Subject: [Ecrit] sourceId in LoST response mappings

From: "Jeff Martin" <[email-masked]>
Date: Thu, 8 May 2008 15:52:27 -0700

The current draft-lost-09 doesn't have much to say about how LoST
clients will use the 'sourceId' in the 'mapping' element of responses.

Section 5.1 is clear that a specific LoST server must use unique
'sourceId's for all the possible mappings it can return.  For example,
if an authoritative LoST server currently holds 10 mappings, there must
be 10 distinct 'sourceId' values -- one for each mapping.

The only other thing draft-09 has to say about 'sourceId' is also in
section 5.1:

    A receiver can replace a mapping with another one having the same
    'source' and 'sourceId' and a more recent time in 'lastUpdated'.

Since there is nothing else about 'sourceId', my interpretation is that
clients only look at 'sourceId' as part of the client's caching
strategy.

This would mean a 'mapping' returned for the same input location would
have the same 'sourceId' even if the mapping changed somehow.  Changes
would be reflected in 'mapping' responses in the 'lastUpdated' field and
the actual values like uri, serviceBoundary, displayName, etc.  In other
words, from the perspective of LoST server code, if a user changes the
uri for a specific mapping, the LoST server code would still return the
existing sourceId when responding with that mapping (which now has a
different uri and more recent 'lastUpdated').  

I searched through the list archives to see if there was any other
discussion on sourceId, and I found the following in a Feb 1, 2007
thread (more than a year ago) where Shida Schubert asked similar
questions, and Hannes Tschofenig responded:

    > >  If user A and B queries for "sos"/"regionA" would 
    > >  they get the same mapping data with same sourceId/version?
    >   
    > Yes. They would receive they same mapping if it did not change in
    > between the two requests if their location falls within the same
region.
    > 
    > >  If above are the same when does sourceId/version ever change?
    >   
    > If the mapping Location+Service -> PSAP (+service
boundary+location
    > profile) association changes.
    >  
    > > Assuming server that caches the data can't change
sourceId/version, 
    > > they need to be the same for both A and B? 
    > >
    > > I think the text needs to be elaborated to clarify what 
    > > I have raised as a question.
    >   
    > I will look at it in order to determine what has to be done.

This thread implies that a sourceId *must* change if anything about the
mapping changed.  If the sourceId must change for every mapping change,
then why does the draft-09 talk about clients looking for the same
'sourceId' with a different 'lastUpdated'?  There would be no way for a
server to ever return a different sourceId/lastUpdated combination if
any change meant a new sourceId.

So which way is it:

- 'sourceId' *must* change for any change to the mapping as discussed on
the mailing list. In which case the draft-09 describes a situation that
is not possible.

- 'sourceId' does not have to change.  In which case the mailing list
discussion reached incorrect conclusions.

Thanks,
-- Jeff

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