Publish-Subscribe (XEP-0060)  §6.5.7 allows to retrieve the "most recent items" and Message Archive Management (XEP-0313)  state in §3.1 that archives are ordered in "chronological order". While this order is straighforward in general use cases, it is sometimes desirable to use a different order, for instance while using Microblogging Over XMPP (XEP-0277) : a spelling mistake correction should not bring an old blog post to the top of retrieved items.
This specification allows to explicitly change business logic to retrieve the items in a different order, in a similar way as the "ORDER BY" clause in SQL.
In XEP-0060, there is no such thing as "updated item". This XEP changes the business logic as follow:
Juliet wants to retrieve plays of her favorite writer, William Shakespeare. She wants to retrieve the 3 most recent ones by date of creation.
To do so, her client do a regular Pubsub request, but adds the <order> element as a children of the <pubsub> element with the "urn:xmpp:order-by:0" namespace and with a 'by' attribute equal to "creation".
The Pubsub service then returns the 3 most recently created plays, first one being the most recent.
Juliet realizes that there is a spelling mistake, it's "Winter's Tale" and not "Wintter's Tale". She fixes it by overwritting the item:
To check that everything is alright, she requests again the last 3 items, but this time by date of modification. To do so, the client proceeds the same way as for date of creation, except that it uses the value "modification" for the 'by' attribute.
The Pubsub service returns again the 3 plays but the "Winter Tales" item has been overwritten recently, while the 2 others have never been overwritten, so it returns the items in the following order, with the most recently modified item on top:
This way, filters can be used with a specific ordering.
This specification can be extended by further XEPs, proposing other kind of ordering in the 'by' attribute (e.g. ordering by filename for a file sharing service). But this is beyond the scope of this XEP, and a client should not assume that other ordering than "creation" and "modification" are available without negociation.
In a similar way, the semantic described here could be reused in other use cases as for Pubsub or MAM, but this would need to be detailed in a separate specification.
If a server supports the "order by" protocol, it MUST advertize it including the "urn:xmpp:order-by:0" discovery feature (see Protocol Namespaces regarding issuance of one or more permanent namespaces) in response to a Service Discovery (XEP-0030)  information request:
Several ordering elements may be used, this allows to solve next levels of ordering in case of conflicts. In this case, the first ordering (i.e. the top most <order> element) is the main one, the second <order> element is used in case of conflicts, then the next one if a new conflict happens and so on.
In case of conflicts, if no new <order> element is specified, the item order is not guaranted and is up to the implementation.
For implementations based on SQL databases, the "ORDER BY" clause can be used to easily implement this specification. Other kind of databases should have similar mechanisms.
This document introduces no additional security considerations above and beyond those defined in the documents on which it depends.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
This specification defines the following XML namespace:
If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.
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Editorial language fixes