[Standards] XEP-136 and XEP-59 implementation comments

Peter Saint-Andre stpeter at stpeter.im
Mon Mar 3 23:18:09 UTC 2008


Finally getting to this. Comments inline.

On 2007-11-04 (!), Alexander Tsvyashchenko wrote:
> 
> Hello All,
> 
> When working on mod_archive_odbc implementation (XEP-136 support for
> ejabberd) and libwsw (the library for XEP-136 support on clients side) I
> discovered different issues with XEP-136 standard which I’d like to
> present here, in the hope that they still can be addressed until XEP-136
> goes “gold”.
> 
> I hope posting those issues here is the right way to start discussion
> about their resolution, if not - I appologize for that and would
> appreciate if someone points me to the right way of doing it.
> 
> For those issues that have proposed solutions, support for these
> solutions was implemented and verified in mod_archive_odbc and libwsw,
> so they’re certainly feasible.
> 
> The HTML version of these comments is available at
> http://www.ndl.kiev.ua/typo/articles/2007/11/14/xep-136-and-xep-59-implementation-comments
> 
> 
> Replication
> ===========
> 
> Replication in XEP-136 has at least two flaws which make its usage
> somewhat problematic.
> 
> Duplicate items
> ---------------
> 
> Basically, it is stated that client should use specifically prepared
> <after>
> RSM tag to specify the point where to start from.
> 
> IMHO this seems to be quite bad decision on its own for at least two
> reasons:
>  1. This violates XEP-59, item 2.2: "The requesting entity MUST treat
> all UIDs
>     as opaque".
>  2. It differs from all other XEP-136 commands, which use "start" / "end"
>     attributes to specify the required range, thus creating unwanted
>     and confusing "special case".
> 
> That being not enough, it creates yet another problem.
> 
> Consider several changes that are done at the same period of time - for
> example,
> as part of "remove range" request, but it can happen even without that
> if several messages come in different conversations at the same time
> and auto-archiving is enabled - and, thus, change times of these
> conversations
> become equal.
> 
> Now if someone issues <modified> query and this query due to RSM <limit>
> clause
> stops at some of the collections with the same changed time, the next
> query will
> either list all collections which were sent to the client already with
> the same
> time - or skip all remaining collections with the same time, as the server
> have no way to determine where to start its answer having only "change
> time"
> value in <after> element.
> 
> Both cases seems to be quite bad, as first one requires the additional
> filtering on client side and in some cases may mean client will enter the
> infinite loop (if <limit> size is less than number of collections with the
> same changed time), and second one means that some data will be just
> missing,
> thus destroying synchronization between client and server.
> 
> Proposal: change "10. Replication" item by removing references to <after>
> and <last> element and stating that start replication date should be
> specified
> using "start" attribute of "modified" command with additional note that
> the collections with changed time exactly equal to "start" time are NOT
> included
> in the result (thus, "start" will effectively work as "after").
> 
> So, rephrasing the query from Example 57:
> 
>     <iq type='get' id='sync1'>
>       <modified xmlns='http://www.xmpp.org/extensions/xep-0136.html#ns'
>                 start='1469-07-21T01:14:47Z'>
>         <set xmlns='http://jabber.org/protocol/rsm'>
>           <max>50</max>
>         </set>
>       </modified>
>     </iq>
> 
> Probably, it may make sense to use "after" attribute in "modified"
> command to
> highlight the difference with "start", I'm not sure which solution is
> better.
> 
> Then "modified" command may work just like any other command and RSM
> will also
> be used consistently to page through results without any ambiguity.
> 
> This change can be done with complete backward compatibility: if server
> discovers that there's <after> RSM element that specifies datetime - use
> old
> mechanism, if <after> is not specified or is not datetime - use the new
> one.

+1 to that modification.

> Who changed it?
> ---------------
> 
> Typically the client will perform replication when it has some local cache
> for collections / messages, to synchronize its cache with server one.
> Therefore, it makes sense that client also use this cache for caching those
> collections client uploads.
> 
> However, implementing it strictly according to XEP-136 means that client
> has no way to determine if the changes received in replication were done
> by this client or not - so, it will have to re-fetch entire collection even
> if <changed> item in replication results was caused by upload from itself,
> thus basically downloading the same collection it just uploaded on the
> server,
> which is stored already in local cache.
> 
> Proposal: extend replication answer to include "by" attribute, which
> specifies
> full JID of entity who made that change. Then client that receives
> replication
> results can verify if the change was done by itself or not, thus discarding
> those changes that are cached locally already.
> 
> Example:
> 
>     <iq type='result' to='romeo at montague.net/orchard' id='sync1' >
>       <modified xmlns='http://www.xmpp.org/extensions/xep-0136.html#ns'>
>         <removed by='romeo at montague.net/pda'
>                  with='balcony at house.capulet.com'
>                  start='1469-07-21T03:16:37Z'/>
> .....
> 
> This change can be done with complete backward compatibility, as it's just
> extends the answer format.

I prefer the collection-versioning proposal you made here:

http://mail.jabber.org/pipermail/standards/2007-November/017199.html

However, I think the server needs to include the version in the
IQ-results it sends to the client after an upload (this way the client
knows the latest version number). I'll add something about that in the spec.

