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From Howard Chu <>
Subject Re: [Replication] Using time offsets instead of absolute time
Date Tue, 20 Jan 2009 03:05:07 GMT
Alex Karasulu wrote:
> Hi all,
> The first thread on replication after Emmanuel's comments started taking
> a turn towards the question of time synchronization across replicating
> servers. Time synchronization is very nice to have and forget about
> while writing the code around replication. And yes we do not have
> control over it since administrators can fudge this up.
> Another alternative would be to have all the servers in the cluster
> communicate with one another on occasion to share their current time to
> establish offsets. These offsets (differences in time) between servers
> in replication agreements can be used to correct for any time variance
> we might encounter. Furthermore if the time is adjusted on the hosts,
> the cluster can adjust to time changes.

Doing this accurately requires the same amount of work as NTP, to factor in 
network roundtrip delays etc.

In one of my prior jobs I took this route for a Kerberos client implementation 
on Windows. (Which, if you're curious, is used by Eudora...) Since Kerberos 
would refuse to authenticate a client if its clock was more than 5 minutes 
different from the server, we simply recorded the clock offset of each KDC in 
the client. But - wtf - we had to use SNTP to maintain the offsets anyway...

   -- Howard Chu
   CTO, Symas Corp. 
   Director, Highland Sun
   Chief Architect, OpenLDAP

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