to the correct ones.

While the basic case of direct parent-child relationships between entities is handled automatically by coherence, more complex hierarchies (with multiple levels) need a little extra work.

An example of such a hierarchy would be a synced Player prefab with a hierarchical bone structure, where you want to place an item (e.g. a flashlight) in the hand:

Player > Shoulder > Arm > Hand

A prefab can only have a single CoherenceSync script on it, so you can't add an additional one to the hand. In this case you need to use the CoherenceNode component. To do so, follow these steps:

1. Add the CoherenceNodecomponent to your child prefab. In the example above, that would be the flashlight you want your player to be able to pick up.

2. Select the two fields path and pathDirtyCounter for syncing.

You don't need to do any changes to the Player prefab, just make sure it has a CoherenceSync script in the root.

This setup allows you to place instances of this prefab anywhere in the hierarchy of another synced object. The one important constraint is that the hierarchies have to be identical on all clients.

This document explains how to set up an ever increasing counter that all clients have access to. This could be used to make sure that everyone can generate unique identifiers, with no chance of ever getting a duplicate.

By being persistent, the counter will also keep its value even if all clients log off, as long as the replication server is running.

The Counter

First, create a script called Counter.cs and add the following code to it:

using UnityEngine;
using Coherence.Toolkit;

public class Counter : MonoBehaviour
{
    public int counter = 0;

    public void NextNumber(CoherenceSync requester)
    {
        requester.SendCommand<NumberRequester>(
            nameof(NumberRequester.GotNumber), 
            MessageTarget.AuthorityOnly,
            counter);
        counter++;
    }
}

Next, add this script to a prefab with CoherenceSync on it, and select the counterand the method NextNumber for syncing in the bindings window. To make the counter behave like we want, mark the prefab as "Persistent" and give it a unique persistence ID, e.g. "THE_COUNTER". Also change the adoption behaviour to "Auto Adopt":

Finally, make sure that a single instance of this prefab is placed in the scene.

NumberRequester

Now, create a script called NumberRequester.cs. This will be an example MonoBehaviour that requests a unique number by sending the command GetNumber to the Counter prefab. As a single argument to this command, the NumberRequester will send an entity reference to itself. This makes it possible for the Counter to send back a response command (GotNumber) with the number that was generated. In this simple example we just log the number to the console.

using UnityEngine;
using Coherence.Toolkit;

public class NumberRequester : MonoBehaviour
{
    CoherenceSync sync;

    private void Awake()
    {
        sync = GetComponent<CoherenceSync>();
    }

    void Update()
    {
        if(sync.isSimulated && Input.GetKeyDown(KeyCode.Return))
        {
            var counter = FindObjectOfType<Counter>();
            var counterSync = counter.GetComponent<CoherenceSync>();
            
            counterSync.SendCommand<Counter>(
                nameof(Counter.NextNumber),
                MessageTarget.AuthorityOnly,
                sync);
        }
    }

    public void GotNumber(int number)
    {
        Debug.Log($"Got number: {number}");
    }
}

To make this script work, add it to a prefab that has the CoherenceSync script and mark the GotNumber for syncing in the bindings window.

LiveQuery

The way you get information about the world is through LiveQueries. We set criteria for what part of the world we are interested in at each given moment. That way, the replicator won’t send information about everything that is going on in the game world everywhere, at all times.

Instead, we will just get information about what’s within a certain area, kind of like moving a torch around to look in a dark cave.

Adding a LiveQuery to the scene

A LiveQuery is a cube that defined the area of interested in a particular part of the world. It is defined by its position and its radius (half the side of the cube). There can be multiple LiveQueries in a single scene.

Moving a LiveQuery

A classic approach is to put a LiveQuery on the camera and set the radius to correspond to the far clipping plane or visibility distance.

Moving the GameObject containing the LiveQuery will also notify the replication server that the query for that particular game client has moved.

Adding a TagQuery

In addition to the LiveQuery, coherence also supports filtering objects by tag. This is useful when you have some special objects that should always be visible regardless of world position.