> XEP-59: detecting the change
> ============================
> 
> During caching in client implementation I faced up with the problem
> that it may be dangerous to fetch collections when client is not
> synchronized
> with the server, as if client maintains some internal state based on
> received
> results and it receives results after some change was made, but it
> doesn't know
> it - it may screw up its internal state.
> 
> Consider the following example: the client builds indices for
> collections that
> are fetched by utilizing RSM "index" attribute, so that it can answer
> locally
> indexed requests. However, these local indices are valid until the change
> is made on server side - after that they should be rebuild using
> replication.
> 
> However, if now the client fetches some collections after the change
> happened
> on the server, and it wasn't able to discover that - it will screw up its
> indices by inserting fetched collections in local cache, as indices may
> be shifted already - and will be shifted once more, when performing
> replication,
> as client cannot detect at replication time what collections were
> fetched before
> replication and what collections after it.
> 
> Please note that this is just one possible scenario, there may be some
> others.
> All such scenarios would require some form of determining whether fetched
> results are "valid", which translates to "were they changed compared to
> some
> fixed time point?"

This is a general problem for Result Set Management. For example, when
using RSM with search (XEP-0055), it is possible for the underlying
representation of the result set to change while the client is paging
through the results (e.g., PageRank changed which item is #14 in the
result set for a Google search). In search we don't care about this
problem so much, but you're right that in message archiving it might matter.

> One possible solution here is to perform replication before and after each
> query to the server, and discard results of query just performed if it is
> found out that change took place - however, this seems to be unacceptably
> high overhead, as instead of 1 query the client has to perform 3 queries.

Ick. :)

> Proposal: add to RSM result the tag "changed", which, when present,
> indicates the datetime of the most recent change of the items affected by
> the query. It typically shouldn't be that problematic to compute this value
> (certainly it wasn't for XEP-136 implementation), and it can be made
> optional,
> as it is done with "index" if in some cases it's hard to calculate it.

Can this be handled via the 'version' attribute in archiving?

> Example:
> 
>     <iq type='result' to='romeo at montague.net/orchard' id='sync1' >
>       <modified xmlns='http://www.xmpp.org/extensions/xep-0136.html#ns'>
>         <removed by='romeo at montague.net/pda'
>                  with='juliet at capulet.com/chamber'
>                  start='1469-07-21T02:56:15Z'/>
>     ...
> 
>         <changed by='romeo at montague.net/orchard'
>                  with='balcony at house.capulet.com'
>                  start='1469-07-21T03:16:37Z'/>
>         <set xmlns="http://jabber.org/protocol/rsm">
>           <first index="0" >63362086582 at 1</first>
>           <last>63362092915 at 51</last>
>           <changed>1469-07-21T04:22:39Z</changed>
>           <count>1372</count>
>         </set>
>       </modified>
>     </iq>
> 
> Inconsistencies or omissions
> ============================
> 
> Start attribute
> ---------------
> 
> Attribute "start" usage seems to be inconsistent:
> 
> * For "8.1 Retrieving a List of Collections" it is "If only 'start' is
> specified then all collections on or after that date should be returned."
> * For "8.3 Removing a Collection" it is "If the end date is in the
> future then
> then all collections after the start date are removed."
> 
> I assume it's just a typo and for 8.3 it should be "on or after" instead of
> "after", no?

Correct. Fixed.

> Remove JID from prefs
> ---------------------
> 
> I'm not sure if this is a problem or not, but it seems there's no way
> to remove JID from user prefs once it is there. I'm not really
> comfortable with
> this as it means even if some JIDs are removed from your roster, all their
> collections are also removed - but still you have them in prefs without the
> possibility to remove them, and this list will grow over time.
> 
> Wouldn't it make sense to specify that uploading item with all tags besides
> JID being empty removes this user from prefs, in the same way it is done
> for links and extra info in chats?

That seems kind of hackish. I see two other options:

1. Modify the complete prefs and remove that item (but I don't see an
example of modifying the complete prefs -- all the examples are more
granular).

2. Define a new <remove/> element, like so:

<iq type='set' id='pref3'>
  <remove xmlns='urn:xmpp:tmp:archive'>
    <item jid='romeo at montague.net'/>
  </remove>
</iq>

> JIDs prefs: ambiguity
> ---------------------
> 
> Possibly related to previous item: what should happen if during prefs
> upload
> some attributes for JID are not specified, and they were present earlier?
> Should they be reset to default values, or remained as they were before
> update?

I think leave them as they were. That's the rule we tend to follow in
other protocols.

> Taking into account previous item, I see two possibilities:
> 
>  1. All missing attributes are remained as is unless none are specified
> - then
>     request is treated as removal request.
>  2. All missing attributes are reset to default values unless none are
>     specified - then request is treated as removal request.
> 
> Second case seems to be more logical for me, as then removal behavior
> follows
> almost automatically from general case.

I would prefer an explicit removal command.

