fmg-simple-adjudicator

Simple Adjudicator

The simple adjudicator is the on-chain contract that is responsible for holding the funds deposited by the parties and for managing on-chain challenges.

The simple adjudicator can only support a single state channel game, meaning that a new on-chain deposit is required for each game played by the participants. We provide this adjudicator as a usable but inefficient proof-of-principal. In the long term, we would want to see adjudicators that support ledger channels and virtual channels.

Usage

Here we go over some example happy-case useage for the simple adjudicator. There are will soon be more comprehensive examples in the tests.

Note: In what follows we interact with a StateWallet object. This is for illustration purposes only - the StateWallet isn't written yet. Ultimately it will be responsible for signing and storing states, as well as making sure that the state transitions are allowed according to the rules of the game being played.

Pre-funding setup

Before putting any funds into the adjudicator both A and B need to be holding signed agreements of their intent to start the game with pre-agreed balances. These agreements give them the capability (through a force-move) to recover any funds that they have deposited in the case that the opponent stalls.

import { Channel } from 'fmg-core';
import { MyGame } from 'my-game';

// the contract describing MyGame should be installed on-chain in a single global location
let myGameContract = MyGameContract.deployed();
// participants are defined by their addresses
let participants = [addressOfA, addressOfB];
// choose the channelNonce to make (myGameContract.address, channelNonce, participants) unique
let channelNonce = 12345678; 
// starting amounts
let aBal = Number(web3.toWei(6, "ether"));
let bBal = Number(web3.toWei(4, "ether"));
let resolution = [aBal, bBal];

// In a's client:
let startPos = { /* ... */ } // the game state that a wants to start in
let channel = new Channel(myGameContract.address, channelNonce, participants);
let state0A = MyGame.preFundSetup({ channel, turnNum: 0, stateCount: 0, resolution, startPos });
let message0A = StateWalletA.sign(state0A);

// a ---> message0A ----> b

// In b's client:
StateWalletB.receive(message0A);
let move0A = MyGame.fromMessage(message0A);
let state1B = MyGame.preFundSetup({ ...move0A, turnNum: 1, stateCount: 1 });
let message1B = StateWalletB.sign(state1B);

// b ---> message1B ----> a

// In a's client:
StateWalletA.receive(message1B);

Funding the channel with the simple adjudicator

At this point both players hold two signed messages signifying their agreement to commence the game in the specified start state. They can now fund the project:

// In a's client:
let adj = await SimpleAdjudicator.new(channel.id).address; // deploy adjudicator with channelId
await web3.eth.sendTransaction({ to: adj, value: aBal }); // fund contract

// a ---> adj ---> b

// In b's client:
assert(Number(web3.eth.getBalance(adj)) == aBal); // check that a deposited the right amount
await web3.eth.sendTransaction({ to: adj, value: bBal }); // fund contract

// b ----> "done" ---> a

// In a's client:
assert(Number(web3.eth.getBalance(adj)) == aBal + bBal); // check that a deposited the right amount

let state2A = MyGame.postFundSetup({ channel, turnNum: 2, stateCount: 0, resolution, startPos });
let message2A = StateWalletA.sign(state2A);

// a ---> message2A ---> b

// In b's client
StateWalletB.receive(message2A);
let state3B = MyGame.postFundSetup({ channel, turnNum: 3, stateCount: 1, resolution, startPos });
let message3B = StateWalletB.sign(state3B);

At this point, the adjudicator is funded and both participants have confirmed. They are now free to proceed with the game, starting from the agreed starting position.