Major performance issue with Abdera (underlying Axiom) object model while
writing Atom DOM to XmlStreamWriter
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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
*Background:*
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.
*Issue:*
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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