source: Dev/branches/rest-dojo-ui/client/dojox/cometd/timesync.js @ 274

dojo.provide("dojox.cometd.timesync");
dojo.require("dojox.cometd._base");

/**
 * this file provides the time synchronization extension to cometd.
 * Timesync allows the client and server to exchange time information on every
 * handshake and connect message so that the client may calculate an approximate
 * offset from it's own clock epoch to that of the server.
 *
 * With each handshake or connect, the extension sends timestamps within the
 * ext field like: <code>{ext:{timesync:{tc:12345567890,l:23,o:4567},...},...}</code>
 * where:<ul>
 *  <li>tc is the client timestamp in ms since 1970 of when the message was sent.
 *  <li>l is the network lag that the client has calculated.
 *  <li>o is the clock offset that the client has calculated.
 * </ul>
 * The accuracy of the offset and lag may be calculated with tc-now-l-o,
 * which should be zero if the calculated offset and lag are perfectly
 * accurate.
 * <p>
 * A cometd server that supports timesync, should respond only if the
 * measured accuracy value is greater than accuracy target. The response
 * will be an ext field like: <code>{ext:{timesync:{tc:12345567890,ts:1234567900,p:123,a:3},...},...}</code>
 * where:<ul>
 *  <li>tc is the client timestamp of when the message was sent,
 *  <li>ts is the server timestamp of when the message was received
 *  <li>p is the poll duration in ms - ie the time the server took before sending the response.
 *  <li>a is the measured accuracy of the calculated offset and lag sent by the client
 * </ul>
 *
 * On receipt of the response, the client is able to use current time to determine
 * the total trip time, from which p is subtracted to determine an approximate
 * two way network traversal time. The measured accuracy is used to adjust the assumption
 * that the network is symmetric for traversal time, so: <ul>
 * <li>lag = (now-tc-p)/2-a
 * <li>offset = ts-tc-lag
 * </ul>
 *
 * In order to smooth over any transient fluctuations, the extension keeps a sliding
 * average of the offsets received. By default this is over 10 messages, but this can
 * be changed with the dojox.cometd.timesync._window element.
 */
dojox.cometd.timesync = new function(){
        this._window = 10;              // The window size for the sliding average of offset samples.
        this._lags = [];                // The samples used to calculate the average lag.
        this._offsets = [];             // The samples used to calculate the average offset.
        this.lag=0;                             // The calculated network lag from client to server
        this.offset = 0;                // The offset in ms between the clients clock and the servers clock.
        this.samples = 0;               // The number of samples used to calculate the offset. If 0, the offset is not valid.
        
        this.getServerTime = function(){ // return: long
                // Summary:
                //      Calculate the current time on the server
                //
                return new Date().getTime()+this.offset;
        }
        
        this.getServerDate = function(){ // return: Date
                // Summary:
                //      Calculate the current time on the server
                //
                return new Date(this.getServerTime());
        }
        
        this.setTimeout = function(/*function*/call, /*long|Date*/atTimeOrDate){
                // Summary:
                //      Set a timeout function relative to server time
                // call:
                //      the function to call when the timeout occurs
                // atTimeOrTime:
                //      a long timestamp or a Date representing the server time at
                //      which the timeout should occur.
                
                var ts = (atTimeOrDate instanceof Date) ? atTimeOrDate.getTime() : (0 + atTimeOrDate);
                var tc = ts - this.offset;
                var interval = tc - new Date().getTime();
                if(interval <= 0){
                        interval = 1;
                }
                return setTimeout(call,interval);
        }

        this._in = function(/*Object*/msg){
                // Summary:
                //      Handle incoming messages for the timesync extension.
                // description:
                //      Look for ext:{timesync:{}} field and calculate offset if present.
                // msg:
                //      The incoming bayeux message
                
                var channel = msg.channel;
                if(channel && channel.indexOf('/meta/') == 0){
                        if(msg.ext && msg.ext.timesync){
                                var sync = msg.ext.timesync;
                                var now = new Date().getTime();
                                var l=(now-sync.tc-sync.p)/2-sync.a;
                                var o=sync.ts-sync.tc-l;
                                
                                this._lags.push(l);
                                this._offsets.push(o);
                                if(this._offsets.length > this._window){
                                        this._offsets.shift();
                                        this._lags.shift();
                                }
                                this.samples++;
                                l=0;
                                o=0;
                                for(var i in this._offsets){
                                        l+=this._lags[i];
                                        o+=this._offsets[i];
                                }
                                this.offset = parseInt((o / this._offsets.length).toFixed());
                                this.lag = parseInt((l / this._lags.length).toFixed());
                                
                        }
                }
                return msg;
        }

        this._out = function(msg){
                // Summary:
                //      Handle outgoing messages for the timesync extension.
                // description:
                //      Look for handshake and connect messages and add the ext:{timesync:{}} fields
                // msg:
                //      The outgoing bayeux message
                
                var channel = msg.channel;
                if(channel && channel.indexOf('/meta/') == 0){
                        var now = new Date().getTime();
                        if(!msg.ext){
                                msg.ext = {};
                        }
                        msg.ext.timesync = {tc:now,l:this.lag,o:this.offset};
                }
                return msg;
        }
};

dojox.cometd._extendInList.push(dojo.hitch(dojox.cometd.timesync, "_in"));
dojox.cometd._extendOutList.push(dojo.hitch(dojox.cometd.timesync, "_out"));