XEP-0413: Order-By

Abstract
This specification allows to change order of items retrieval in a Pubsub or MAM query
Author
Jérôme Poisson
Copyright
© 1999 – 2019 XMPP Standards Foundation. SEE LEGAL NOTICES.
Status

Experimental

WARNING: This Standards-Track document is Experimental. Publication as an XMPP Extension Protocol does not imply approval of this proposal by the XMPP Standards Foundation. Implementation of the protocol described herein is encouraged in exploratory implementations, but production systems are advised to carefully consider whether it is appropriate to deploy implementations of this protocol before it advances to a status of Draft.
Type
Standards Track
Version
0.1.0 (2019-02-04)
Document Lifecycle
  1. Experimental
  2. Proposed
  3. Draft
  4. Final

1. Introduction

Publish-Subscribe (XEP-0060) [1] §6.5.7 allows to retrieve the "most recent items" and Message Archive Management (XEP-0313) [2] state in §3.1 that archives are ordered in "chrnological 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) [3]: a spell 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.

2. Requirements

3. Glossary

In XEP-0060, there is no such thing as "updated item". This XEP changes the business logic as follow:

4. Use Cases

4.1 Retrieve Items By Date of Creation

Juliet wants to retrieve plays of his 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 add the <order> element as a children of the <pubsub> element with a namespace of "urn:xmpp:order-by:0" and with a 'by' attribute equal to "creation".

Example 1. Retrieving items ordered by date of creation
<iq type='get'
    from='juliet@capulet.lit/balcony'
    to='pubsub.shakespeare.lit'
    id=''>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <items node='plays' max_items='3'/>
    <order xmlns='urn:xmpp:order-by:0' by='creation'/>
  </pubsub>
</iq>
  

Pubsub service then returns the 3 plays created the most recently, first one being the most recent

Example 2. Service returns all items
<iq type='result'
    from='pubsub.shakespeare.lit'
    to='juliet@capulet.lit/balcony'
    id=''>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <items node='plays'>
      <item id='153214214'>
        <entry xmlns='http://www.w3.org/2005/Atom'>
          <title>Henry VIII</title>
        </entry>
      </item>
      <item id='623423544'>
        <entry xmlns='http://www.w3.org/2005/Atom'>
          <title>Tempest</title>
        </entry>
      </item>
      <item id='452432423'>
        <entry xmlns='http://www.w3.org/2005/Atom'>
          <title>Wintter's Tale</title>
        </entry>
      </item>
    </items>
  </pubsub>
</iq>

4.2 Retrieve Items By Date of Modification

Juliet realize that there is a spelling mistake, it's "Winter's Tale" and not "Wintter's Tale". She fixes it by overwritting the item:

Example 3. Juliet Overwritte the Item to Fix It
<iq type='set'
    from='juliet@capulet.lit/balcony'
    to='pubsub.shakespeare.lit'
    id='orderby2'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <publish node='plays'>
      <item id='452432423'>
        <entry xmlns='http://www.w3.org/2005/Atom'>
          <title>Winter's Tale</title>
        </entry>
      </item>
    </publish>
  </pubsub>
</iq>

To check that everything is alright, she request again the last 3 items, but this time by date of modification. To do so, he client proceed the same way as for date of creation, except that it uses the value "modification" for the 'by' attribute

Example 4. Retrieving items ordered by date of modification
<iq type='get'
    from='juliet@capulet.lit/balcony'
    to='pubsub.shakespeare.lit'
    id='orderby3'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <items node='plays' max_items='3'/>
    <order xmlns='urn:xmpp:order-by:0' by='modification'/>
  </pubsub>
</iq>
  

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 most recently modified item on top:

Example 5. Service returns all items
<iq type='result'
    from='pubsub.shakespeare.lit'
    to='juliet@capulet.lit/balcony'
    id='orderby3'>
  <pubsub xmlns='http://jabber.org/protocol/pubsub'>
    <items node='plays'>
      <item id='452432423'>
        <entry xmlns='http://www.w3.org/2005/Atom'>
          <title>Winter's Tale</title>
        </entry>
      </item>
      <item id='153214214'>
        <entry xmlns='http://www.w3.org/2005/Atom'>
          <title>Henry VIII</title>
        </entry>
      </item>
      <item id='623423544'>
        <entry xmlns='http://www.w3.org/2005/Atom'>
          <title>Tempest</title>
        </entry>
      </item>
    </items>
  </pubsub>
</iq>