To create a TagQuery, right click a GameObject in the scene and select coherence -> TagQuery from the context menu.

All networked GameObjects with matching tags will now be visible to the client. The coherence tag can be any string and can be configured separately from the Unity tag in the Advanced Settings section of the CoherenceSync component.

Tags and TagQueries can be updated at any time while the application is running, either from the Unity inspector or setting CoherenceSync.tag and CoherenceTagQuery.tag with code.

Currently, only a single tag per GameObject and TagQuery is supported. To include objects with different tags, you can create multiple TagQuery objects for each tag.

CoherenceSync configuration

The CoherenceSync editor interface allows us to define the Lifetime of a networked object. The following options are available:

• Session Based. No persistence. The entity will disappear when the client or simulator disconnects.

• Persistent. The entity will remain on the server until a simulating client deletes it.

Persistence UUID

Persistent objects need to be identified so that the system can know how to treat duplicate persistent objects.

Manually assigning a UUID means that each instance of this persistent object prefab is considered the same object regardless of where on the network it is instantiated. So, for example, if two clients instantiate the same prefab object with the same persistence UUID then only one is considered official and the other is replaced by the replication server.

The CoherenceUUID behaviour is used to uniquely identify a prefab.

It has several functions: you can Generate new ID for your object, and you can set auto-generate UUID on the scene to true, so each time object will receive a new ID.

Deleting a persistent object

A persistent object can be deleted only by the client or simulator that has authority over it. For indirect remote deletion, see the section about network commands.

Deleting a persistent object is done the same as with any network object - by destroying its GameObject.

Destroy(coherenceSync.gameObject);

Bandwidth is limited

No matter how fast the internet becomes, conserving bandwidth will always be important. Some game clients might be on poor mobile networks with low upload and download speeds, or have high ping to the replication server and/or other clients, etc.

Additionally, sending more data than is required consumes more memory and unnecessarily burdens the CPU and potentially GPU, which could add to performance issues, and even to quicker battery drainage.

Optimization techniques

In order to optimize the data we are sending over the network, we can employ various techniques built into the core of coherence.

• Delta-compression (automatic). When possible, only send differences in data, not the entire state every frame.

• Compression and quantization (automatic and configurable). Various data types can be compressed to consume less bandwidth that they naturally would.

• Simulation frequency (configurable). Most entities do not need to be simulated at 60+ frames per second.

• Levels of detail (configurable). Entities need to consume less and less bandwidth the farther away they move from the observer.

• Area of interest. Only replicate what we can see.

Why does world size matter?

Positions and livequeries in the world are compressed, and the compression is defined by maximum scale and bit count. Larger scale at the same bit count means lower precision, and vice-versa.

Defining world size

A default maximum world size of 2400 means that only values from [-2400, 2400] will be supported for all spatial axes.

If our game scenes are larger than 2400 x 2 (4800) units across, we can increase this value in the Settings window or in the schema as illustrated below.

Don't forget to extend the LiveQuery scale to match the world position scale.

# Default world positions:

component WorldPosition
  value Vector3 [bits "24", scale "5000"]
  

# LiveQuery:

component WorldPositionQuery
  position Vector3 [bits "24", scale "5000"]
  radius Float [bits "24", scale "5000"]

Motivation

coherence can support large game worlds with many objects. Since the amount of data that can be transmitted over the network is limited, it's very important to only send the most important things.

You already know a very efficient tool for enabling this – the LiveQuery. It ensures that a client is only sent data when an object in its vicinity has been updated.

Often though, there is a possibility for an even more nuanced and optimized approach. It is based on the fact that we might not need to send as much data for an entity that is far away, compared to a close one. A similar technique is often used in 3D-programming to show a simpler model when something is far away, and a more detailed when close-up.

This idea works really well for networking too. For example, when another player is close to you it's important to know exactly what animation it is playing, what it's carrying around, etc. When the same player is far off in the horizon, it might suffice to only know it's position and orientation, since nothing else will be discernible anyways.

To use this technique we must learn about something called archetypes.

