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Syncing Game State

Underlying all multiplayer gaming is syncing game information. This page will use the simple example of Tic Tac Toe to explain how the game state is synced across players using Rune's custom predict-rollback netcode.

Separation into Game Logic and Rendering

Multiplayer games are generally separated into game logic and rendering. This separation has many benefits, including being able to run dedicated servers that only have game logic. Rune multiplayer games are also separated into logic and rendering.

Game Logic

The logic is stored in a single file, logic.js, and initialized by running Rune.initLogic() with minPlayers, maxPlayers, setup and actions. The minPlayers and maxPlayers values ensure the game only have to consider a number of players between those two values. All other cases are Rune, incl. automatically making remaining people in the room spectators (more info in Joining and Leaving).

The setup function returns the initial values for the game state, which is the game information that’s synced across players. In the case of Tic Tac Toe, the game state describes who’s turn it is and which of the 9 cells have been filled with an X or an O. The setup function gets the players argument with info about the players at the time of starting the game.

The player can perform one of the actions to modify the game state. An action is a function that take as input an object with arguments. The function is also provided the game state and playerId containing the ID of the player, who performed the action. In the case of Tic Tac Toe, the only needed action is to mark a cell with an X or O.

Only logic.js file can modify the game state. The setup and actions functions cannot make use of any data from outside of their function definition and can’t modify anything outside the function definition (i.e. they’re pure functions w/o side effects).

The remaining parts of the code are hopefully self-explanatory. Some code like the isVictoryOrDraw function is left out as it’s not important in this context.

minPlayers: 2,
maxPlayers: 2,
setup: () => {
return {
cells: Array(9).fill(null),
// Allow either player to start
lastPlayerTurn: undefined,
actions: {
markCell: ({ cellId }, { game, playerId }) => {
// Check it's not the other player's turn and unmarked cell
if (game.lastPlayerTurn !== playerId || game.cells[cellId]) {
throw Rune.invalidAction()

// Update cell and switch turn
game.cells[cellId] = playerId
game.lastPlayerTurn = playerId

// Determine if game has ended
const winner = isVictoryOrDraw(game)
if (winner !== undefined) {


The game state should be rendered for the player to interact with. That’s the responsibility of client.js, which calls Rune.initClient with a onChange callback function. Whenever an action is performed, the onChange function is called with read-only info for updating the game experience (animations, graphics, UI, sound effects). The onChange has all the info you might need to update your game, including the action / event that triggered it, the old and new game states, info about the players, etc.

The client.js also binds the UI to call the actions. For instance, for Tic Tac Toe, tapping on a cell would trigger Rune.actions.markCell({ cellId }).

const onChange = ({
}) => {
// TODO: Update animations, graphics, UI, sound effects

Rune.initClient({ onChange })

High-Level Game Syncing Flow

Rune does a lot of magic behind the scenes to sync the game state. Here’s a simplified overview of how it works:

  1. A client performs an action by interacting with the game (e.g. clicking a cell in Tic Tac Toe). The client optimistically updates game state by calling the associated action function (i.e. clickCell in the case of Tic Tac Toe) and calls onChange to update the graphics etc.
  2. The action is immediately sent to the server. The server runs the associated actions function provided by the game, thereby checking that the action is valid and whether the game ends.
  3. If the action is valid, the server updates its groundtruth game state and sends the action out to all connected clients. If the action is not valid, it’s ignored.
  4. Each client computes the new game state using the action payload and the associated function in actions. It’s much cheaper bandwidth-wise to send the action than the entire game state. The client who sent the action will also receive the same action payload from the server as an acknowledgement message.


  • Game logic must be written in a subset of JavaScript, see Server-Side Logic. The client rendering the game can use anything.
  • Max 10 actions per player per second.
  • Actions must be synchronous, execute in <10 ms and consume <1 MB memory.
  • The onChange function must be synchronous. It may trigger async functions if needed, but cannot await them.
  • The game state must be <1 MB and any action payload below <100 KB to avoid unnecessary network bandwidth usage.
  • The game state must be JSON-serializable (e.g. no classes/functions) so it can be sent over the network.
  • The logic.js file must be <1 MB as it will be fetched by the server and run inside a VM.

These restrictions are necessary to make great multiplayer games using predict-rollback netcode. Here's more info about why predict-rollback netcode is the future of multiplayer games. It's possible to make all kinds of amazing games using this approach (see the list of supported games for inspiration).

StateSync Event

Games running on Rune should support initializing the game at any possible moment as someone can join as a spectator/player at any time. This could happen e.g. at the start of the game, in the middle of a match, or after game over. This initialization is done using the stateSync event. Additionally, the stateSync event is also used when restarting the game, reconnecting after an unexpected disconnect, or if the game crashes.

Your game must support this stateSync event. If you built your game in a reactive way (i.e. it always rerenders according to onChange's game argument), then you don't need to worry about stateSync event. If your game has side effects, then you might need to specifically handle this event.

You can test your game by adding players/spectators joining at various times during your game session. See Simulating Multiplayer for more info on how we simulate a multiplayer experience when developing.