Xml – One xml namespace equals one and only one schema file

schemaxmlxsd

…or Why do these files validate in Visual Studio 2010 but not with xmllint¹?

I'm currently working against a published xml schema where the original author's habit is to break down the schemas into several .xsd-files, but where some schema files have the same targetNamespace. Is this really "allowed"?

Example (extremely simplified):

File    targetNamespace    Contents
------------------------------------------------------------
b1.xsd  uri:tempuri.org:b  complex type "fooType"
b2.xsd  uri:tempuri.org:b  simple type "barType"

a.xsd   uri:tempuri.org:a  imports b1.xsd and b2.xsd
                           definition of root element "foo", that
                           extends "b:fooType" with an attribute
                           of "b:barType"

(Complete file contents below.)

Then I have an xml file, data.xml, with this content:

<?xml version="1.0"?>
<foo bar="1" xmlns="uri:tempuri.org:a" xmlns:xs="http://www.w3.org/2001/XMLSchema" />

For a long time, I have believed that all of this was correct, since Visual Studio apparently allows this schema style. However, today I decided to set up a command line utility for validating xml files, and I chose xmllint.

When I ran xmllint --schema a.xsd data.xml, I was presented with this warning:

a.xsd:4: element import: Schemas parser warning : Element '{http://www.w3.org/2001/XMLSchema}import':
Skipping import of schema located at 'b2.xsd' for the namespace 'uri:tempuri.org:b', since this
namespace was already imported with the schema located at 'b1.xsd'.

The fact that the import of b2.xsd was skipped obviously leads to this error:

a.xsd:9: element attribute: Schemas parser error : attribute decl. 'bar', attribute 'type':
The QName value '{uri:tempuri.org:b}barType' does not resolve to a(n) simple type definition.

If xmllint is correct, there would be an error in the published specs I'm working against. Is there? And Visual Studio would be wrong. Is it?

I do realize the difference between xs:import and xs:include. Right now, I just don't see how xs:include could fix things, since:

b1.xsd and b2.xsd have the same targetNamespace
they both differ in targetNamespace from a.xsd
and they do not (need to) know about each other

Is this a flaw in the original schema specification? I'm beginning to think that the third bullet point is crucial. Should the fact that they don't know about each other have led to placing them in different namespaces to begin with?

b1.xsd:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema targetNamespace="uri:tempuri.org:b" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:complexType name="fooType" />
</xs:schema>

b2.xsd:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema targetNamespace="uri:tempuri.org:b" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:simpleType name="barType">
    <xs:restriction base="xs:integer" />
  </xs:simpleType>
</xs:schema>

a.xsd:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema targetNamespace="uri:tempuri.org:a" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:b="uri:tempuri.org:b">
  <xs:import namespace="uri:tempuri.org:b" schemaLocation="b1.xsd" />
  <xs:import namespace="uri:tempuri.org:b" schemaLocation="b2.xsd" />
  <xs:element name="foo">
    <xs:complexType>
      <xs:complexContent>
        <xs:extension base="b:fooType">
          <xs:attribute name="bar" type="b:barType" />
        </xs:extension>
      </xs:complexContent>
    </xs:complexType>
  </xs:element>
</xs:schema>

Notes:

¹⁾ I'm using the Windows port of libxml2/xmllint found at www.zlatkovic.com.

Best Answer

The crux of the problem here is what does it mean when you have two different <import> elements, when both of them refer to the same namespace.

It helps to clarify the meaning when you consider that the schemaLocation attribute of <import> is entirely optional. When you leave it out, you're just saying "I want to import schema of namespace XYZ into this schema". The schemaLocation is just a hint as to where to find the definition of that other schema.

The precise meaning of <import> is a bit fuzzy when you read the W3C spec, possibly deliberately so. As a result, interpretations vary.

Some XML processors tolerate multiple <import> for the same namespace, and essentially amalgamate all of the schemaLocation into a single target.

Other processors are stricter, and decide that only one <import> per target namespace is valid. I think this is more correct, when you consider that schemaLocation is optional.