Archetypes

An archetype is a component that can be added to any prefab with the CoherenceSync component. It contains a list of the various levels of detail (LODs) that this particular prefab can have.

There must always exist a LOD 0, this is the default level and it always has all components enabled (it can have per-field overrides though, see below.)

There can be any number of subsequent LODs (e.g. LOD 1, LOD 2, etc.) and each one must have a distance threshold higher than the previous one. The coherence SDK will try to use the LOD with the highest number, but that is still within the distance threshold.

One each LOD, there are two options for optimizing data being transferred:

1. Components can be turned off, meaning you won't receive any updates from it.

2. It's fields can be configured to use fewer bits, usually leading to less fine-grained information. The idea is that this won't be noticeable at the distance of the LOD.

Scale, bits and range

coherence allows us to define the scale of numeric fields and how many bits we want to allocate to them.

Here are some terms we will be using:

• Scale. The minimum/maximum value of the field before it overflows. A scale of 2400 means the number can run from -2400 to 2400.

• Bits. The number of bits (octets) user for the field. When used for vectors, the number defined the number of bits used for each component (x, y and z). A vector3 set to 24 bits will consume 3 * 24 = 72 bits.

• Range. For integer values, we define a minimum and maximum possible value (e.g. Health can lie between 0 and 100).

coherence allows us to define these values for specific components and fields. Furthermore, we can define levels of detail so that precision and therefore bandwidth consumption falls with the distance of the object to the point of observation.

Defining levels of detail

On each LOD you can configure the individual fields of any component to use less data. You can only decrease the fidelity, so a field can't use more data on a lower (more far away) LOD. The Archetype editor interface will help you to follow these rules.

In order to define levels of detail, we have to add a CoherenceArchetype component to a prefab with CoherenceSync defined field bindings.

Clicking on the Edit LOD button opens the Archetype Editor (and adds a CoherenceArchetype component if there wasn't one already).

We can override the base component settings even without defining further levels of detail.

Clicking on Add new Level Of Detail will add a new LOD. We can now define the distance at which the LOD starts. This is the minimum distance between the entity and the center of the LiveQuery at which the new level of detail becomes active (i.e. the replicator will start sending data as defined here at this distance).

You can also disable components at later LOD levels if they are not needed. In the example above, you can see that the entire Animator component is disabled beyond the distance of 100 units. At 100 units (a.k.a. meters), we usually do not see animation details, so it does not make sense to replicate this data.

The Data Cost Overview shows us that this takes the original 416 bits down to just 81 bits at LOD level 2.

Field overrides per type

The primitive types that coherence supports can be configured in different ways:

Float, Vector2 & Vector3

These three types can all be configured in the same way:

1. By setting the scale, which affects the maximum and minimum value that the data type can take on. For example, a scale of 100 means that a float ranges from -100 to 100.

2. By setting the precision, which defines the greatest deviation allowed for the data type. For example, a precision of 0.5 means that a float of value 10.0 can be transmitted as anything from 9.5 to 10.5 over the network.

Based on the scale and the desired precision, a bit count will be calculated. The default precision and scale (which happens to be 2400) gives a bit count of 24. This means that for a Vector3 a total of 72 bits will be used, 24 x 3.

Integers

Integers can be configured to any span (that fits within a 32-bit int) by setting its minimum and maximum value.

For example, the member variable age in a game about ancient trolls might use a minimum of 100 and a maximum of 2000. Based on the size of the range (1900 in this case) a bit-count will be calculated for you.

Quaternions

Right now quaternions (used for rotations) do not support field overrides, but this will be fixed in the near future.

Other types

All other types (strings, booleans, entity references) have no settings that can be overridden, so your only option for optimizing those are to turn them off completely at lower LODs.

Using LODs with connected entities

If a LODed game object is parented to another synced object, the child will base its LOD level on the world position of its parent. This means that the (local) position of the LODed child does not have any effect on its LOD, until it is unparented.

Also – to save bandwidth, detection of LOD changes on the client only happens when the entity sends a component update. This means that a child object might appear to be using a nonsensical LOD until it changes in some way, for example by modifying its position.

