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apps | annot

An XML framework for interactive publishing

This article describes the systems developed for publishing articles at ApacheTutor. The goal was a highly lightweight but extensible framework for interactive authoring and publication, offering full guarantees of Accessibility and Quality. We discuss reasons why existing blog and wiki software was not considered adequate, and describe the design and implementation of this application, including new modules, an XML namespace, and a DTD.

Introduction

The concept of ApacheTutor is one of an interactive, online book. High quality content (which is gradually being written) should be accompanied by genuine interactivity. Subject to permissions, reviewers, editors and readers should be able to attach notes to an article, and authorised editors should be able to work on the document text itself. The system is required to manage this process, including retaining a full revision history.

Coincidentally, while the site was in preparation, I was asked to do the technical review for a forthcoming book. This required me to find out about and use broadly comparable capabilities in a modern wordprocessor, which permits an editor to make changes or enter notes anywhere, while the software takes care of attaching my name and the date to my edits. But publishing on the Web opens this process up, and permits not only expert reviewers but also general readers to participate. So if anything is inaccurate or unclear, there is a readily-available builtin mechanism to ask for review and clarification.

Wearing my other hat, I am concerned about web standards and accessibility, and I am not prepared to accept sub-standard publishing 'solutions'. ApacheTutor is required to ensure that all HTML delivered over the web is both valid and fully accessible. To the best of my knowledge, this is the first and only web-based system that can offer such a guarantee while permitting editors the flexibility of working on 'raw' markup.

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Design Goals

Our design goals fall into three categories: functional (required capabilities), System (few dependencies, high performance) and Web Standards Compliance. To meet these requirements, we can also consider non-requirements. The system should offer strong support for technical and scholarly publishing to support authors, editors and readers. But there is no expectation that it will be used in high-volume discussions.

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Interactive Features

Permissions Framework: The system should permit or deny privilege to annotate or edit it, and should different levels of access to different users, including anonymous users where appropriate. It should be possible for each document to have its own permissions.
Editing facilities: Subject to permissions, it should be possible to edit the markup of a section and to add an annotation (margin-note) to a section.
Granular comment framework: It should be possible to edit or annotate sections individually, and to attach annotations to the section to which they apply. Structurally each section is an independent unit.
Concurrency Management: It is essential that concurrency is properly managed. In addition to locking a document for updates, any change or annotation to a section must be coordinated to avoid the risk of conflicting changes. Changes to different sections of a document are not considered to be in conflict, even if made simultaneously.
Versioning: Strong versioning (such as provided by CVS) is not a requirement, but it should at least be possible for a document owner and/or system administrator to restore an older version if required. It should mark annotations as old once the section has been updated to reflect the comments.

General

Simplicity: This should not require the addition of any substantial new software that might prove problematic to introduce on an operational server or risk compromising performance on older hardware.
Lightweight application: Reading a document should be a very low overhead on the server, preferably avoiding any such overheads as a database query or multiple filesystem accesses. Write operations should also be fast but are considered less critical.
Extensibility: Many planned capabilities are not yet implemented, but the design must accommodate introducing them at a later date without disrupting the existing site.

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Web Standards

Guaranteed Valid Markup: A system that permits an editor to enter markup risks collecting garbage, whether through ignorance or simple typos. The system must prevent that happening.
Accessibility and Graceful Degredation: Although the system may take advantage of graphics, stylesheets and clientside scripting to deliver a complete presentation, all contents must be available and clearly presented even in a text-only rendering such as Lynx.

The Solution

The basic solution adopted is to make each article a single XML file, containing all the data and metadata relevant to the article (except history information). The XML format adopted contains three types of data:

XHTML (namespace http://www.w3.org/1999/xhtml) for display
DublinCore (namespace http://purl.org/dc/elements/1.1/) for metadata
Annot (namespace http://apache.webthing.com/annot) for annotation and document management information. This is the only new markup introduced by this system.

Documents are presented using Apache's namespace support (mod_xmlns), based on a SAX parser that dispatches to different namespace handlers. A new namespace module mod_annot_view was developed to handle the annotations namespace. A second module mod_annot was developed to edit the articles. Other elements of the site development are a DTD used to validate document edits, and site design elements including CSS and Javascript.

The core of the system is therefore the design of the annotations namespace and document structure, and the implementation of the editor.

The Document Structure

The XML source is delivered on the Web as XHTML, which is required to be valid and accessible. To implement this with a fast SAX parser requires that the XML be ordered as it will be served, and that document-level metadata appear at the top of the file.

The structure of the XHTML served is constrained by accessibility to follow the logical order of the material presented. Apart from that, it needs hooks for CSS and Javascript to enhance the presentation.

The document root element is html. It is followed by annotation metadata describing ownership and access control. The data that will be served on the Web comprise the article's title, an Abstract, and one or more Section. Each section comprises a section title, section contents (html), and annotations, in that order.

All interactive features are at the level of the Section, which contains relevant metadata including in particular a last-modified timestamp and editor (this information is updated only by edits, not by annotations).

