[K12OSN] Not recreating the wheel

Christopher K. Johnson ckjohnson at gwi.net
Fri May 14 12:05:56 UTC 2004

Jim Wildman wrote:

>On Thu, 13 May 2004, Lewis Holcroft wrote:
>Wow.  That was quite a diagram!
>I would not do the nfs over the isdn lines.  rsync will work fine to
>keep the boxes in sync for basic file stuff, either pushing from the
>master for pulling from the client end.  For the people who move around,
>just set an rsync job for them that runs more often than they can get
>between the 2 closest buildings.  It doesn't have to be fast, just
>faster than them.
>While folks like to talk about instantenous access to everything, in
>reality the cost to go from "you will never wait more than 15 minutes" to
>"instaneous" can be rather prohibitive.
The tricky thing about using rsync, is that you need to make it work in 
the correct direction.  When not logged in at an alternate location it 
needs to sync all other sites to the person's primary site.  When logged 
in at an alternate location it needs to sync all other sites to that 
alternate copy of their home directory.  And if you do it with cron jobs 
there is a problem with changes after logging off at an alternate site 
being sync'd the wrong way.  So you need to  manage some state 
indicator  (a file touched at logon and cleared by cron job only if the 
person is no longer logged on) that will enable the cron job to do the 
right thing that one last time after logoff.

I'm not saying it can't be done.  In fact provided a time lag for travel 
between sites I'm sure it could be done.  You just have to tackle these 
issues to make it work right.

By the way, what will remain problematic in the rsync scenario is if 
someone actually does log on remotely to a system.  The worst case is 
when they simultaneously log on to more than one system.  What should 
the rsync cron jobs do then and how will they detect that condition to 
accomodate it?  Perhaps you need to tell people "don't log on to more 
than one server at a time".

   "Spend less!  Do more!  Go Open Source..." -- Dirigo.net
   Chris Johnson, RHCE #807000448202021

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