LODs in the schema

When we bake, information from the CoherenceArchetype component gets written into our schema. Below, you can see the setup presented earlier reflected in the resulting schema file.

archetype FemZombie
  lod 0
    WorldPosition 
      value 
    WorldOrientation 
      value 
    FemZombie_UnityEngine_Animator 
      InputHorizontal [bits "11", scale "1"]
      InputVertical [bits "11", scale "1"]
      InputMagnitude [bits "11", scale "1"]
      TurnOnSpotDirection [bits "11", scale "1"]
      ActionState [bits "15", range-min "0", range-max "10"]
  lod 1 [distance "50"]
    WorldPosition 
      value [bits "18", scale "2400"]
    WorldOrientation 
      value 
    FemZombie_UnityEngine_Animator 
      InputHorizontal [bits "11", scale "1"]
      InputVertical [bits "11", scale "1"]
      InputMagnitude [bits "11", scale "1"]
      TurnOnSpotDirection [bits "11", scale "1"]
      ActionState [bits "15", range-min "0", range-max "10"]
      IdleRandom [bits "15", range-min "-9999", range-max "-9999"]
      RandomAttack [bits "15", range-min "-9999", range-max "-9999"]
      AttackID [bits "15", range-min "-9999", range-max "-9999"]
      DefenseID [bits "15", range-min "-9999", range-max "-9999"]
      RecoilID [bits "15", range-min "-9999", range-max "-9999"]
      ReactionID [bits "15", range-min "-9999", range-max "-9999"]
      HitDirection [bits "15", range-min "-9999", range-max "-9999"]
  lod 2 [distance "100"]
    WorldPosition 
      value [bits "10", scale "2400"]
    WorldOrientation 
      value 

Caveats

The most unintuitive thing about archetypes and LOD-ing is that it doesn't affect the sending of data. This means that a "fat" object with tons of fields will still tax the network and the replication server if it is constantly updated, even if it uses a very optimized Archetype.

Also, it's important to realize that the exact LOD used on an entity varies for each other client, depending on the position of their query (or the closest one, if several are used.)

Sometimes you want to synchronize entities that are connected to other entities. These relationships can be references between entities, but they can also involve direct parent-child relationship between game objects, or more nuanced use cases.

Here's a guide for what technique to use, depending on the situation:

• If you have an entity that needs to keep a nullable reference to another entity, use a normal . This includes any existing MonoBehaviour that has GameObject or Transform fields that you want to synchronize over the network.

• If the entities are placed in a hierarchy, use the techniques for .

Entity references let you set up references between entities and have those be synchronized, just like other value types (like integers, vectors, etc.)

To use Entity references, simply select any fields of type GameObject, Transform, or CoherenceSync for syncing in the Bindings window:

The synchronization works both when using reflection and in baked sync scripts.

Limitations

It's important to know about the situations when an entity reference might become null, even though it seems like it should have a value:

• The owner of the entity reference might sync the reference to the Replication Server before syncing the referenced entity. This will lead to the Replication Server storing a null reference. If possible, try setting the entity references during gameplay when the referenced entities have already existed for a while.

In any case, it's important to use a defensive coding style when working with entity references. Make sure that your code can handle missing entities and nulls in a graceful way.

Overview

Objects with the CoherenceSync component can be connected to other objects with CoherenceSync components to form a parent-child relationship. For example, an object can be linked to a hand, a hand to an arm, and the arm to a spine.

Usage

Creating an entity hierarchy is very simple. All you need to do is to add a GameObject with a CoherenceSynccomponent as a direct child of another GameObject with a CoherenceSynccomponent. You can add and remove parent-child relationships at runtime (even from the editor).

Destruction or disconnection of the parent object will also destroy and remove all children of this object. Those objects state needs to be treated on the client side to be reinstantiated on the next connection.

Hierarchies with intermediary transforms

Sometimes, it is not practical to add CoherenceSyncobjects to all the links in the chain. For example, if a weapon is parented to a hand controlled by an Animator, we do not need to synchronize the entire skeleton over the network. In that case, see .

