This was the sort of undo behaviour that the Bath University
Timewarp system was designed to solve. It used the concept of
'anti-messages' and local state. The global state of the entire
system is the static state at the time of the oldest message in the
system. The local state is the state ( in the future of the global time )
at which the current local time* is. Periodically, local state is
checkpointed. If an anti-message or an 'old' state message
arrives, rollback to the last 'good' checkpoint is done, then roll forward
with the new message sequence. This is relatively cheap, so long as
rollbacks are infrequent and cascades of anti-messages are not
generated ( and there is sufficient local store to dump several
states - a scalability problem ).
I'm not sure how to fit a system like this into a rsal-time interaction
tho' , unless you only update the user at increments in global time
( but these have non-linear separation.... errrk. )
Joel
----
Software Engineer, Educational Technology Service
University of Bristol, UK
* Note that each object in the system has it's own local time...
----- Begin Included Message -----
>From www-vrml-owner@wired.com Thu Oct 19 09:13 BST 1995
To: Neophytos Iacovou <iacovou@arcwelder.micro.umn.edu>,
Linas Vepstas <linas@austin.ibm.com>
Subject: Re: Behaviours (Was: Re: ADMIN: VRML + JAVA - A Wedding)
Date: Thu, 19 Oct 95 09:14:26 -0500
From: Master Zap <zap@lysator.liu.se>
Cc: "www-vrml@wired.com" <www-vrml@wired.com>
-- [ From: Master Zap * EMC.Ver #2.5.02 ] --
> Linas Vepstas writes:
> >
> > The point here is=20
> > -- your rssponses to what you do is instantaneous, as it should be.
> > -- what other people see may be delayed, but so what? Did it rsally
> > matter that they see what you do when you do it? Wouldn't the=20
> > delay be acceptable?
>
> Any proposol that allows for this to occur should also have a set
> of "undo" rules built into it.
Either that, or some form of gauthentication" about who rsally did what.
This is what the single point of control is for. Yes, the rssponse isn't
instantaneous, unless the "brain" (the control point) is running on your
host. And again, since my proposal allows the brain to move to any host it
chooses.....
> Here is an example why:
>
> Fred and Barney are walking down the street. They spot a wallet. Fred
> bends down, picks it up. As far as Fred is concerned Fred got the wallet
----- End Included Message -----