Revision as of 12:41, 14 December 2012

VTD-XML Investigation

VTD-XML (http://vtd-xml.sourceforge.net/) is a high-performance XML processing model that deals with XML in a binary form, instead of the traditional text form. VTD stands for Virtual Token Descriptor.

VTD-XML parses an XML document and builds an internal data structure representing the entire XML document in byte[] form. Each "token" of the XML document is represented as the following 64-bit integer:

• It is a binary format specification, not an API specification
• A VTD record is a primitive data type (integer multiple of 32 bits) that encodes the following parameters of a token in an XML file:
  • Starting offset
  • Length
  • Nesting depth
  • Token type
• VTD requires that XML document be maintained intact in memory.
• Our current VTD record layout further specifies the following:
  • Use 64 bits as the primitive type (b63~b0)
  • Big endian
  • Starting offset: 30 bits (b29 ~ b0) maximum value is 2^30 -1 = 1G -1
  • Length: 20 bits (b51 ~ b32) maximum value is 2^20-1 = 1M -1
    • For some token type
      • Prefix length: 9 bits (b51~ b43) max value 511
      • Q-name length: 11 bits (b42 ~ b 32) max value 1023
  • Depth: 8 bits (b59~b52) max value is 2^8-1 = 255
  • Token type: 4 bits (b63~b60)
  • Reserved bit: 2 bits (b31: b30)

VTD-XML Core Concepts

Unmarshalling a VTD-XML document

VTDGen vg = new VTDGen();
 
// from existing byte[]
// true indicates namespace aware
vg.setDoc(byte[]); vg.parse(true);
 
// - or - 
 
// from file
vg.parseFile("old.xml",false)