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<?xml version='1.0' encoding='UTF-8'?>
<!--
* Copyright 2000-2004 The Apache Software Foundation.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
-->
<!DOCTYPE faqs SYSTEM 'dtd/faqs.dtd'>
<faqs title='Performance FAQs'>
<faq title='General Performance'>
<q>General Performance</q>
<a>
<p>
Don't use XML where it doesn't make sense. XML is not a panacea.
You will not get good performance by transferring and parsing a
lot of XML files.
</p>
<p>Using XML is memory, CPU, and network intensive.</p>
</a>
</faq>
<faq title='Parser Performance'>
<q>Parser Performance</q>
<a>
<p>
Avoid creating a new parser each time you parse; reuse parser
instances. A pool of reusable parser instances might be a good idea
if you have multiple threads parsing at the same time.
</p>
<p>
The parser configuration may affect the performance of the parser.
For example, if you are interested in evaluating the parser performance
with DTDs you may want to use the <code>DTDConfiguration</code>
(<em>Note</em>: you can build Xerces with DTD-only support using
<em>dtdjars</em> build target).
</p>
</a>
</faq>
<faq title='Parsing Documents Performance'>
<q>Parsing Documents Performance</q>
<a>
<p>
There are a variety of things that you can do to improve the
performance when parsing documents:
</p>
<ul>
<li>
Convert the document to US ASCII ("US-ASCII") or Unicode
("UTF-8" or "UTF-16") before parsing. Documents written using
ASCII are the fastest to parse because each character is
guaranteed to be a single byte and map directly to their
equivalent Unicode value. For documents that contain Unicode
characters beyond the ASCII range, multiple byte sequences
must be read and converted for each character. There is a
performance penalty for this conversion. The UTF-16 encoding
alleviates some of this penalty because each character is
specified using two bytes, assuming no surrogate characters.
However, using UTF-16 can roughly double the size of the
original document which takes longer to parse.
</li>
<li>
Explicitly specify "US-ASCII" encoding if your document is in
ASCII format. If no encoding is specified, the XML specification
requires the parser to assume UTF-8 which is slower to process.
</li>
<li>
Avoid external entities and external DTDs. The extra file
opens and transcoding setup is expensive.
</li>
<li>
Reduce character count; smaller documents are parsed quicker.
Replace elements with attributes where it makes sense. Avoid
gratuitous use of whitespace because the parser must scan past it.
</li>
<li>
Avoid using too many default attributes. Defaulting attribute
values slows down processing.
</li>
</ul>
</a>
</faq>
<faq title='XML Application Performance'>
<q>XML Application Performance</q>
<a>
<p>
When writing an XML application there are a number of choices you can make that effect
performance. Some of the things which will affect the performance of your application
are described below.
</p>
<ul>
<li><strong>Grammar Caching</strong> -- if you do need validation, consider
using grammar caching to reduce the cost of validation by allowing the parser
to skip grammar loading and assessment. See this <link idref="faq-grammars">FAQ</link>
on how to perform grammar caching with Xerces.
</li>
<li><strong>Validation</strong> -- if you don't need validation (and infoset augmentation)
of XML documents,
don't include validators (DTD or XML Schema) in the pipeline.
Including the validator components in the pipeline will result in a performance
hit for your application: if a validator component is present in the pipeline,
Xerces will try to augment the infoset even if the validation feature is set to false.
If you are only interested in validating against DTDs don't include
XML Schema validator in the pipeline.
</li>
<li> <strong>DOCTYPE</strong> -- if you don't need validation,
avoid using a DOCTYPE line in your XML document.
The current version of the parser will always read the DTD if the DOCTYPE line
is specified even when validation feature is set to false.
</li>
<li> <strong>Deferred DOM</strong> --
by default, the DOM feature <em>defer-node-expansion</em> is true,
causing DOM nodes to be expanded as the tree is traversed.
The performance tests produced by <em>Denis Sosnoski</em> showed that Xerces DOM with
deferred node expansion offers poor performance and large memory size
for small documents (0K-10K). Thus, for best performance when using Xerces DOM
with smaller documents you should disable the deferred node expansion feature.
For larger documents (~100K and higher) the deferred DOM offers
better performance than non-deferred DOM but uses a large memory size.
</li>
<li><strong>SAX</strong> --
if memory usage using DOM is a concern, SAX should be considered;
the SAX parser uses very little memory and notifies the
application as parts of the document are parsed.
</li>
</ul>
<p>
For more detailed information on best practices for writing XML applications
you may want to read the following series of articles:
</p>
<ol>
<li><jump href="http://www-106.ibm.com/developerworks/xml/library/x-perfap1.html">Write XML documents and develop applications using the SAX and DOM APIs</jump></li>
<li><jump href="http://www-106.ibm.com/developerworks/xml/library/x-perfap2.html">Reuse parser instances with the Xerces2 SAX and DOM implementations</jump></li>
<li><jump href="http://www-106.ibm.com/developerworks/xml/library/x-perfap3.html">XNI, Xerces2 features and properties, and caching schemas</jump></li>
</ol>
<p>
These three articles discuss general performance tips in addition to ones specifically pertaining to Xerces2.
</p>
</a>
</faq>
</faqs>
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