[Standards-JIG] JEP-0163 (PEP) retrieve items

[Ian Paterson]

JEP-0163 currently specifies that PEP services should not allow item 
retrieval (see the list of "smart defaults" in Section 6). [The 
functionality may be emulated by sending a pair of subscribe/unsubscribe 
requests, but that is not simple, efficient or elegant.]

Item retrieval functionality is trivial to implement, and it would allow PEP 
to act as a very simple generic protocol building-block for 
personal-publishing - where the transmition and reception of notifications 
upon every login are not desired.

For example, Peter's User Fingerprint proto-JEP 
(http://www.jabber.org/jeps/inbox/fingerprint.html) is an efficient way to 
enable subscribers to understand if they already have a local copy of 
another user's public key. In this case notification subscriptions will be 
unnecessary, so there is no need to push the whole public key to subscribing 
contacts every time they become available.

Item retrieval would also facilitate other use cases, especially when the 
"open" access model is used - and node subscribers do not send presence to 
the publisher. For examples:

1. If a node subscriber prefers not to (or is unable to) persist it's own 
copy of the last published item payload between sessions, then item 
retrieval would be the only solution.

2. If the user subscribed to the node is using a different client (not the 
one that subscribed to the node/namespace) at the time when an item is 
republished, then the notification message will be delivered to the wrong 
client (that client may not even support the subscribed namespace). So the 
client that subscribed will never receive the notification, and, without 
item retrieval, it has no nice way of obtaining the republished item. (It 
seems likely that I'm missing something here?)

- Ian