> Resource modification when auto archiving
> -----------------------------------------
> 
> When performing auto archiving it's possible that the initial message
> may be
> not enough to determine full JID of the recipient - if the conversation
> is initiated by the client whose server performs auto archiving and the
> client
> does not know what resource it should use, it will send the message to
> bare JID,
> thus initiating auto archiving for collection with bare JID.
> 
> However, when reply message is received, we know now the full JID -
> thus, it
> could make sense to adjust initial collection, changing its JID to full
> JID,
> otherwise we will either start new collection after message is received -
> or continue recording in bare JID collection, thus effectively eliminating
> resource usage in "with" attribute altogether.

That makes sense.

> Of course, the possibility here would be to just drop all resources from
> JIDs
> and store only bare JIDs, but that seems to be too limiting and
> inconvenient.

I'm not so sure.

There is also the case of sending a message to the bare JID and the
receiving server sends that message to all resources. Then the recipient
could reply from multiple resources, thus starting multiple
conversations! I'm not sure how to handle that. Probably it's best to
save each conversation separately but each conversation / collection has
the same start message (however they might have different threads).

> The proposal here is to specify the algorithm the implementation should use
> to perform tracking of conversations, making the best effort to determine
> and correct JIDs when additional info becomes available.
> 
> In mod_archive_odbc I've implemented tracking algorithm, but it's
> possible I've
> missed some points due to not really good knowledge of XMPP standards or
> lack of experience with XMPP-related developments. I will provide
> description
> of the algorithm in appendix - please, fill free to comment on it or
> take it
> as the basis for inclusion to XEP-136, if it appears to be useful.
> 
> Various small notes
> ===================
> 
> Duplicate messages times
> ------------------------
> 
> In "5.3 Uploading Messages to a Collection" it's specified that "If the
> collection already exists then the server
> MUST append the messages to the existing collection." However, it's not
> said
> what should be done if time for some of the messages is equal to time of
> those
> messages existing already in collection.
> 
> I assume that from "append the messages" clause it follows that
> duplicate entities
> should be created, but it could be good to mention to avoid ambiguities.

By "duplicate entities" do you mean <from/> or <to/> elements with the
same dateTime?

> Malformed XML in examples
> -------------------------
> 
> "Example 21. Private chat linked to later groupchat" and
> "Example 24. Private chat with attributes form" contain malformed XML:
> first message in chats starts as "to", but closes as "from".

Fixed.

> List collections for Bare JID / Domain
> --------------------------------------
> 
> There seems to be no way to list collections solely for service JID,
> as according to XEP-136 it's treated as domain JID request.
> 
> For example, when trying to list all collections for icq.example.com
> you will get instead all collections of all users at icq.example.com - even
> if you wanted to receive collections ONLY for icq.example.com
> 
> I do not think this is major problem, as it can be filtered out on
> client side -
> the only drawback is high amount of extra traffic, so, probably, it can
> be left as it is, but adding some notice in specification on that subject
> could be nice.

Hmm. That's the matching process we use in Multi-User Chat (XEP-0045)
and Privacy Lists (XEP-0016) and so on. I don't see this as a big
problem (you don't really chat with services directly), but it we find
out that it causes problems in reality we can fix it later.

> File format
> -----------
> 
> From my experience it seems that limiting one file to conversation with
> just
> one JID is too restrictive - dealing with one single file for all JIDs
> is much
> more convenient in many cases than with a bunch of files.
> 
> On the other hand, I can imagine when it's better to separate it to
> small files.
> 
> Therefore, probably the restriction could be just removed by allowing
> having
> "with" attribute in "chat" items stored in file and making "with"
> attribute for
> "archive" tag optional? This doesn't seem like a big change, but will
> certainly
> make this file format more useable for those cases when one big file is
> preferred,
> such as backup, or import / export.

I've removed the text about the file format. Let's leave that up to the
implementation.

> Appendix: conversations tracking
> ================================
> Here is the approach that is used in mod_archive_odbc.
> 
> It's assumed that information about all active collections being
> recorded is stored in dictionary.
> 
> The dictionary has two levels: first level key is bare JID, second level
> key is the thread. If thread is not present, {no_thread, Resource} is used
> instead.
> 
> Algorithm for deciding on collection to use when some message is
> received is as follows:
> 
> 1. If thread is specified in the message - just use both-levels keys
> normally, reusing some existing collection if there's a match or
> creating new one if no matching collection found.
> 2. If no first-level key exists for this JID: create new collection +
> two level keys with existing information, second level key will be
> {no_thread, Resource} with Resource being either empty or non-empty.
> 3. Otherwise use first-level key to get access to second-level keys, then:
>     * If resource IS specified: search for matching resource through
> second-level keys:
>         - if found - just use the appropriate collection.
>         - if not, search for second-level key with empty resource. If
> found, use its collection and rewrite key's & collection's empty
> resource to the new one. If not - create new collection + key.
>     * If resource IS NOT specified: use the most recent second-level key
> or create new collection if none exists.

That seems reasonable.

> Good luck!                                     Alexander

Thanks for your feedback, and sorry for taking so long to reply!

Peter

-- 
Peter Saint-Andre
https://stpeter.im/


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