This document is about: FUSION 2

This page is a work in progress and could be pending updates.

Fusion 2 Introduction


For a list of changes from Fusion 1.1 to 2.0 check this page

Fusion is a new high performance state synchronization networking library for Unity. Fusion is built with simplicity in mind to integrate naturally into the common Unity workflow, while also offering advanced features like data compression, client-side prediction and lag compensation out of the box.

Under the hood, Fusion relies on a state-of-the-art compression algorithm to reduce bandwidth requirements with minimal CPU overhead. Data is transferred as partial chunks with eventual consistency. A fully configurable area-of-interest system is supplied to allow support for very high player counts.

The Fusion API is designed to be similar to regular Unity MonoBehaviour code. For example, RPCs and network state is defined with attributes on methods and properties of MonoBehaviours with no need for explicit serialization code, and network objects can be defined as prefabs using all of Unity's most recent prefab features like nesting and variants.

Inputs, Networked Properties and RPCs provide the foundation for writing gameplay code with Fusion.

overview of the core fusion apis
Overview of the Core Fusion APIs

Using a Networked Property in Fusion:


[Networked] public int lives { get; set; }

Using Network Input for client-side predicted movement:


public override void FixedUpdateNetwork
    if (GetInput(out NetworkInputData data))
        data.direction.Normalize(); // normalize to prevent cheating with impossible inputs
        _characterController.Move(5 * data.direction * Runner.DeltaTime);

Declaring a Remote Procedure Call (RPC) in Fusion:


[Rpc(RpcSources.InputAuthority, RpcTargets.StateAuthority)]
public void RPC_Configure(string name, Color color)
    playerName = name;
    playerColor = color;

Choosing the Right Mode

Fusion supports two fundamentally different network topologies with the same API as well as a single player mode with no network connection.

The first step when starting with Fusion is to chose between Server/Host and Shared mode.

The Quadrant provides a good starting point for deciding what mode is right for your application.

the quadrant

Full Size


If you already have a good idea for what mode to use you can get started with the tutorials:

If you're not sure yet, keep reading!

Quadrant Comparisons

In the Quadrant, four options are displayed:

  • Fusion - Dedicated Server
  • Fusion - Client Host
  • Fusion - Shared
  • Quantum

These are compared over the following categories:

  • Cheat Protection: How easy is it for players to cheat?
  • Server Complexity: How complex is it to set up online capabilities for this solution?
  • Mobile Quality: How well does this solution work for mobile games and applications?
  • Cost Efficiency: How expensive is this solution?

Every project has its own needs, so it's important to compare and understand the differences between these choices and the frameworks available through Photon. Note, this documentation focuses solely on Photon Fusion, so if you want to read more about Photon Quantum, you can do so here.

Cheat Protection

Cheating players, capable of ruining any experience for players if left unchecked, are just an unfortunate reality of online gaming. When it comes to cheating, Fusion's solutions have varying levels of protection by default:

  • Fusion - Dedicated Server: Cheat protection is strong because the dedicated server has state authority over the game. If a player tries to cheat, the server can validate and prevent or react to a client who is trying to do so.
  • Fusion - Client Host: Because the host player also acts as the server, they can, in theory, cheat if the proper checks are not made within the game itself. For example, when sending inputs, a hacked game could possibly send values that would not be possible in normal gameplay, so extra work is needed to prevent this.
  • Fusion - Shared: Because players have State Authority over objects they control, cheating can occur without extra precautions being taken which can require the use of custom plugins.

Server Complexity

When comparing Fusion's three solutions through this lens, using a Dedicated Server with Photon Fusion is really the only unique solution since the others work through Photon Cloud, meaning server orchestration, matchmaking, and other online features are provided by Photon. Setting up a dedicated server requires the headless server version of the game to run on machines run by the user in-house or by a third party such as Amazon Web Services, which will potentially require more setup and costs.

Mobile Quality

Mobile games with online multiplayer are popular but can be challenging due to devices' limited capabilities and wireless connection.

  • Fusion - Dedicated Server: Can be costly if a lot of players are connecting depending on the third party, dedicated server provider.
  • Fusion - Client Host: Because one of the players will also be acting as the server, having a poor connection can make this a difficult and unreliable solution for mobile.
  • Fusion - Shared: Due to Fusion's performance and speed, Shared Mode works well since the connection is to the cloud and not to other players directly.

Cost Efficiency

Creating an online game requires servers and cloud space, neither of which are free. All three of Fusion's solutions depend on CCU costs; however and similar to Server Complexity, the cost of using Server Mode will depend on the third party server running headless instances and can exceed the plans currently offered by Photon.

Topology Differences


Full Size

Server Mode

In Server Mode the server has full and exclusive State Authority over all objects, no exceptions.

Clients can only modify networked objects by sending their input to the server (and having the server react to that input) or by requesting a change using an RPC.

The server application is built from the Unity project and runs a full headless Unity build. This headless build needs to be hosted on a server machine or a cloud hosted server. Photon does not provide servers for hosting a dedicated Fusion server application.

Client Side Prediction

Client Side Prediction is a popular multiplayer architecture in which clients use their inputs to predict their own movement while awaiting confirmation from the server. This hides latency and allows the gameplay to feel snappy.

In Fusion's Server Mode, any changes a client makes directly to the networked state are only a local prediction, which will be overridden with actual authoritative snapshots from the server once those are received. This is known as reconciliation, as the client is rolled back to the server-provided state and re-simulated forward to the local (predicted) tick.

If previous predictions were accurate, this process is seamless. If not, the state will be updated, and because the network state is separate from the rendering state, the rendering may either snap to this new state or use various forms of interpolation, error correction and smoothing to reduce the visual artifacts caused by the correction.

Host Mode

In Host Mode, the host acts as both a server and a client. The host has a local player, polls input for it and interpolates the rendering as expected of a client.

Overall, this mode is equivalent to the Server Mode but is much cheaper to run as no dedicated server hosting costs are incurred. However, this comes at the price of the state authority's trustworthiness—in other words, a rogue host can cheat.

When running hosted mode from behind a firewall or a router, the Photon cloud transparently provides UDP punch through or package relay as needed.

Since the session is owned by the host, it will be lost if the host disconnects. Fusion does provide a host migration mechanism to allow transferring of state authority to a new client in situations like this, however this is not automatic in Host Mode (unlike in Shared Mode) and requires special handling in the client code.

Shared Mode

In Shared Mode, authority over network objects is distributed among all clients. Specifically, each client initially has State Authority over objects they spawn, but are free to release that State Authority to other clients. Optionally, clients may be allowed to take State Authority at will.

Features such as client side prediction and rollback are not available in this mode. The simulation always moves forward at the same tick rate on all clients.

The Shared Mode network session is owned by the Photon cloud and remains alive as long as any client is connected to it. The Photon cloud serves as a package relay and has full access to the network state with no need to run Unity, allowing for lightweight server logic and data validation (e.g. cheat protection) to be implemented without the need to spin up dedicated server hardware.

For those coming from Photon Unity Networking (PUN), Shared Mode is similar in many ways, albeit more feature complete, faster, and with no run-time allocation overhead.

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