LWW Register

Last Write Wins Register

class LWWRegister<T> {
  readonly id: string;
  state: [peer: string, timestamp: number, value: T];

  get value() {
    return this.state[2];
  }

  constructor(id: string, state: [string, number, T]) {
    this.id = id;
    this.state = state;
  }

  set(value: T) {
    // set the peer ID to the local ID, increment the local timestamp by 1 and set the value
    this.state = [this.id, this.state[1] + 1, value];
  }

  merge(state: [peer: string, timestamp: number, value: T]) {
    const [remotePeer, remoteTimestamp] = state;
    const [localPeer, localTimestamp] = this.state;

    // if the local timestamp is greater than the remote timestamp, discard the incoming value
    if (localTimestamp > remoteTimestamp) return;

    // if the timestamps are the same but the local peer ID is greater than the remote peer ID, discard the incoming value
    if (localTimestamp === remoteTimestamp && localPeer > remotePeer) return;

    // otherwise, overwrite the local state with the remote state
    this.state = state;
  }
}

class LWWMap<T> {
  readonly id: string;
  #data = new Map<string, LWWRegister<T | null>>();

  constructor(id: string, state: State<T>) {
    this.id = id;

    // create a new register for each key in the initial state
    for (const [key, register] of Object.entries(state)) {
      this.#data.set(key, new LWWRegister(this.id, register));
    }
  }

  get value() {
    const value: Value<T> = {};

    // build up an object where each value is set to the value of the register at the corresponding key
    for (const [key, register] of this.#data.entries()) {
      if (register.value !== null) value[key] = register.value;
    }

    return value;
  }

  get state() {
    const state: State<T> = {};

    // build up an object where each value is set to the full state of the register at the corresponding key
    for (const [key, register] of this.#data.entries()) {
      if (register) state[key] = register.state;
    }

    return state;
  }

  has(key: string) {
    return this.#data.get(key)?.value !== null;
  }

  get(key: string) {
    return this.#data.get(key)?.value;
  }

  set(key: string, value: T) {
    // get the register at the given key
    const register = this.#data.get(key);

    // if the register already exists, set the value
    if (register) register.set(value);
    // otherwise, instantiate a new `LWWRegister` with the value
    else this.#data.set(key, new LWWRegister(this.id, [this.id, 1, value]));
  }

  delete(key: string) {
    // set the register to null, if it exists
    this.#data.get(key)?.set(null);
  }

  merge(state: State<T>) {
    // recursively merge each key's register with the incoming state for that key
    for (const [key, remote] of Object.entries(state)) {
      const local = this.#data.get(key);

      // if the register already exists, merge it with the incoming state
      if (local) local.merge(remote);
      // otherwise, instantiate a new `LWWRegister` with the incoming state
      else this.#data.set(key, new LWWRegister(this.id, remote));
    }
  }
}

An Interactive Intro to CRDTs | jakelazaroff.com

CRDTs don't have to be all academic papers and math jargon. Learn what CRDTs are and how they work through interactive visualizations and code samples.

https://jakelazaroff.com/words/an-interactive-intro-to-crdts/