The Annotations Namespace

The annotations namespace http://apache.webthing.com/annot is introduced by this application, and comprises application-specific metadata and structure:

editor: element listing a named editor, and noting owner
protection: sets protection for annot or edit to No (not allowed), Yes (allowed for anyone), or Auth (allow only identified/authorised users)
lock: Sets a temporary lock against any edits or annotations
article: container for all article content, with title and hooks for templated header and footer. Contains abstract and one or more section.
abstract: Document abstract (html markup)
section: container for a section, with title, editor and timestamp information, containing content element and zero or more annots.
content: Contents of the section. (Editable HTML markup).
annot: An annotation (margin note), with identifier of who made it and when. Plain text.

Note by anonymous, Sun May 11 10:59:45 2008

Displaying an Article

Articles are served using the generic namespace processor mod_xmlns, together with the mod_xhtml XHTML namespace implementation and a new module mod_annot_view, implementing the annotations namespace. The document is displayed in order: title, abstract, sections, within a page template incorporating a sitewide header and footer, and document information. Each section is displayed in order: section title, content, annotations, together with Annotate and Edit buttons if the user has the relevant permissions.

On the browser side (and subject to browser support for CSS and javascript), annotations are transformed into margin notes and collapsed to icons. Selecting an icon will display the annotation, while a simple mouseover will show who made the annotation and when.

The Editor

The editor supports two handlers: editing or annotating a section. Each operation has two parts: displaying a Form, and submitting the Form to commit the update.

This is not considered so performance-critical ad displaying an article, as it is likely to be a relatively infrequent (and non-cacheable) operation. Nevertheless, it is implemented as a module mod_annot. But mod_annot does allow itself the overhead of parsing the document to a DOM and using XPATH expressions to check document metadata and extract the section we are working on. If a document is not well-formed, the parse will fail and an HTTP Error 500 will be returned.

On all requests, mod_annot first checks that the user is authorised for the attempted operation and that the document is not locked against any update. If this check fails an HTTP 401 or 503 error response will be returned as appropriate.

After these checks, one of two things happen. If the request was an HTTP GET, it will display an editor form. If the request was a POST, it will submit the form, making the update.

In either case, the next operation is to extract the section we are working on from the DOM. This relies on the user submitting valid inputs to the editor: if the inputs are not acceptable to the system (i.e. if someone is trying to tamper with it), it will return an HTTP 400 error. If the section cannot be extracted despite well-formed inputs, it will return an HTTP 500 error: this could happen either on a tamper attempt or because the section was updated between loading a page and selecting the editor operation.

Either the annotation or edit forms are displayed within a general page template. The annotation form displays the section with an empty text box to enter the user's comment, while the edit form displays the section title and the section contents (markup) in form fields to be edited.

Submitting either of the forms causes an update to the document. So in the case of a POST operation, the editor will take an exclusive lock to prevent a race condition. It will also check the section timestamp to ensure that the section is still the same as what the user is working on, and hasn't been updated by someone else in the meantime, returning an HTTP 409 error on failure.

An Annotation operation will insert a new annot node in the section, escaping any markup to plain text.

An Edit operation will first validate the markup, returning an HTTP 422 error on failure (if no section DTD is supplied, it would merely check well-formedness). If successful, it will replace the section title and contents in the DOM with the new values entered, together with the timestamp and editor id.

The DOM will then be saved to a new file, and moved atomically to the filename used to display the document. Finally, an internal redirect returns the updated document to the user.

Appendix: httpd.conf


LoadModule	xmlns_module		modules/mod_xmlns.so
LoadModule	xhtml_module		modules/mod_xhtml.so
LoadModule	annotate_module		modules/mod_annot.so
LoadModule	annot_view_module	modules/mod_annot_view.so
LoadModule	dublincore_module	modules/mod_dublincore.so

<VirtualHost 195.224.39.74>
ServerName	www.apachetutor.org
DocumentRoot	/usr/www/atutor

AnnotSectionDTD file:///usr/www/atutor/section.dtd
AliasMatch	/([a-z]+)\/$	/usr/www/atutor/$1.html
AliasMatch	/auth/([a-z]+)\/$ /usr/www/atutor/$1.html
Alias /auth	/usr/www/atutor

<Directory /usr/www/atutor>
	AddType text/html;charset=utf-8 .edit .annot
	AddHandler	annot-edit	.edit
	AddHandler	annot-annot	.annot
	AnnotHeader	/path/to/header
	AnnotFooter	/path/to/footer
	<FilesMatch "^[a-z-]+$">
		SetOutputFilter	 xmlns
		ForceType		text/html;charset=utf-8
		XMLNSUseNamespace	http://apache.webthing.com/annot on view
		XMLNSUseNamespace	http://www.w3.org/1999/xhtml on 1.0
		XMLNSUseNamespace	http://purl.org/dc/elements/1.1/ on hide
	</FilesMatch>
</Directory>

<Location /auth>
	AuthType	Basic
	AuthName	"Apache Tutorials"
	Require		valid-user
	#(details chopped)
</Location>

</VirtualHost>