timesync

    1.0.8 • Public • Published

    timesync

    Time synchronization between peers.

    Usage scenarios:

    • master/slave: Clients synchronize their time to that of a single server, via either HTTP requests or WebSockets.
    • peer-to-peer: Clients are connected in a (dynamic) peer-to-peer network using WebRTC or WebSockets and must converge to a single, common time in the network.

    Install

    Install via npm:

    npm install timesync
    

    Usage

    A timesync client can basically connect to one server or multiple peers, and will synchronize it's time. The synchronized time can be retrieved via the method now(), and the client can subscribe to events like 'change' and 'sync'.

    // create a timesync instance
    var ts = timesync({
      server: '...',  // either a single server,
      peers: [...]    // or multiple peers
    });
     
    // get notified on changes in the offset
    ts.on('change', function (offset) {
      console.log('offset from system time:', offset, 'ms');
    }
     
    // get the synchronized time
    console.log('now:', new Date(ts.now()));

    Example

    Here a full usage example with express.js, showing both server and client side. timesync has build-in support for requests over http and can be used with express, a default http server, or other solutions. timesync can also be used over other transports than http, for example using websockets or webrtc. This is demonstrated in the advanced examples.

    More examples are available in the /examples folder. Some of the examples use libraries like express or socket.io. Before you can run these examples you will have to install these dependencies.

    server.js

    var express = require('express');
    var timesyncServer = require('timesync/server');
     
    // create an express app
    var port = 8081;
    var app = express();
    app.listen(port);
    console.log('Server listening at http://localhost:' + port);
     
    // serve static index.html
    app.get('/', express.static(__dirname));
     
    // handle timesync requests
    app.use('/timesync', timesyncServer.requestHandler);

    index.html

    <!DOCTYPE html>
    <html>
    <head>
      <!-- note: for support on older browsers, you will need to load es5-shim and es6-shim -->
      <script src="https://cdnjs.cloudflare.com/ajax/libs/es5-shim/4.0.5/es5-shim.min.js"></script> 
      <script src="https://cdnjs.cloudflare.com/ajax/libs/es6-shim/0.23.0/es6-shim.min.js"></script> 
     
      <script src="/timesync/timesync.js"></script> 
    </head>
    <script>
      // create a timesync instance
      var ts = timesync.create({
        server: '/timesync',
        interval: 10000
      });
     
      // get notified on changes in the offset
      ts.on('change', function (offset) {
        document.write('changed offset: ' + offset + ' ms<br>');
      });
     
      // get synchronized time
      setInterval(function () {
        var now = new Date(ts.now());
        document.write('now: ' + now.toISOString() + ' ms<br>');
      }, 1000);
    </script> 
    </html>

    API

    Client

    Construction

    An instance of timesync is created as:

    var ts = timesync(options);

    Options

    The following options are available:

    Name Type Default Description
    delay number 1000 Delay in milliseconds between every request sent.
    interval number or null 3600000 Interval in milliseconds for running a synchronization. Defaults to 1 hour. Set to null to disable automatically running synchronizations (synchronize by calling sync()).
    now function Date.now Function returning the local system time.
    peers string[] or string [] Array or comma separated string with uri's or id's of the peers to synchronize with. Cannot be used in conjunction with option server.
    repeat number 5 Number of times to do a request to every peer.
    server string none Url of a single server in case of a master/slave configuration. Cannot be used in conjunction with option peers.
    timeout number 10000 Timeout in milliseconds for requests to fail.

    Methods

    Name Return type Description
    destroy() none Destroy the timesync instance. Stops automatic synchronization. If timesync is currently executing a synchronization, this synchronization will be finished first.
    now() number Get the synchronized time. Returns a timestamp. To create a Date, call new Date(time.now()).
    on(event, callback) Object Register a callback handler for an event. Returns the timesync instance. See section Events for more information.
    off(event [, callback]) Object Unregister a callback handler for an event. If no callback is provided, all callbacks of this event will be removed. Returns the timesync instance. See section Events for more information.
    sync() none Do a synchronization with all peers now.

    To be able to send and receive messages from peers, timesync needs a transport. To hook up a transport like a websocket or http requests, one has to override the send(id, data) method of the timesync instance, and has to call ts.receive(id, data) on incoming messages.

