Chapter 4 XML Processing

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because such processing is potentially very CPU, memory, and input/output or

network intensive. 

Why does XML document processing potentially impact performance so sig 

nificantly? Recall that processing an incoming XML document consists of multi 

ple steps, including parsing the document; optionally validating the document

against a schema (this implies first parsing the schema); recognizing, extracting,

and directly processing element contents and attribute values; or optionally

mapping these components to other domain specific objects for further process 

ing. These steps must occur before an application can apply its business logic to

the information retrieved from the XML document. Parsing an XML document

often requires a great deal of encoding and decoding of character sets, along with

string processing. Depending on the API that is used, recognition and extraction

of content may consist of walking a tree data structure, or it may consist of inter 

cepting events generated by the parser and then processing these events according

to some context. An application that uses XSLT to preprocess an XML document

adds more processing overhead before the real business logic work can take place.

When the DOM API is used, it creates a representation of the document a DOM

tree in memory. Large documents result in large DOM trees and corresponding

consumption of large amounts of memory. The XML data binding process has, to

some extent, the same memory consumption drawback. Many of these constraints

hold true when generating XML documents.

There are other factors with XML document processing that affect perfor 

mance. Often, the physical and logical structures of an XML document may be

different. An XML document may also contain references to external entities.

These references are resolved and substituted into the document content during

parsing, but prior to validation. Given that the document may originate on a

system different from the application's system, and external entities and even

the schema itself may be located on remote systems, there may be network over 

head affecting performance. To perform the parsing and validation, external enti 

ties must first be loaded or downloaded to the processing system. This may be a

network intensive operation, or require a great deal of input and output operations,

when documents have a complex physical structure.

In summary, XML processing is potentially CPU, memory, and network

intensive, for these reasons:

  It may be CPU intensive. Incoming XML documents need not only to be 

parsed but also validated, and they may have to be processed using APIs which 

may themselves be CPU intensive. It is important to limit the cost of validation