Level of detail considerations

If the child object is using LODs, it will base its distance calculations on the world position of its parent. For more details, see the documentation.

What is a simulator?

A simulation server or simulator is a version of the game client without the graphics ("headless client") optimized and configured to perform server-side simulation of the game world. When we say something is simulated on the server, we mean it is simulated on one or several simulators.

Why do we need a simulator?

A simulator can have various uses, including:

• Server-side simulation of game logic that cannot be tampered with

• Offloading processing from game clients

• Splitting up a large game world with many entities between them

Here are some examples of things a simulator could be taking care of:

• Running all the important game logic

• Running NPC AI

• Simulating the player character (by receiving only inputs from the clients through )

How many simulators can we have per project?

We can have as many simulators as we like. They will connect to the replication server like any other game client.

Enabling the Simulator

coherence allows us to use multiple simulators to split up a large game world with many entities between them. This is called spatial load balancing. Please refer to the section about simulators for more information.

When using the Simulator Upload Wizard in Unity, you can specify a "simulator slug". This is simply a unique identifier for simulator. This value is automatically saved in RuntimeSettings when an upload is complete, and Room creation requests will use this value to identify which simulator should be started alongside your room.

The simulator slug can be any string value, but we recommend using something descriptive. If the same slug is used between two uploads, the later upload will overwrite the previous simulator.

A list of uploaded simulators and their corresponding slugs can be found in the Developer Portal:

World simulators are started and shutdown with the world. They can be enabled and assigned in the Worlds section of the portal.

World simulation servers are started with the command line parameters described in the Simulators section.

Simulators per room can be enabled in the dashboard for the project. The simulator used is matched according to the simulator slug in the RuntimeSettings scriptable object file. This is set automatically when you upload a simulator.

For each new room, a simulator will be created with the command line parameters described in the Simulators section. The simulator is shutdown automatically when the room is closed.

Transforms

Welcome to the first scene of the coherence Network Playground. This scene will show you how easy it is to set up networking in your Unity project and sync GameObject transforms across the network.

In this example, each client will have a player character to move by clicking on the map to make the entity move to that location. Each client will only have control of its local entity.

General Set Up

In the Hierarchy of the Scene you can see three core Prefabs.

Coherence Entity Character is the Prefab that will change per Scene with different functionality. It has a standard CharacterControllerand Rigidbody as well as an Agent script which will handle movement through the Input Manager in the Core Scene Setup prefab.

In This Scene...

Coherence Entity Character (always change the prefab, not the instance) is located in the Resources folder. The UnityEngine.Transform and position are ticked to sync. All other settings (persistence and authority) use the default settings. This entity will be session based, no authority handover and no adoption will take place, when a client leaves.

The On Network Instantiation event is used to change the color of the mesh and recalculate the RigidBody collisions. That's it.

Build and Try

You can build this Scene via the Build Settings. Run the local Replication Server through the Window -> Coherence -> Settings window and see how it works. You can try running multiple clients rather than just two and see replicating for each.

The Network Playground project is a series of scenes with different features of coherence for you to pick through and learn.

They will teach you the fundamentals that should set you on your way to creating a multiplayer game.

Scenes

Each Scene in this Network Playground Project shows you something new:

• Scene 1. Synchronizing Transforms

• Scene 2. Physics

• Scene 3. Persistence

• Scene 4. Synchronizing Animations and Custom Variables

• Scene 5. AI Navigation

• Scene 6. Commands

• Scene 7. Team based

• Scene 8. Connected Entities

Each scene comes with a few helpful core components.

Core Scene setup

This Prefab stores all the generic things that make up these simple Scenes.

• Main Camera

• Global Volume

• Directional Light

• Environment (Ground, Walls etc)

• Navigation Interface

  • To move between the Scenes without having to enter and exit playmode, useful when testing the standalone build.

• Input Manager

  • Input Manager that uses Raycasting to send a message to the GameObject (with a collider) that it hit.

Coherence Setup

This Prefab includes all of the things that make coherence work.

