Validator.nu Web Service Interface

This is a inline-commentable updated wiki copy of the original article.

Motivation

First, I assume there is some level of interest in doing RELAX NG / Schematron validation and HTML5 conformance checking. Next, it would be nice to enable applications that deal with documents to make these checks automatically in addition to having the functionality available for human operators as a Web app. For example, a content management system might check the input it is given.

Java apps could just integrate a private copy of the Free Software back end of the validation / conformance checking service. However, non-Java apps would benefit from having the validation / conformance checking service running out of process and having an interface for talking to the out-of-process Java service. The service instance could be hosted publicly or as a local copy. Even some Java developers would elect to use such a service instead of integrating the back end as part of their own app.

Input Modes

The schemas are expected to be relatively static. Therefore, I think preloading them into the service or letting the service retrieve them is sufficient. Identification by URI works in both cases.

What needs different input modes is the document that is checked.

I think the following modes would make sense:

• Document URI as a GET parameter; the service retrieves the

document by URI (already implemented).

• Document in a data: URI as a GET parameter.
• Document POSTed as the HTTP entity body (the preferred Web

service mode; already implemented).

• Document POSTed as an application/x-www-form-urlencoded

form field value.

• Document POSTed as a multipart/form-data file

upload.

In the first three modes, additional parameters would be communicated in the URI query string. In the last two modes, additional parameters would be communicated like corresponding from fields are communicated as application/x-www-form-urlencoded and multipart/form-data.

I don’t particularly like the last two modes, but they are needed to address feature requests and for parity with other services. Also, unlike the first three modes, the last two modes need companion UI changes, which is not nice. As a further complication, the last two don’t come naturally with a Content-Type for dispatching to an HTML5 parser or to an XML parser.

All these input modes would share the same “service endpoint URI” (and the same servlet class). The different cases can be distinguished from the HTTP method and in the POST cases from the Content-Type request header.

Output Modes

A Web service probably calls for an XML output format for maximal tool chain integration even though the current HTML output format makes sense for browsers and can carry all the necessary data.

I think the following modes would make sense:

• HTML with microformat-style class annotations

(already implemented except the annotation granularity could be better).

• XHTML with microformat-style class annotations (already implemented).
• XML format designed specifically for Validator.nu.
• JSON
• Human-readably plain text (already implemented)
• Emacs-compatible formatted text with one item per line
• Relax-compatible
• Unicorn-compatible
• W3C Validator-compatible SOAP
• EARL

For the HTML and XHTML output formats, there could be an option for suppressing the input form. The output default should be HTML for the browser-targeted input formats. However, the custom XML format might be a reasonable default when the input document was POSTed as the entity body.