sync.js

/*
  Create two peers, each with one Automerge doc
  peer2's doc is a clone of peer1's to ensure common history
  Each peer has a syncState representing the estimated state of the other peer
  Each peer has a FIFO list "inbox" emulating a network transport
  
  Every 500ms, simulate the network for each peer in turn, via update():
  - check for any messages in the peer's inbox
    - call Automerge.receiveSyncMessage for each message
    - and update doc & sync accordingly
    - then clear the inbox
   - check whether we think we need to send sync message to peer via generateSyncMessage ***
    - update sync accordingly (??)
    - if there was a message, push it into the peer's inbox for the next update()
    
   *** Why does it continue to think there are sync updates required, 
       even though both docs are already in sync?
       
   Notes: 
   it doesn't matter whether we poll peer1 first or peer2, the same thing happens
*/
import Automerge from "https://cdn.skypack.dev/automerge@1.0.1-preview.4";

let peer1 = {
  name: "peer1",
  doc: Automerge.init(),
  sync: Automerge.initSyncState(), // for syncing to peer2
  inbox: [] // FIFO for simulated network transport
};

let peer2 = {
  name: "peer2",
  // peer2's doc is a duplicate of peer1's doc:
  doc: Automerge.clone(peer1.doc),
  sync: Automerge.initSyncState(), // for syncing to peer1
  inbox: [] // FIFO for simulated network transport
};

// check if a peer has changes to send to the other peer
// and if so, post them in the other's inbox
function send(self, other) {
  // do we have any changes (or requests) for the other peer?
  let [newsync, msg] = Automerge.generateSyncMessage(self.doc, self.sync);
  self.sync = newsync;
  // if we have a message to send:
  if (msg) {
    // always peer2
    // console.log('peer sending', self.name)
    // send msg to peer by appending to peer's inbox
    other.inbox.push(msg);
    // log that we sent a message:
    document.getElementById("log").innerText +=
      "\n" +
      "send " +
      self.name +
      "->" +
      other.name +
      " msg: " +
      msg.toString();
  }
}

// check if a peer has any messages in its inbox,
// and apply them if it does
// then clear the inbox
function receive(self, other) {
  // for each message received from the other peer:
  for (const msg of self.inbox) {
    // always peer1
    // console.log("peer receiving", self.name)
    // log that we received a message
    document.getElementById("log").innerText +=
      "\n" +
      "recv " +
      self.name +
      "<-" +
      other.name +
      " msg: " +
      msg.toString();
    // apply the change & update our doc & syncState:
    const [doc, newsync, patch] = Automerge.receiveSyncMessage(
      self.doc,
      self.sync,
      msg
    );
    self.doc = doc;
    self.sync = newsync;
    send(self, other)
    
  }
  // clear inbox:

  self.inbox = [];
}

// both peers initiate a sync
send(peer1, peer2);
send(peer2,peer1);

// simulate network activity
function update(self, other) {
  receive(self, other);
}

setInterval(function () {
  // simulate network activity for both peers
  // doesn't matter which order you do these, same result

  update(peer2, peer1);
  update(peer1, peer2);

  
  // show current state of docs
  document.getElementById("result").innerText =
    "peer1.doc = " +
    JSON.stringify(peer1.doc) +
    "\npeer2.doc = " +
    JSON.stringify(peer2.doc);

  // show current sync states:
  document.getElementById("result").innerText +=
    "\npeer1.sync = " +
    JSON.stringify(peer1.sync, null, "  ") +
    "\npeer2.sync = " +
    JSON.stringify(peer2.sync, null, "  ");

  // verify that the two peers' docs have different actorIDs:
  document.getElementById("result").innerText +=
    "\npeer1-actor: " + Automerge.getActorId(peer1.doc);
  document.getElementById("result").innerText +=
    "\npeer2-actor: " + Automerge.getActorId(peer2.doc);
}, 500);