[PubSub] Timestamp on items and versioning

Robin Collier robincollier at hotmail.com
Wed Oct 28 09:50:58 CDT 2009




> Date: Wed, 28 Oct 2009 15:09:48 +0100
> From: koski.tuomas at gmail.com
> To: pubsub at xmpp.org
> Subject: Re: [PubSub] Timestamp on items and versioning
> 
> Hi,
> 
> Peter wrote:
> > On 10/27/09 8:31 PM, Matthew Wild wrote:
> >> 2009/10/28 Brian Cully <bcully at gmail.com>:
> >>> On 27-Oct-2009, at 18:25, Tuomas Koski wrote:
> >>>
> >>>> 2009/10/1 Fabio Forno <fabio.forno at gmail.com>:
> >>>>> so +1 for modtime
> >>>> we seem to have reached a overall understanding that a attribute
> >>>> called "stamp" could be added to the item element. Example:
> >>>>
> >>>> <item stamp='20091027225837256'>
> >>>        It appears so, although I'm still vehemently against it. I loathe
> >>> timestamps and would only recommend them in the payload because they're
> >>> vague and error-prone.
> 
> The original idea to use the "modtime" (as defined in
> http://tools.ietf.org/html/rfc2244#section-3.1.1) is that it's unique.
> So the good point at the moment is that everyone understands the
> problem of the "timestamp".
> 
> How ever I think we are maybe solving different problems and
> fulfilling different needs here. Please read below after Peters
> comment.
> 
> >>>> On 27-Oct-2009, at 18:25, Tuomas Koski wrote:
> >>>> If we are going to do this, should we then also allow the usage of
> >>>> time stamp when requesting items from the node
> >>>> (http://xmpp.org/extensions/xep-0060.html#subscriber-retrieve-requestall)?
> >>>        We should use opaque versions, akin to rosters. This has no vagary,
> >>> is not as error-prone, and will scale to the foreseeable future, where
> >>> timestamps simply do not (what happens when you get three updates in a given
> >>> micro or nanosecond?) The worst case is what to do when a publish comes in
> >>> between requesting items and getting the result, in which case subsequent
> >>> requests for differences from a version will return an ever-diminishing set,
> >>> the most likely case being the empty set. It allows more optimizations and
> >>> does not rely on concepts of time which are wildly variable.
> >>>
> >> I haven't re-read the thread (though I might). However we had pretty
> >> much the same debate over roster versioning... what harm does it do to
> >> take the same approach and make the version an opaque string? Then the
> >> server can use a timestamp (more fool it) or something more 'smart'.
> >
> > Using the same approach for pubsub and roster versioning seems sensible.
> >
> > Peter
> 
> I think Robin Collier's case (I hope I'm not wrong) is, that he needs
> to find a way for clients to handle the <item>s in a correct order. If
> this is really the case (and I have not understood anything wrong),
> based on a quick thought I also think that the "roster version" -way
> of handling this sounds like the correct way to go. What could be a
> good attribute name for this?

Although I am not familiar with the discussion on roster versioning, it
looks like what you are saying is correct.  

The point I raised in my last message though was that the timestamping
is already required for any implementeing service, and I guess I fail to see 
why it cannot server both purposes.  As it is, there seems to be several 
suggestions (like your own) that would required the timestamp on the item.

> 
> My use-case then again would be more of a following: Mobile client is
> subscribed to a node that has a presence_based_delivery set to true.
> This is done not to receive "offline messages" in "uncontrolled way"
> when the client goes online. However, when my client logs in, I am
> still interested of those items published to the nodes while I was
> offline. I need to be able to receive them in a controlled way. I only
> want to have the items since I last time were logged in: in other
> words I need to receive all the possible items that were published
> since the last item I have.
> 
> So I assume I could do something like this (just an idea):
> 
> <iq type='get'
>  from='francisco at denmark.lit/barracks'
>  to='pubsub.shakespeare.lit'
>  id='items1'>
>   <pubsub xmlns='http://jabber.org/protocol/pubsub'>
>    <items node='princely_musings'>
>     <set xmlns='http://jabber.org/protocol/rsm'>
>      <after>id_of_my_last_item</after>
>     </set>
>    </items>
>   </pubsub>
> </iq>
> 
> When the server will receive this it must be able to return the items
> in a correct order. So we come back in a point that the items must
> have an order in the client (what is really the current "latest" item
> so that it can handle items in a correct order) and in the server side
> (it will send the id's in correct order, for example in
> "pagination"-case).
> 
> Do you guys think that these two problems should be discussed in
> separate threads or are we talking in the end about the same issue?
> 
> 
> Cheers,
> --
> xmpp:tuomas at xmpp.lobstermonster.org
 		 	   		  
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