Question

Our C++ application reads configuration data from XML files that look something like this:

<data>
 <value id="FOO1" name="foo1" size="10" description="the foo" ... />
 <value id="FOO2" name="foo2" size="10" description="the other foo" ... />
 ...
 <value id="FOO300" name="foo300" size="10" description="the last foo" ... />
</data>

The complete application configuration consist of ~2500 of these XML files (which translates into more than 1.5 million key/value attribute pairs). The XML files come from many different sources/teams and are validated against a schema. However, sometimes the <value/> nodes look like this:

<value name="bar1" id="BAR1" description="the bar" size="20" ... />

or this:

<value id="BAT1" description="the bat" name="bat1"  size="25" ... />

To make this process fast, we are using Expat to parse the XML documents. Expat exposes the attributes as an array – like this:

void ExpatParser::StartElement(const XML_Char* name, const XML_Char** atts)
{
 // The attributes are stored in an array of XML_Char* where:
 //  the nth element is the 'key'
 //  the n+1 element is the value
 //  the final element is NULL
 for (int i = 0; atts[i]; i += 2) 
 {
  std::string key = atts[i];
  std::string value = atts[i + 1];
  ProcessAttribute (key, value);
 }
}

This puts all the responsibility onto our ProcessAttribute() function to read the ‘key’ and decide what to do with the value. Profiling the app has shown that ~40% of the total XML Parsing time is dealing with these attributes by name/string.

The overall process could be sped up dramatically if I could guarantee/enforce the order of the attributes (for starters, no string comparisons in ProcessAttribute()). For example, if ‘id’ attribute was always the 1st attribute we could deal with it directly:

void ExpatParser::StartElement(const XML_Char* name, const XML_Char** atts)
{
 // The attributes are stored in an array of XML_Char* where:
 //  the nth element is the 'key'
 //  the n+1 element is the value
 //  the final element is NULL
 ProcessID (atts[1]);
 ProcessName (atts[3]);
 //etc.
}

According to the W3C schema specs, I can use <xs:sequence> in an XML schema to enforce the order of elements – but it doesn’t seem to work for attributes – or perhaps I’m using it incorrectly:

<xs:element name="data">
 <xs:complexType>
  <xs:sequence>
   <xs:element name="value" type="value_type" minOccurs="1" maxOccurs="unbounded" />
  </xs:sequence>
 </xs:complexType>
</xs:element>

<xs:complexType name="value_type">
 <!-- This doesn't work -->
 <xs:sequence>
  <xs:attribute name="id" type="xs:string" />
  <xs:attribute name="name" type="xs:string" />
  <xs:attribute name="description" type="xs:string" />
 </xs:sequence>
</xs:complexType>

Is there a way to enforce attribute order in an XML document? If the answer is “no” – could anyone perhaps suggest a alternative that wouldn’t carry a huge runtime performance penalty?

Answer 1

According to the xml specification,

the order of attribute specifications in a start-tag or empty-element tag is not significant

You can check it at section 3.1