    Name Return type Description
    send(to, data, timeout) : Promise none Send a message to a peer. to is the id of the peer, and data a JSON object containing the message. Must return a Promise which resolves when the message has been sent, or rejects when sending failed or a timeout occurred.
    receive(from, data) none Receive a message from a peer. from is the id of the sender, and data a JSON object containing the message.

    timesync sends messages using the JSON-RPC protocol, as described in the section Protocol.

    Events

    timesync emits events when starting and finishing a synchronization, and when the time offset changes. To listen for events:

    ts.on('change', function (offset) {
      console.log('offset changed:', offset);
    });

    Available events:

    Name Description
    change Emitted when the offset is changed. This can only happen during a synchronization. Callbacks are called with the new offset (a number) as argument.
    error Emitted when an error occurred. Callbacks are called with the error as argument.
    sync Emitted when a synchronization is started or finished. Callback are called with a value 'start' or 'end' as argument.

    Properties

    Name Type Description
    offset number The offset from system time in milliseconds.
    options Object An object holding all options of the timesync instance. One can safely adjust options like peers at any time. Not all options can be changed after construction, for example a changed interval value will not be applied.

    Server

    timesync comes with a build in server to serve as a master for time synchronization. Clients can adjust their time to that of the server. The server basically just implements a POST request responding with its current time, and serves the static files timesync.js and timesync.min.js from the /dist folder. It's quite easy to implement this request handler yourself, as is demonstrated in the advanced examples.

    The protocol used by the server is described in the section Protocol.

    Load

    The server can be loaded in node.js as:

    var timesyncServer = require('timesync/server');

    Methods

    Name Return type Description
    createServer() http.Server Create a new, dedicated http Server. This is just a shortcut for doing http.createServer( timesyncServer.requestHandler ).
    attachServer(server, [path]) http.Server Attach a request handler for time synchronization requests to an existing http Server. Argument server must be an instance of http.Server. Argument path is optional, and is /timesync by default.

    Properties

    Name Type Description
    requestHandler function A default request handler, handling requests for the timesync server. Signature is requestHandler(request, response). This handler can be used to attach to an expressjs server, or to create a plain http server by doing http.createServer( timesyncServer.requestHandler ).

    Protocol

    timesync sends messages using the JSON-RPC protocol. A peer sends a message:

    {"jsonrpc": "2.0", "id": "12345", "method": "timesync"}

    The receiving peer replies with the same id and its current time:

    {"jsonrpc": "2.0", "id": "12345", "result": 1423151204595}

    The sending peer matches the returned message by id and uses the result to adjust it's offset.

    Algorithm

    timesync uses a simple synchronization protocol aimed at the gaming industry, and extends this for peer-to-peer networks. The algorithm is described here:

    A simple algorithm with these properties is as follows:

    1. Client stamps current local time on a "time request" packet and sends to server
    2. Upon receipt by server, server stamps server-time and returns
    3. Upon receipt by client, client subtracts current time from sent time and divides by two to compute latency. It subtracts current time from server time to determine client-server time delta and adds in the half-latency to get the correct clock delta. (So far this algorithm is very similar to SNTP)
    4. The first result should immediately be used to update the clock since it will get the local clock into at least the right ballpark (at least the right timezone!)
    5. The client repeats steps 1 through 3 five or more times, pausing a few seconds each time. Other traffic may be allowed in the interim, but should be minimized for best results
    6. The results of the packet receipts are accumulated and sorted in lowest-latency to highest-latency order. The median latency is determined by picking the mid-point sample from this ordered list.
    7. All samples above approximately 1 standard-deviation from the median are discarded and the remaining samples are averaged using an arithmetic mean.

    This algorithm assumes multiple clients synchronizing with a single server. In case of multiple peers, timesync will take the average offset of all peers (excluding itself) as offset.

    Tutorials

    Resources

    Build

    To build the library:

    npm install
    npm run build
    

    This will generate the files timesync.js and timesync.min.js in the folder /dist.

    To automatically build on changes, run:

    npm run watch
    

    Install

    npm i timesync

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    2,623

    Version

    1.0.8

    License

    MIT

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    Total Files

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    • josdejong