• Interface

  • Canvas UI that handles Connection/Disconnection dialog and what address to connect.

• Event System

  • Event system to interact with any UI in the scene.

• coherence Live Query

  • Game Object/Component with a query radius that coherence uses to ask the server "What is happening in the query radius?" so it does not query unnecessarily big areas of large worlds. You can find more information here.

• coherence Mono Bridge

  • GameObject/Component to transform the Mono based CoherenceSync component to archetypes so all data can be processed as ECS code.

coherenceSync (on coherence Entity Prefab)

We use this component on anything that we require to be networked, whether this is a playable character, NPC (non playing character), an object (ball, weapon, banana, car, plant etc) or any Game/Input Managers that require to sync data between Clients/Simulators/Servers.

It scans the inspector for attached components and allows us to sync those component values (as long as they are public) and communicate them to other clients. It also allows us to set individual Prefabs persistent, authoritative and network connect/disconnect settings. There's much more information on the CoherenceSync page

Physics

This scene will show you how to use coherence to sync GameObject transforms and Physics objects across the network.

In this example, each client will have its own player character to move. Left-clicking on the map will make the entity move to that location. Right-clicking will spawn local physics based objects that all player characters can interact with. Each client will only have control over its local entity.

General Set Up

In the Hierarchy of the Scene you can see three core Prefabs:

Core Scene Setup and Coherence Setup are present in all scenes and described in detail on Start Tutorial page.

Coherence Entity is the prefab that will change per Scene with different functionality. It has a standard CharacterControllerand Rigidbody as well as an Agent script which will handle movement functionality through the Input Manager in the Core Scene Setup prefab.

Coherence Connection Events handles overall Scene connectivity. Additionally, it removes all Entities with coherenceSync from the Scene to demo disconnection/reconnection via the Interface without refreshing the Scene.

In This Scene...

Coherence Entity Character (always change the prefab, not the instance) is located in the Resources folder. The UnityEngine.Transform and position are ticked to sync. All other settings (persistence and authority) use the default settings. This entity will be session based, no authority handover and no adoption will take place, when a client leaves.

The On Network Instantiation event is used to change the colour of the mesh.

The Physics Entity Spawner is a simple script to instantiate a Coherence Entity Physics Prefab with a coherenceSync component that replicates the transform and position. The component also changes the material based on if it is locally simulated or synced over the network.

Build and Try

You can build this Scene via the Build Settings. Run the local Replication Server through the Window -> Coherence -> Settings window and see how it works. You can try running multiple clients rather than just two and see replicating for each.

AI Navigation, Session Based NPC's

This scene will show you how easy it is to set up Networking in your Unity project and sync non player characters that move around the world via Unity's Navmesh system.

In this example each client can spawn as many navigation agents as they wish, and these navigation agents will move intermittently to different locations on the grid. All navigation agents will be replicated across clients with a specific colors signifying if they are local or networked.

General Set Up

In the Hierarchy of the Scene you can see three core Prefabs:

Coherence Connection Events handles overall Scene connectivity. Additionally, it removes all Entities with coherenceSync from the Scene to demo disconnection/reconnection via the Interface without refreshing the Scene.

In This Scene...

Spawner is a simple script to instantiate a Coherence Entity Nav Agent prefab with a coherenceSync component that replicates the transform and position. The component also changes the material based on if it is locally simulated or synced over the network.

Coherence Entity Nav Agent has a Nav Mesh Agent component controlled via the Navigation Agent script which every few seconds sets a new destination on the grid. It's not required to sync anything other than the Transform; position, rotation parameter as the Nav Mesh Agent settings only need to simulate locally.

Build and Try

You can build this Scene via the Build Settings. Run the local Replication Server through the Window -> Coherence -> Settings window and see how it works. You can try running multiple clients rather than just two and see replicating for each.

Animation & Variables

This scene will show you how easy it is to set up Networking in your Unity project and sync GameObject transforms, animations via the Animator parameters and customer variables.