4.3 Use with MAM

With Message Archive Management (XEP-0313) [2] the logic is the same, but the <order> element is added as a child of the <query> element:

Example 6. MAM Pubsub Query with Ordering
<iq to='pubsub.shakespeare.lit' type='set' id='orderby4'>
  <query xmlns='urn:xmpp:mam:2' queryid='123' node='plays'>
    <order xmlns='urn:xmpp:order-by:0' by='creation'/>
  </query>
</iq>

This way, filters can be used with specific ordering

4.4 Reversing the Order

The ordering can be reversed by using the mechanisms already provided by Result Set Management (XEP-0059) [4]

4.5 Extending this Specification

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 XEPs, 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 re-used in other use cases as for Pubsub or MAM, but this would need to be detailed in a separate specification.

5. Discovering Support

If a server supports the "order by" protocol, it MUST report that fact by including a service discovery feature of "urn:xmpp:order-by:0" (see Protocol Namespaces regarding issuance of one or more permanent namespaces) in response to a Service Discovery (XEP-0030) [5] information request:

Example 7. Service Discovery information request
<iq from='example.org'
    id='disco1'
    to='example.com'
    type='get'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>
Example 8. Service Discovery information response
<iq from='example.com'
    id='disco1'
    to='example.org'
    type='result'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
    ...
    <feature var='urn:xmpp:order-by:0'/>
    ...
  </query>
</iq>

6. Business Rules

Several ordering elements may be used, this allow 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 the second <order> element is used in case of conflict, then the next one if a new conflict happen and so on.

In case of conflict, if no new <order> element is specified, the item order is not guarented and is up to the implementation

7. Implementation Notes

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

8. Security Considerations

This document introduces no additional security considerations above and beyond those defined in the documents on which it depends.

9. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [6].

10. XMPP Registrar Considerations

10.1 Protocol Namespaces

This specification defines the following XML namespace:

10.2 Protocol Versioning

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.

11. XML Schema

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:xmpp:order-by:0'
    xmlns='urn:xmpp:order-by:0'
    elementFormDefault='qualified'>

  <xs:element name='order' maxOccurs='unbounded'>
      <xs:complexType>
          <xs:attribute name='by' type='xs:string' use='required'/>
      </xs:complexType>
  </xs:element>

</xs:schema>

Appendices

Appendix A: Document Information

Series
XEP
Number
0413
Publisher
XMPP Standards Foundation
Status
Experimental
Type
Standards Track
Version
0.1.0
Last Updated
2019-02-04
Approving Body
XMPP Council
Dependencies
XMPP Core, XEP-0060, XEP-0313
Supersedes
None
Superseded By
None
Short Name
NOT_YET_ASSIGNED
Source Control
HTML

This document in other formats: XML  PDF

Appendix B: Author Information

Jérôme Poisson
Email
goffi@goffi.org
JabberID
goffi@jabber.fr

Copyright

This XMPP Extension Protocol is copyright © 1999 – 2018 by the XMPP Standards Foundation (XSF).

Permissions

Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.

Disclaimer of Warranty

## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##

Limitation of Liability

In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.

IPR Conformance

This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).

Visual Presentation

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Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Appendix E: Discussion Venue

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.

Errata can be sent to <editor@xmpp.org>.

Appendix F: Requirements Conformance

The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

Appendix G: Notes

1. XEP-0060: Publish-Subscribe <https://xmpp.org/extensions/xep-0060.html>.

2. XEP-0313: Message Archive Management <https://xmpp.org/extensions/xep-0313.html>.

3. XEP-0277: Microblogging over XMPP <https://xmpp.org/extensions/xep-0277.html>.

4. XEP-0059: Result Set Management <https://xmpp.org/extensions/xep-0059.html>.

5. XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.

6. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.

Appendix H: Revision History

Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/

  1. Version 0.1.0 (2019-02-04)
    Accepted by vote of Council on 2018-01-09.
    XEP Editor (jsc)
  2. Version 0.0.1 (2019-01-05)

    First draft.

    jp

END