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Peer-to-peer

Danger

P2P leaks the client's IP addresses when connecting to them. If someone knows your broker server and P2P ID, they know your IP address and can use it to DDoS or geolocalize you. Make sure to properly inform your players and not to use lobby/matchmaking systems alongside P2P, as those may share the player's P2P ID with unknown peers.

Warning

This is an experimental feature, it is subject to change and may contain bugs. Your feedback is welcome to improve this feature.

GDevelop supports peer-to-peer (P2P) connections to enable basic multiplayer games. This works through the concept of remote events. Connect multiple instances of the game using their ID, before remotely triggering conditions on the other instances.

Danger

This extension is not suitable for all types of multiplayer games and is potentially unsafe against cheating. As every instance of the game is connected directly to each other, there is no machine that can be considered trusted more than another.

As such, it is hard to verify if someone is cheating (unlike dedicated servers). P2P supports a maximum of 250 simultaneous connections and is therefore also not suited for very large multiplayer-focused games like MMOs.

Selecting a broker server

A running game, called a client, must connect to other clients before being able to send data. For this, it needs a way to self-identify and find other clients. To do so a broker server must be configured. It's a server with a fixed, well-known address that stores all the addresses of the connected clients and gives them to each client so that they can connect to each other.

There are two options for setting up a broker server:

  • Setting up a custom server (recommended), which can be run on a local computer as a test.
  • Using a default, public server.

Set up a custom (local) server

A local server can be set up easily. Install Node.js will need to be installed. The LTS version is recommended.

Open a command line. To do so on Windows:

  • Press the Windows and R keys. This will open the run window
  • Type cmd in the popup.
  • Press the enter key.

After opening a command line, complete the following:

  • If this is the first time completing these steps, to install the server type npm install peer -g

  • After the server is installed, and every subsequent time (to start the server), type peerjs -p

    • Any port that is not already used by the computer can be used. Other options can be passed to the PeerJS server, see its website for more information. After the above has been completed, the Connect to a broker server action is used to connect to the server.

  • localhost can be used as the host address to point to the local server.

Tip

Note that this is a local server, so it will only work on your machine. When releasing your game you will need to deploy one to a Node.js compatible hosting, like Heroku.

Use the default server

Warning

It is not recommended to use that server, you should use your own if possible. The default server is not operated by GDevelop and GDevelop team is not responsible if anything goes wrong using that server.

You can also use the default server provided by PeerJS. To use that server use the action "Connect to the default broker server".

Connecting

To connect instances, you need to enter their ID in the other instances. The ID can be found with the expression P2P::GetID(). To connect, use the "Connect to another client" action and pass as a parameter the ID of another instance. Both instances will then connect automatically.

Connecting is not instantaneous. Before sending or receiving anything, make sure the extension is initialized by checking the Is P2P ready condition, and detect newly established connections with the Peer Connected condition (the ID of the peer that just connected is available with the P2P::GetLastConnectedPeer() expression).

Changing the ID generation

The default P2P ID generation is very long to avoid conflicts, but if you want to have an easily shareable ID, it is not ideal.

The simplest option is the Override the client ID action, which lets you pick your own ID (for example a short room code). It must be called before connecting to a broker server, and the ID must not already be used by another connected client.

Alternatively, you can configure a custom ID generation on your custom P2P broker by following the instructions on the peerjs-server documentation.

Interacting with connected games

Once you got connected, you can trigger remote events. You can select another specific game instance (using its id) or send an event to all connected instances.

An event is just a name of your choosing (like "playerMoved" or "chatMessage"). On the receiving side, use the Event triggered by peer condition with the same name to react to it: it is true for one frame each time that event is received.

Sending and receiving data along with an event

When triggering an event, you can attach some data to it:

  • The Trigger event on ... (extra data) actions send a piece of text. Read it back on the receiving side with the P2P::GetEventData("eventName") expression.
  • The (variable) variants send a whole variable, including structures and arrays. Read it back with the Get event data (variable) action, which copies the received content into a variable of your choice.

To know which peer sent the event you just received, use the P2P::GetEventSender("eventName") expression, which returns that peer's ID. This is useful to reply only to the sender, or to keep track of which player did what.

Choosing if you want to activate data loss mode

You might be wondering what the "data loss" parameter is for. Due to how GDevelop is made, only one occurrence of a remote event can be handled when the event sheet is executed (this happens roughly 60 times per second). To help optimize events execution, we provide the choice to use the dataloss mode.

  • With the no dataloss mode, every remote event is queued, and on every frame, if there is one in the queue, we take the oldest one and handle it. This makes sure every data is processed/taken into account.
  • With the dataloss mode activated, it doesn't queue the data but only stores the latest occurrence of the remote event. This means only the latest data is processed and outdated data will be discarded.

Here are two examples:

  • if you use a synchronized score counter, you don't want to lose any data, as missing only one point of the counter would desynchronize the counters, so the dataloss mode would be deactivated.
  • If you want to synchronize positions, only the last position sent is relevant, not older positions. In this case, you would activate the dataloss mode to prevent delays/lags.

Handling disconnections and errors

Network connections are unreliable, so a game should react to peers leaving:

  • The Peer disconnected condition is true for one frame when a peer disconnects. The P2P::GetLastDisconnectedPeer() expression returns its ID, so you can, for example, remove that player's character from the scene.
  • The An error occurred condition is true for one frame when something goes wrong (for example when the broker server is unreachable). Use the P2P::GetLastError() expression to get a description of the error to log or display to the player.

You can also close connections yourself with the Disconnect from a peer, Disconnect from all peers, Disconnect from broker and Disconnect from all actions. Disconnecting from the broker keeps existing peer connections alive but prevents new peers from connecting to you.

Connecting over the internet

By default, peers try to connect directly to each other and share their IP addresses to do so (see the security warning at the top of this page). For games played over the internet rather than a local network, this direct connection often fails because of routers and firewalls (NAT).

To make connections more reliable, you can provide your own STUN and TURN servers with the Use a custom ICE server action (call it before connecting to the broker; it can be used several times to add multiple servers).

If you would rather never expose players' IP addresses, use the Disable IP address sharing action before connecting. All traffic is then relayed through a TURN server instead of going peer-to-peer directly, so you must add at least one TURN server as an ICE server for connections to work.

Reference

All actions, conditions and expressions are listed in the peer-to-peer reference page.