this blog is girtby.net

Posted
26 December 2006 @ 9pm

Comments Off

Content Export with Atom

Draft version 0.3, 2006-09-27

Alastair Rankine alastair@girtby.net

NOTE: This is a tentative proposal. Nothing is concrete, let alone implemented. I just wanted to get something out there that people can mull over. Comments definitely appreciated, either in email to me or the atom-syntax list, or in the accompanying blog post..

Introduction

With the explosion in adoptation of software for weblogging and similar forms of online publishing, there is now a significant quantity of content contained in the persistent storage formats used by such software. This content cannot currently be extracted easily, as the storage formats are typically either proprietary or undocumented or just rapidly changing. The effect on the user is that it becomes a difficult proposition to migrate this content from one system to another. Additionally, software authors are faced with a combinatorial explosion, where each new system must understand all of the storage formats of those that came before in order to maximise the potential to attract users from those systems.

What is needed is a common interchange format, which can be used when content authors migrate from one system to another. This should allow the content to be represented with maximum fidelity and minimise the amount of manual re-entry required.

The goal of this document is to specify how to adapt the Atom syndication format [RFC 4287] to make it suitable as a common interchange format. Atom is currently deployed as a syndication format, but with only relatively minor extensions and usage conventions, can be applied to the task of content exchange between blogging engines. This was originally envisioned as a goal for the Atom protocol.

Scope

This document describes how existing Atom constructs can be used to represent content. More specifically, it defines conventions for existing Atom constructs, but always in a manner that is conformant to the letter and spirit of [RFC 4287] and derived standards.

The emphasis of this document is on the content stored by a given software system, and not its configuration. In other words it is not anticipated that this export format entirely captures the configuration of a given blogging system. For example, configuration of display themes, spam protection, or database are deliberately excluded from this format. As a consequence, this format is not deemed suitable for backup purposes for a given blogging system.

It is also assumed that media files referenced by links from within the content are not required to be represented in this format. The same applies to resources refered to from “enclosure” links.

Dependencies

This document is dependent on the following:

  • Atom syndication format (defined in [RFC 4287])
  • Atom threading extensions (defined in [RFC 4685])
  • Atom Publishing Protocol (currently in draft)

Conventions

The atom: prefix is used to refer to the Atom namespace http://www.w3.org/2005/Atom.

The thr: prefix is used to refer to the Atom feed thread namespace http://purl.org/syndication/thread/1.0.

The app: prefix is used to refer to the Atom Publishing Protocol namespace http://purl.org/atom/app#.

The URL prefix http://purl.org/syndication/export/ is temporary only, pending assignment. This is used below to construct schemes for specifying certain information using Atom categories.

It is assumed that the implementation is already capable of generating Atom feed documents, and hence described below are the differences between these documents and the blog export document. The term “similar to feed” is used when a given Atom element should be populated in a similar manner to an Atom feed document, and generally the differences will be listed.

The Atom feed thread extensions [RFC 4685] are preferred to represent discussions. However in the absence of a complete implementation of these extensions, a workaround is provided below.

The Atom Publishing Protocol [APP-id] is referenced by this specification but a complete implementation is not required by Atom export implementations. However some exported information is specified using elements from the Atom Publishing Protocol, as referenced below.

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC 2119].

Atom Export Documents

An Atom export document is an Atom feed document, except that it contains entries for all content on the blog. This includes all articles, published or unpublished. It also includes any managed pages, comments, trackbacks and pingbacks. All such entries are included in the export document.

No ordering is specified for the individual content items in the feed, except where dependencies exist between entries. The thr:in-reply-to element is used to define dependencies, as described below. Thus, such elements SHOULD be used to refer to previous entry elements (in document order).

Example

The following is an example of an Atom export document.

The Brillant Paula Bean tag:example.com,2005:Typo Typo Paula paula@example.com 2006-08-07T07:15:35-04:00

Paula http://example.com/id/articles/123 2006-08-07T06:07:00-04:00 2006-08-07T07:15:35-04:00 Get Paula

I just wrote this brillant code. I just wrote this brillant Java code. Have a look!

public String getPaula() {
    return paula;
}</content>

Anonymous http://example.com/id/comments/1532 2006-08-07T06:07:30-04:00 2006-08-07T07:15:35-04:00 WTF!?! First post!

