org.cometd.server.ext

Class TimesyncExtension

java.lang.Object

org.cometd.server.ext.TimesyncExtension

All Implemented Interfaces:

BayeuxServer.Extension

public class TimesyncExtension
extends Object
implements BayeuxServer.Extension

Timesync extension (server side).

With each handshake or connect, the extension sends timestamps within the ext field like: {ext:{timesync:{tc:12345567890,l:23,o:4567},...},...} where:

• tc is the client timestamp in ms since 1970 of when the message was sent.
• l is the network lag that the client has calculated.
• o is the clock offset that the client has calculated.

A CometD server that supports timesync, can respond with an ext field like: {ext:{timesync:{tc:12345567890,ts:1234567900,p:123,a:3},...},...} where:

• tc is the client timestamp of when the message was sent,
• ts is the server timestamp of when the message was received
• p is the poll duration in ms - ie the time the server took before sending the response.
• a is the measured accuracy of the calculated offset and lag sent by the client

The relationship between tc, ts, o and l on the server is given by ts=tc+o+l (the time the server received the message is the client time plus the offset plus the network lag). Thus the accuracy of the o and l settings can be determined with a=tc+o+l-ts.

When the client has received the response, it can make a more accurate estimate of the lag as l2=(now-tc-p)/2 (assuming symmetric lag). A new offset can then be calculated with the relationship on the client that ts=tc+o2+l2, thus o2=ts-tc-l2.

Since the client also receives the a value calculated on the server, it should be possible to analyse this and compensate for some asymmetry in the lag. But the current client does not do this.

Field Summary

Fields

Modifier and Type	Field and Description
static String	LAG_ATTRIBUTE

Constructor Summary

Constructors

Constructor and Description
TimesyncExtension()

Method Summary

Modifier and Type	Method and Description
int	getAccuracyTarget() timesync responses are not set if the measured accuracy is less than the accuracyTarget.
boolean	rcvMeta(ServerSession from, ServerMessage.Mutable message) Blocking version of BayeuxServer.Extension.incoming(ServerSession, ServerMessage.Mutable, Promise) for meta messages.
boolean	sendMeta(ServerSession session, ServerMessage.Mutable message) Blocking version of BayeuxServer.Extension.outgoing(ServerSession, ServerSession, ServerMessage.Mutable, Promise) for meta messages.
void	setAccuracyTarget(int target) timesync responses are not set if the measured accuracy is less than the accuracyTarget.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface org.cometd.bayeux.server.BayeuxServer.Extension

incoming, outgoing, rcv, send

Field Detail

LAG_ATTRIBUTE

public static final String LAG_ATTRIBUTE

See Also:

Constant Field Values

Constructor Detail

TimesyncExtension

public TimesyncExtension()

Method Detail

getAccuracyTarget

public int getAccuracyTarget()

timesync responses are not set if the measured accuracy is less than the accuracyTarget.

Returns:

accuracy target in ms (default 25ms)

setAccuracyTarget

public void setAccuracyTarget(int target)

timesync responses are not set if the measured accuracy is less than the accuracyTarget.

Parameters:

target - accuracy target in ms

rcvMeta

public boolean rcvMeta(ServerSession from,
                       ServerMessage.Mutable message)

Description copied from interface: BayeuxServer.Extension

Blocking version of BayeuxServer.Extension.incoming(ServerSession, ServerMessage.Mutable, Promise) for meta messages.

Specified by:

rcvMeta in interface BayeuxServer.Extension

Parameters:

from - the session that sent the message

message - the incoming message

Returns:

whether message processing should continue

sendMeta

public boolean sendMeta(ServerSession session,
                        ServerMessage.Mutable message)

Description copied from interface: BayeuxServer.Extension

Blocking version of BayeuxServer.Extension.outgoing(ServerSession, ServerSession, ServerMessage.Mutable, Promise) for meta messages.

Specified by:

sendMeta in interface BayeuxServer.Extension

Parameters:

session - the session the message is sent to, or null for a publish.

message - the outgoing message

Returns:

whether message processing should continue