Feeds in the data model

So, yesterday we were discussing the possibility of moving the Mugshot
client to use the data model rather than custom notification; a lot of
the information that the Mugshot client uses fits well into the data
model ... information about users, groups, even each individual stacker
block. But there are two things that fit less well:

 - The stack itself
 - The contents of a chat room

In both of these cases, just using a list-valued property wouldn't work
well ... we never want *all* the blocks in your stack or *all* the
messages in the chatroom, just the more recent entries.

The question that comes to mind then is: are these two special cases in
a world of things that can't be handled well by the data model? Or is
there some simple addition to the data model that would handle both of
these cases and lots more similar things besides.

What I think the two things above are is "feeds". Property of a feed:

 - The feed is ordered
 - Items are typically added at or near the end of the feed
 - The basic access operation is "give me N items from the end of 
   the feed"
 - The complete set of items in the feed is potentially very large

Feeds are very familiar concepts via RSS and Atom these days. Along with
the two examples above, we can come up with many more ... from email
inboxes to upcoming concerts. Anything, in fact, that an RSS feed is
used for these days.

The Mugshot stack extends the feed concept a bit with the concept of 

 - An item in the feed can be removed from the feed and reinserted
   at or near the end of the feed.

This sort of idea comes up elsewhere; for example in a threaded or
conversation view of an email inbox, a thread can be moved to the end
when new messages arrive in that thread.

More concretely, I'm thinking that in a data model, a property that that
is feed-valued would have values that are pairs of resources and
timestamps. (So, I've specialized the abstract concept above a bit to
say that the ordering of a feed is by timestamp, and the "end" of a feed
is the most recent items.)


<group xmlns="http://mugshot.org/p/o/group";>
  <chatMessages m:type="f*" m:resource="/o/chatMessage/513423" m:ts="1193931374313"/>
  <chatMessages m:type="f*" m:resource="/o/chatMessage/513426" m:ts="1193932521431"/>

Fetch strings are already specified to allow "chatMessages(max=50) +",
the ordering defined for a feed gives an exact definition of what that
maximum means. A feed valued property would also have a default maximum
if one wasn't specified explicitly in the fetch string.

I haven't yet completely figured out how feeds look like on the server
side. Basically, I think you'd specify a property:

 Feed<ChatMessageDMO> getChatMessages(int start, int max)

And that returns a Feed custom container object that contains *at least*
(but not necessarily only) the feed item items given in the range.

It might be useful to extend ReadWriteSession.changed() to allow saying
not just "chatMessages changed", but to give details of the change, so
we can avoid having to requery past messages and do a diff. That would
take some care to make sure that it was safe with respect to
concurrency, transactions, and threading.

- Owen

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