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From "Andreas Veithen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (ABDERA-267) Major performance issue with Abdera (underlying Axiom) object model while writing Atom DOM to XmlStreamWriter
Date Wed, 06 Oct 2010 12:20:30 GMT

    [ https://issues.apache.org/jira/browse/ABDERA-267?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12918491#action_12918491

Andreas Veithen commented on ABDERA-267:

Abhishek's analysis is correct, and I would like to add some more explanations (wearing my
Axiom developer hat):

* Some background information about the isSetPrefixBeforeStartElement thing can be found in
[1]. It is actually not Axiom that got it wrong, but a particular version of a particular
StAX implementation. In Axiom 1.2.9, we isolated that code into what we call a StAX dialect
so that it is only ever triggered when that particular StAX implementation is used.

* On the other hand, the performance issue related to isSetPrefixBeforeStartElement  was already
addressed in 1.2.8 (see WSCOMMONS-325). So upgrading to that version instead of 1.2.9 may
provide a quick fix for the issue.

* In Axiom 1.2.9, the factories returned by StAXUtils are indeed immutable. The reason is
that these factories are cached and therefore shared with other frameworks using Axiom. E.g.
if Abdera and Axis2 are used in the same application, they would get the same factory instances
from Axiom. Therefore we can't allow modifications of these factory instances, because this
has the potential of causing subtle issues. In 1.2.9, we also introduced new features that
allow to customize the factories in a controlled way, but for XMLOutputFactory instances,
this is only implemented in the current trunk.

* Since Abdera extends Axiom implementation classes (namely the LLOM implementation), it is
indeed strongly coupled and therefore very sensitive to changes in Axiom. Probably the best
way to avoid these issues is to have the Abdera trunk depend on the Axiom trunk so that incompatibilities
are detected early by the Abdera build in Hudson.

I will see what needs to be done in the Abdera and Axiom code to make them again play nicely

[1] http://markmail.org/thread/37y3qjcfucb6756o

> 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
> *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 javax.xml.stream.XmlOutputFactory and javax.xml.stream.XmlInputFactory,
that Axiom 1.2.9 creates, are now immutable and Abdera code is trying to set some {{javax.xml.stream.*}}
properties in them. To fix this, I updated StaxStreamWriter.createXMLStreamWriter and FOMParser.getXMLInputFactory
to NOT set any property. For our purposes this seems to work fine so far, but perhaps needs
a bit more research to make the real fix.
> *Action items:*
>  1. Upgrade to Axiom 1.2.9
>  2. Fix places where Abdera (esp. parser module) is trying to modify the immutable XmlInputFactory
and XmlOutputFactory as returned by Axiom.

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