In addition to the VS and xmllint examples you gave, Xerces-J is also super-strict, and ignores subsequent <import> for the same target namespace, giving much the same error as xmllint does. XML Spy, on the other hand, is much more permissive (but then, XML Spy's validation is notoriously flaky)

To be safe, you should not have these multiple imports. A given namespace should have a single "master" document, which in turn has an <include> for each sub-document. This master is often highly artificial, acting only as a container. for these sub-documents.

From what I've seen, this generally consists of "best practise" for XML Schema when it comes to maximum tool compatibility, but some will argue that it's a hack that takes away from elegant schema design.

Meh.

Native XML Extensions

I prefer using one of the native XML extensions since they come bundled with PHP, are usually faster than all the 3rd party libs and give me all the control I need over the markup.

DOM

The DOM extension allows you to operate on XML documents through the DOM API with PHP 5. It is an implementation of the W3C's Document Object Model Core Level 3, a platform- and language-neutral interface that allows programs and scripts to dynamically access and update the content, structure and style of documents.

DOM is capable of parsing and modifying real world (broken) HTML and it can do XPath queries. It is based on libxml.

It takes some time to get productive with DOM, but that time is well worth it IMO. Since DOM is a language-agnostic interface, you'll find implementations in many languages, so if you need to change your programming language, chances are you will already know how to use that language's DOM API then.

A basic usage example can be found in Grabbing the href attribute of an A element and a general conceptual overview can be found at DOMDocument in php

How to use the DOM extension has been covered extensively on StackOverflow, so if you choose to use it, you can be sure most of the issues you run into can be solved by searching/browsing Stack Overflow.

XMLReader

The XMLReader extension is an XML pull parser. The reader acts as a cursor going forward on the document stream and stopping at each node on the way.

XMLReader, like DOM, is based on libxml. I am not aware of how to trigger the HTML Parser Module, so chances are using XMLReader for parsing broken HTML might be less robust than using DOM where you can explicitly tell it to use libxml's HTML Parser Module.

A basic usage example can be found at getting all values from h1 tags using php

XML Parser

This extension lets you create XML parsers and then define handlers for different XML events. Each XML parser also has a few parameters you can adjust.

The XML Parser library is also based on libxml, and implements a SAX style XML push parser. It may be a better choice for memory management than DOM or SimpleXML, but will be more difficult to work with than the pull parser implemented by XMLReader.

SimpleXml

The SimpleXML extension provides a very simple and easily usable toolset to convert XML to an object that can be processed with normal property selectors and array iterators.

SimpleXML is an option when you know the HTML is valid XHTML. If you need to parse broken HTML, don't even consider SimpleXml because it will choke.

A basic usage example can be found at A simple program to CRUD node and node values of xml file and there is lots of additional examples in the PHP Manual.

3rd Party Libraries (libxml based)

If you prefer to use a 3rd-party lib, I'd suggest using a lib that actually uses DOM/libxml underneath instead of string parsing.

FluentDom - Repo

FluentDOM provides a jQuery-like fluent XML interface for the DOMDocument in PHP. Selectors are written in XPath or CSS (using a CSS to XPath converter). Current versions extend the DOM implementing standard interfaces and add features from the DOM Living Standard. FluentDOM can load formats like JSON, CSV, JsonML, RabbitFish and others. Can be installed via Composer.

HtmlPageDom

Wa72\HtmlPageDom is a PHP library for easy manipulation of HTML documents using DOM. It requires DomCrawler from Symfony2 components for traversing the DOM tree and extends it by adding methods for manipulating the DOM tree of HTML documents.

phpQuery (not updated for years)

phpQuery is a server-side, chainable, CSS3 selector driven Document Object Model (DOM) API based on jQuery JavaScript Library written in PHP5 and provides additional Command Line Interface (CLI).

Also see: https://github.com/electrolinux/phpquery

Zend_Dom

Zend_Dom provides tools for working with DOM documents and structures. Currently, we offer Zend_Dom_Query, which provides a unified interface for querying DOM documents utilizing both XPath and CSS selectors.

QueryPath

QueryPath is a PHP library for manipulating XML and HTML. It is designed to work not only with local files, but also with web services and database resources. It implements much of the jQuery interface (including CSS-style selectors), but it is heavily tuned for server-side use. Can be installed via Composer.

fDOMDocument

fDOMDocument extends the standard DOM to use exceptions at all occasions of errors instead of PHP warnings or notices. They also add various custom methods and shortcuts for convenience and to simplify the usage of DOM.

sabre/xml

sabre/xml is a library that wraps and extends the XMLReader and XMLWriter classes to create a simple "xml to object/array" mapping system and design pattern. Writing and reading XML is single-pass and can therefore be fast and require low memory on large xml files.