In this example, each client will have its own player character to move, the beloved Unity RobotKyle asset. Type your name into the connect dialog box and connect. You can move around with WASD. When another player connects, their name is carried across and their transform and animation state is replicated.

General Set Up

In This Scene...

Coherence Kyle is taken from the Unity asset "Robot Kyle", with added components Rigidbody, Character Controller, and Animator with the two animation states - Idle and Walk. The animation states are controlled by a speed parameter from the Agent script. The scene also contains a Name Badge script which gets the Connect Dialog GameObject from the Core Scene Setup and sets the color depending if it's local or networked.

Attached to Coherence Kyle is a coherenceSync component which replicates the parametersTransform; position and rotation, Animator; Speed[float] and NameBadge; Name [string]. The authority and persistence settings are set to their default values and the On Network Instantiation event is used to change the color of the networked entities.

Build and Try

You can build this Scene via the Build Settings. Run the local Replication Server through the Window -> Coherence -> Settings window and see how it works. You can try running multiple clients rather than just two and see replicating for each.

In This Scene...

This scene demonstrates usage of connected entities.

This scene shares the moving entities that were in a few previous scenes but also has added functionality.

Right-clicking on a non-local entity causes the local entity to start moving towards it while also displaying an arrow pointed at the target. When the entity reaches this target it will parent itself under it as a child, setting the target as its connected entity.

From this point onward, until the entity disconnects by moving or otherwise, the entity will be smaller and parented to this target. When the target moves the local entity will move with it as one.

This type of connection can also cause issues. For example if the client controlling the parent entity disconnects, destroying its entity, any client whose entity was a child of that destroyed entity will have his entity destroyed as well.

Build and Try

You can build this Scene via the Build Settings. Run the local Replication Server through the Window -> Coherence -> Settings window and see how it works. You can try running multiple clients rather than just two and see replicating for each.

Build a simulator to be uploaded to the cloud

A simulator build is a built Unity Player for the Linux 64-bit platform that you can upload to coherence straight from Unity Editor.

On Unity's menu bar, navigate to coherence -> Simulator -> Build Wizard.

From within the Build Wizard you can build and upload simulators.

The Info tab provides information and requirements to build simulators properly.

The Build tab creates valid simulator builds from Build Configuration Assets.

Creating a Build Configuration Asset

You can create them via Assets -> Create -> coherence -> Simulator Build Configuration.

A newly created build configuration looks like this:

There are several settings you might want to change.

• Specify the scenes you want to get in the build via the Scene List component.

• Specify a Company Name and a Version from the General Settings component (optional).

• Additionally, you can add our OptimizeForSize component (find it using Add Component). Specify which optimizations you want to use to reduce the final build size from the Optimize For Size component (optional).

Reducing simulator build size (experimental ⚠️)

You can add an OptimizeForSize component to your build configuration via the Add Component in the build configuration inspector. It looks like this:

Select the desired optimizations depending on your needs.

Upload your simulator

Once you have created a valid simulator build, you can upload it to coherence.

If you built your simulator using the Build tab, you should have a valid path to your simulator build set already. If you haven't or want to use a different path, use the Browse button.

You'll see in the developer dashboard when your simulator is ready and running.

Am I a simulator?

Ask Coherence.SimulatorUtility.IsSimulator.

There are a few ways you can tell coherence if the instance should behave as a simulator:

• Specifying a ConnectionType when connecting via Coherence.Network.Connect.

• COHERENCE_SIMULATOR preprocessor define.

• --coherence-simulation-server command-line argument.

• Toggling on Simulator in the sample connection dialog.

Connect and ConnectionType

The Connect method on Coherence.Network accepts a ConnectionType parameter.

COHERENCE_SIMULATOR

Whenever the project compiles with the COHERENCE_SIMULATOR preprocessor define, coherence understands that the game will act as a simulator.

Command-line argument

Launching the game with --coherence-simulation-server will let coherence know that the loaded instance must act as a simulator.

Server-side simulation

You can define who simulates the object in the CoherenceSync inspector.

Auto-reconnect

If you need a different solution, take a look at its implementation use plug your own.