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From "Abhishek Shadangi (JIRA)" <j...@apache.org>
Subject [jira] Created: (ABDERA-267) Major performance issue with Abdera (underlying Axiom) object model while writing Atom DOM to XmlStreamWriter
Date Fri, 01 Oct 2010 19:54:32 GMT
Major performance issue with Abdera (underlying Axiom) object model while writing Atom DOM
to XmlStreamWriter

                 Key: ABDERA-267
                 URL: https://issues.apache.org/jira/browse/ABDERA-267
             Project: Abdera
          Issue Type: Improvement
    Affects Versions: 1.1
         Environment: N/A
            Reporter: Abhishek Shadangi

Abdera object model (OM) is based off of Axiom OM. In FOMDocument class (which extends Axiom's
OMDocumentImpl), method {{toWrite(java.io.Writer)}} makes a call to {{this.internalSerialize(javax.xml.stream.XmlStreamWriter)}}.
After this point Abdera delegate the XML stream writing to Axiom. 

Axiom 1.2.7, has a serious performance issue if one is using likes of woodstox's implementation
(com.ctc.wstx.sw.SimpleNsStreamWriter - http://woodstox.codehaus.org/3.2.9/javadoc/index.html),
which I believe is the default writer used by Axiom.

The {{internalSerialize}} call eventually makes it to {{org.apache.axiom.om.impl.util.OMSerializerUtil.serializeStartpart(OMElement
element, String localName, XMLStreamWriter writer)}}. Now, here starts the major performance
hog. This makes a call to {{isSetPrefixBeforeStartElement(XmlStreamWriter)}}, which handles
one of the MOST controversial part of the XmlStreamWriter spec - http://download.oracle.com/javase/6/docs/api/javax/xml/stream/XMLStreamWriter.html.

They fairly recognized this in the javadocs for {{OMSerializerUtil.isSetPrefixBeforeStartElement}}
in Axiom version *1.2.7*. But they got the implementation all wrong. The implementation looks
for the property {{javax.xml.stream.XMLStreamWriter.isSetPrefixBeforeStartElement}} in the
writer, which throws an IllegalArgumentException if this property is not found, which IS NOT
FOUND. The implementation then catches this exception and returns false. This happens for
EVERY single element written to output stream. This exception handling is way too much expensive
and when it happens per element in XML per request, it takes MOST part of the processing time
every request. More on this specific property - http://publib.boulder.ibm.com/infocenter/realtime/v2r0/index.jsp?topic=/com.ibm.rt.doc.20/user/xml/xlxpj_reference.html.

Since Abdera is meant for Atom feed, when there are concurrent requests the overall latency
per request increases more than linearly and after a while it becomes unusable for high load.

Axiom realized this flaw in their logic and fixed it in *1.2.9*. Hence, Abdera SHOULD consider
upgrading it's code to use Axiom 1.2.9 instead of 1.2.7, and this performance issue will be
gone. For our use, I explicitly made this change to use the new Axiom library but there are
other dependencies which is preventing me to make the upgrade that easily.

Most importantly, after upgrading the JAR, I get the following exception.:

    Caused by: java.lang.IllegalStateException: This factory is immutable
	    at org.apache.axiom.util.stax.wrapper.ImmutableXMLOutputFactory.setProperty(ImmutableXMLOutputFactory.java:39)
	    at org.apache.abdera.parser.stax.StaxStreamWriter.createXMLStreamWriter(StaxStreamWriter.java:106)
	    at org.apache.abdera.parser.stax.StaxStreamWriter.setOutputStream(StaxStreamWriter.java:113)

This is because the javax.xml.stream.XmlOutputStream, that Axiom 1.2.9 now returns, is immutable
and Abdera code is trying to set some property in it.

*Action items:*
 1. Upgrade to Axiom 1.2.9
 2. Fix places where Abdera (esp. parser) is trying to modify the immutable XmlObjectFactory
returned by Axiom.

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