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?
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