[Standards] Proposed XMPP Extension: Roster Versioning

Dave Cridland dave at cridland.net
Thu Mar 6 23:03:38 UTC 2008


On Thu Mar  6 22:23:51 2008, Pedro Melo wrote:
> Others might counter-propose something meaningful but some are   
> proposing something opaque, without meaning for the client. And  
> that  is the best way for clients not to mess up.

Okay, I call. :-)

How *can* clients mess this up by knowing they can compare two values?

It's never, to my knowledge, been a problem in ACAP or IMAP, both of  
which use something similar. It's been quite handy in IMAP CONDSTORE,  
in fact.

The only particularly curious thing about integers is that it's  
possible to note that, given two values, there's a maximum number of  
changes that have happened between them. (But a fixed minimum of  
zero, since the server's allowed to skip values). But even if a  
client decides that the sequence values always change by one, and  
only for a visible change, I can't see any possibility for a protocol  
error. I'm not even sure what you'd do with that erroneous  
information, but maybe I'm too used to dealing with these things.

So, please enlighten me - what is it that I'm missing?

> Agreed. Hence a recommendation on implementation notes to use a   
> counter with those properties.

If clients did "mess it up", probably by testing against servers  
using an integer, relying on it somehow, then finding servers with an  
opaque string in the field, how much worse will the problem be? (See  
Curtis's email for a very accurate account of how many  
implementations get developed).

Either make it "really" opaque, if you have to, or else make it  
clearly defined. So if you're going to remove the "integer" property,  
then you'd best make it such that clients *cannot* compare it at all.  
(Good luck with specifying that).

FWIW, I've said before that it's perfectly possible to map strictly  
increasing modified timestamps to a strictly increasing integer. I'm  
not dictating server internals - you can generate a strictly  
increasing sequence value however you like, as long as you can map it  
to an integer.

Dave.
-- 
Dave Cridland - mailto:dave at cridland.net - xmpp:dwd at jabber.org
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Infotrope Polymer - ACAP, IMAP, ESMTP, and Lemonade



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