Daily WTF http://thedailywtf.com/ http://example.com/id/trackbacks/3123 2006-08-07T06:07:30-04:00 2006-08-07T07:15:35-04:00 Trackback from Daily WTF

Paula shows off her brillant code

Element Definitions

Unless stated otherwise, all elements are populated similarly to the feed.

atom:feed Element

Populated similarly to the feed. The following differences apply:

  • An instance of an Atom Publishing Protocol Category Document SHOULD be included. This defines the list of valid categories which may appear in subsequent atom:entry elements.

  • An atom:contributor element is REQUIRED for each defined author. This MAY include registered users which do not have authoring privileges.

atom:entry Element

Populated similarly to the feed. The following differences apply:

  • An entrytype category SHOULD be used to indicate the type of entry. For example, the entrytype category can be used to specify a static page instead of a regular blog article, or to disambiguate a trackback from a comment.

  • atom:category elements SHOULD be used similarly to the feed, but where these do not match the defined categories in the atom:feed element, they can be assumed to be “tags”. See discussion for app:categories in [APP-id].

  • app:control elements MAY be used to indicate the control status for an entry. In particular, draft status may be indicated by the app:draft sub-element.

  • A contentsyntax category MAY be used to indicate a syntax for the content element. This MUST only be present when the atom:content element contains text (as indicated by the text attribute), and should be used only when a registered MIME type is not available. See further discussion below regarding atom:content.

Note that content may be inline or out of line, but the former is preferred for obvious reasons.

atom:content Element

This SHOULD be populated with the “raw” content of the item, as it was originally entered by the author. Note this differs from current syndication usage, where the “cooked” (typically HTML) format is used, but it is intended to conform to the usage of this element within the Atom Publishing Protocol.

For example, Markdown and Textile are both commonly-used syntaxes to enable the generation of HTML using textual conventions instead of explicit markup. Although either of these can be used to generate HTML, it is not (in general) feasible to reconstruct the original textual source from HTML. Hence it is highly preferred that all Atom export implementations that implement custom syntax for their content allow exporting in that syntax.

Registered MIME types SHOULD be used in the type attribute, as discussed in [RFC 4287]. Where these are not available (such as for Markdown and Textile, as of this writing), the contentsyntax category SHALL be employed (see below).

There is a potential for interoperability problems here. Specifically, if the syntax chosen by the exporter is not recognised by the importer, or the target publishing platform itself. In this case it is suggested that implementations support a “cooked” mode for content as well. This should be indicated by a suitable MIME type for the type attribute, and the absence of the contentsyntax category.

An xml:space="preserve" attribute MUST be used if whitespace is significant in the content. This is necessary even when the signficance of the whitespace is unambiguously specified by a contentsyntax category.

contentsyntax category

This is an atom:category, constructed as follows.

  • The scheme attribute MUST be set to http://purl.org/syndication/export/contentsyntax.

  • The term attribute MUST be set to a URL uniquely identifying the syntax (or autopara as noted below)

  • The label attribute SHOULD be set to the name of the syntax.

For example, the Markdown syntax would be specified as follows:

A special case is the commonly-encountered syntax which supports a subset of HTML, and additionally inserts <p> and <br/> elements according to whitespace. This syntax is unlikely to be standardised, nor be identifiable from a single URL, hence instead will be identified by term="autopara" instead of a URL.

entrytype category

This is an atom:category, constructed as follows.

  • The scheme attribute MUST be set to http://purl.org/syndication/export/entrytype.

  • The term attribute MUST be set to one of the following:

    • page for static pages;
    • comment for manually-entered comments;
    • trackback for trackbacks
  • The label attribute is optional and undefined.

When comment or trackback is used, it can be assumed that the entry contains feedback on a previously-encountered entry.

References

[RFC 2119] Key words for use in RFCs to Indicate Requirement Levels

[RFC 4287] The Atom Syndication Format

[RFC 4685] Atom Threading Extensions

[APP-id] The Atom Publishing Protocol

Acknowledgements

Thanks to Aristotle Pagaltzis and Scott Laird for their comments on drafts of this document.

Revision History

  • 2006-09-27: RFC 2119 conformance, various minor cleanups.

  • 2006-09-18: Added requirement for xml:space in atom:content where necessary. Removed all export: elements, instead used APP and Thread extensions where appropriate, and category elements otherwise.

  • 2006-08-14: Initial draft


Back to Home