FluidXML

FluidXML is a PHP library for manipulating XML with a concise and fluent API. It leverages XPath and the fluent programming pattern to be fun and effective.

3rd-Party (not libxml-based)

The benefit of building upon DOM/libxml is that you get good performance out of the box because you are based on a native extension. However, not all 3rd-party libs go down this route. Some of them listed below

PHP Simple HTML DOM Parser

An HTML DOM parser written in PHP5+ lets you manipulate HTML in a very easy way!

Require PHP 5+.

Supports invalid HTML.

Find tags on an HTML page with selectors just like jQuery.

Extract contents from HTML in a single line.

I generally do not recommend this parser. The codebase is horrible and the parser itself is rather slow and memory hungry. Not all jQuery Selectors (such as child selectors) are possible. Any of the libxml based libraries should outperform this easily.

PHP Html Parser

PHPHtmlParser is a simple, flexible, html parser which allows you to select tags using any css selector, like jQuery. The goal is to assiste in the development of tools which require a quick, easy way to scrape html, whether it's valid or not! This project was original supported by sunra/php-simple-html-dom-parser but the support seems to have stopped so this project is my adaptation of his previous work.

Again, I would not recommend this parser. It is rather slow with high CPU usage. There is also no function to clear memory of created DOM objects. These problems scale particularly with nested loops. The documentation itself is inaccurate and misspelled, with no responses to fixes since 14 Apr 16.

Ganon

A universal tokenizer and HTML/XML/RSS DOM Parser

   Ability to manipulate elements and their attributes

```
   Supports invalid HTML and UTF8
```

   Can perform advanced CSS3-like queries on elements (like jQuery -- namespaces supported)

A HTML beautifier (like HTML Tidy)
```
   Minify CSS and Javascript
```

   Sort attributes, change character case, correct indentation, etc.

Extensible

   Parsing documents using callbacks based on current character/token

   Operations separated in smaller functions for easy overriding

Fast and Easy

Never used it. Can't tell if it's any good.

HTML 5

You can use the above for parsing HTML5, but there can be quirks due to the markup HTML5 allows. So for HTML5 you want to consider using a dedicated parser, like

html5lib

A Python and PHP implementations of a HTML parser based on the WHATWG HTML5 specification for maximum compatibility with major desktop web browsers.

We might see more dedicated parsers once HTML5 is finalized. There is also a blogpost by the W3's titled How-To for html 5 parsing that is worth checking out.

WebServices

If you don't feel like programming PHP, you can also use Web services. In general, I found very little utility for these, but that's just me and my use cases.

ScraperWiki.

ScraperWiki's external interface allows you to extract data in the form you want for use on the web or in your own applications. You can also extract information about the state of any scraper.

Regular Expressions

Last and least recommended, you can extract data from HTML with regular expressions. In general using Regular Expressions on HTML is discouraged.

Most of the snippets you will find on the web to match markup are brittle. In most cases they are only working for a very particular piece of HTML. Tiny markup changes, like adding whitespace somewhere, or adding, or changing attributes in a tag, can make the RegEx fails when it's not properly written. You should know what you are doing before using RegEx on HTML.

HTML parsers already know the syntactical rules of HTML. Regular expressions have to be taught for each new RegEx you write. RegEx are fine in some cases, but it really depends on your use-case.

You can write more reliable parsers, but writing a complete and reliable custom parser with regular expressions is a waste of time when the aforementioned libraries already exist and do a much better job on this.

Also see Parsing Html The Cthulhu Way

Books

If you want to spend some money, have a look at

PHP Architect's Guide to Webscraping with PHP

I am not affiliated with PHP Architect or the authors.

XML + Schema + Namespaces. No matching global declaration available for the validation root

In your xml file, try with

xmlns:buildings="http://example/buildings/1.0/"

with a / final, as in your xsd declaration : xs:schema targetNamespace="http://example/buildings/1.0/"