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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.

To prevent IP sharing, use the Disable IP address sharing action before connecting to the broker. This forces all traffic through a TURN relay server (add one with the Use a custom ICE server action) at the cost of higher latency.

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.

Configure ICE servers (for internet play)

By default, P2P uses public STUN servers to establish connections. For games played over the internet (not just LAN), it is strongly recommended to add at least one self-hosted STUN server and one TURN relay server using the Use a custom ICE server action. This action can be called multiple times to add multiple servers. It must be called before connecting to the broker.

TURN servers are required when players are behind strict firewalls or NAT that prevent direct connections. Without a TURN server, some players may be unable to connect.

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 Use custom 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 "Use the default 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 other instance" action and pass as a parameter the ID of another instance. Both instances will then connect automatically. You can then send an event from one instance to the other one to make sure that the connection is established.

Wait for the Is P2P ready condition to become true before reading the ID or connecting, as initialization happens asynchronously after connecting to the broker server.

Use the Peer Connected condition (and P2P::GetLastConnectedPeer() expression) to detect when a remote peer has initiated a connection to you. Similarly, the Peer disconnected condition fires when a connection drops, and P2P::GetLastDisconnectedPeer() returns the ID of the peer that left. The An error occurred condition combined with P2P::GetLastError() lets you display or log connection problems.

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. You can use the Override the client ID action to set a custom ID before connecting to the broker — useful for room-code-style matchmaking. Alternatively, configure custom ID generation on your broker by following the instructions on the peerjs-server documentation.

Disconnecting

Use Disconnect from a peer to close a connection with one specific client, Disconnect from all peers to close all peer connections while keeping the broker connection alive, or Disconnect from all to fully shut down both peer connections and the broker connection.

Interacting with connected games

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

Events carry an optional string payload retrieved with P2P::GetEventData("eventName") and an optional sender ID retrieved with P2P::GetEventSender("eventName"). If you need to transfer structured data (arrays, structures), use the variable variants of the send actions (Trigger event on all connected clients (variable) / Trigger event on a specific client (variable)) and retrieve the data with the Get event data (variable) action, which populates a scene variable with the received value.

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.

Reference

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