Tool to synchronize events between Logux logs

Usage no npm install needed!

<script type="module">
  import loguxSync from 'https://cdn.skypack.dev/logux-sync';


Logux Sync Cult Of Martians

A tool for synchronizing actions between Logux logs. It could synchronize logs on different machines through network, or on same machine.

Also it does authentication, actions filtering, timestamp fixing and connection diagnostics.

Synchronization protocol specification: logux/protocol.

In most use cases, you don’t need to create Sync, high-level Logux tools will do it for you. But you will have access to Sync API from that tools.

import BrowserConnection from 'logux-sync/browser-connection'
import ClientSync from 'logux-sync/client-sync'
import Reconnect from 'logux-sync/reconnect'

const connection = new BrowserConnection('wss://logux.example.com')
const reconnect  = new Reconnect(connection)
const sync = new ClientSync('user' + user.id + ':' + uuid, log1, connection, {
  subprotocol: '3.0.0',
  credentials: user.token,
  outFilter: action => Promise.resolve(action.sync)

import { ServerSync, ServerConnection } from 'logux-sync'

wss.on('connection', function connection (ws) {
  const connection = new ServerConnection(ws)
  const sync = new ServerSync('server', log2, connection, {
    subprotocol: '3.1.0',
    outFilter: action => access(action),
    auth: token => checkToken(token)
Sponsored by Evil Martians


Logux protocol can work with any data encoding format (e.g. JSON or XML) and via any data transfer channel (e.g. WebSockets Secure or AJAX with HTTP “keep-alive”).

You can create special connection classes for different channels and encoding formats and use them with Logux Sync.

import BrowserConnection from 'logux-websocket/browser-connection'
const connection = new BrowserConnection(serverUrl)
const sync = new ClientSync(nodeId, log, connection, opts)

Connection instance should provide connect() and disconnect() methods and connect, disconnect and message events in NanoEvents API.


Some old proxy servers may block unsafe WebSocket protocol. It is one of the reasons why we highly recommend to use wss:// over ws://.

WebSocket Secure is a “HTTPS” for WebSockets. First of all, it increases user security. On the other hand, it also protects you from proxy-related problems.


Logux Sync has special wrapper classes, which automatically reestablish lost connections:

const reconnect = new Reconnect(connection, {
  minDelay: 0,
  attempts: 10

Client and Server

There is not that much difference between ClientSync and ServerSync. For one, the client will send connect, when connection will be started. Also, the server will destroy itself after the connection is closed.

Messages are same for client and server. However, if you want a different behaviour, you can take BaseSync class and make your own roles (e.g. for a multi-master synchronization).

Node ID

Node ID are used in the action ID generator and it uniqueness is very important for correct timing and log synchronization.

Ensure using unique node IDs. For example, your back-end application may use a counter to generate short and unique IDs. You can put this name in a <meta> tag for using it in the client JS code.

If you can’t generate short unique IDs, UUID will be best way.

Current node ID will be saved to the localNodeId property. Remote node ID will be saved to remoteNodeId.

console.log('Start synchronization with ' + client.remoteNodeId)

Subprotocol Versions

Subprotocol is an application protocol, which you build on top of Logux. It consists of actions and expected reactions on them.

In future you may want to change some action types or options. But some clients could still be using old code for some time after an update.

It’s a good reason to specify a subprotocol version in SemVer format using Logux Sync.

new ClientSync(nodeId, log, connection, {
  subprotocol: '3.1.0'

Logux will send this version from the client to the server and from the server to the client. Remote node subprotocol will be saved as remoteSubprotocol:

if (!semver.satisfies(sync.remoteSubprotocol, '4.x')) {

You can check if the subprotocol is supported in the connect event and send a wrong-subprotocol response in case of a wrong subprotocol version:

import SyncError from 'logux-sync/sync-error'
sync.on('connect', () => {
  if (!semver.satisfies(sync.remoteSubprotocol, '>= 4.0.0')) {
    throw new SyncError(sync, 'wrong-subprotocol', {
      supported: '>= 4.0.0',
      used: sync.remoteSubprotocol


Authentication is built-in into Logux protocol. Both client and server can have credentials data (yet in most use cases only client will have it). Both can as well have an auth callback for authenticating.

new ClientSync(nodeId, log, connection, {
  credentials: user.token

Credentials can be stored as a string, number, object or an array.

Authentication callback should return a promise with true or false. All messages from this node will wait until the authentication is finished.

new ServerSync('server', log, connection, {
  auth: token => {
    return findUserByToken(token).then(user => {
      return !!user

Time Fixing

Some clients may have wrong time zone and fixed it by setting a wrong time. Other client may have a small (±10 minutes) time mistake or just ignore the computer’s time.

Nevertheless, the correct time is vitally important for CRDT and other log based operations.

This is why you can enable time fixing using the fixTime option. ClientSync enables it by default.

Logux Sync will calculate a round-trip time and compare client and server times in order to calculate the time difference between them.

This fix will be applied to the actions time timestamps before sending them to the server or receiving them from the server to a client log.


At every moment, the client-server interaction can be in one of 5 possible states:

  • disconnected: there is no connection, nor new actions for synchronization.
  • wait: new actions are awaiting synchronization, but there is no connection.
  • connecting: connection was established and we wait for the node's response.
  • sending: new actions were sent, waiting for the answer.
  • synchronized: all actions are synchronized, and the connection is active.

You can get the current state accessing the state property or subscribing to it changes using the state event:

client.on('state', () => {
  if (client.state === 'disconnected' || client.state === 'sending') {
  } else {


After receiving connect and connected messages, nodes will synchronize actions.

Every node has lastSent and lastReceived properties. They contain the latest added time of sent and received actions.

If the node will go offline, lastSent and lastReceived properties will be used on the next connection for finding new actions for synchronization.

In most cases, you don’t need to synchronize all actions. Some client actions are local (like clicks or animation updates). Some server actions, however, are now allowed to be shown for every client.

So client and server have inFilter and outFilter options. This callbacks should return Promises resolving with true or false.

In the outFilter, you can specify the action to send:

new ClientSync(nodeId, log, connection, {
  outFilter: action => Promise.resolve(action.sync)

In the inFilter you can specify the actions to be received:

new ServerSync(nodeId, log, connection, {
  inFilter: action => doesUserHaveWriteAccess(action)

Also, you can change actions before sending or adding them to the log using inMap and outMap options.


Sometimes the connection can go down without emitting a disconnected event. So there is an explicit answer for every message in order to ensure that it was received.

You can set a milliseconds timeout option and if the answer will not received after this time, Logux Sync will close the connection and throw an error.

new ClientSync(nodeId, log, connection, {
  timeout: 5000

To be sure that connection is working and you get the latest state from the server, Logux Sync can send ping messages. Set milliseconds ping option specifying how often it should test the connection:

new ClientSync(nodeId, log, connection, {
  timeout: 5000,
  ping: 10000

All synchronization errors will be thrown to be shown in the terminal. You can process this errors and disable throwing by catch() method:

client.catch(error => {


When you will not need synchronization, call the destroy() method. It will remove every listener and disable the synchronization.


LocalPair is a loopback connection pair synchronization on the same machine.

import { LocalPair } from 'logux-sync'
const pair = new LocalPair()
const client = new ClientSync('client', log1, pair.left)
const server = new ServerSync('server', log2, pair.right)

But we also have special TestPair with test helpers:

import { TestPair } from 'logux-sync'

it('should send messages', () => {
  const pair = new TestPair()
  const client = new ClientSync('client', log1, pair.left)

  return pair.wait().then(() => {
    return pair.wait()
  }).then(() => {
      ['connect', …]