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From "Abhishek Shadangi (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 15:37:30 GMT

    [ https://issues.apache.org/jira/browse/ABDERA-267?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12918556#action_12918556
] 

Abhishek Shadangi commented on ABDERA-267:
------------------------------------------

Thanks Andreas for giving Axiom developer's perspective :) Good to learn further about the
real cause. I realized the rationale behind immutability of XMLInputFactory and XMLOutputFactory
when I thought more about it. Thanks for clarifying though.

I was about to upload the EXACT SAME patch here :). However, there is ONE problem with this
fix. The StAXUtils allows creating either a standalone or per classloader instances of immutable
XMLInputFactory and XMLOutputFactory. So if say Axis2 already initialized them, the StAXParserConfiguration
and StAXWriterConfiguration injected into the getXML*Factory method will never load these
properties. Moreover, the create APIs in StAXUtils are NOT public, so we can't instantiate
a new instance (perhaps a static instance) to load specific properties regardless if there
was a singleton or classloader instance already instantiated. 

I don't quite understand the rationale behind 2 things:
    1. ONLY allowing singleton or per classloader instances of XMLInputFactory and XMLOutputFactory
    2. Uselessness of StAXParserConfiguration and StAXWriterConfiguration in get APIs if an
immutable instance has already been created

I personally think Axiom should provide further APIs to allow diverse usage of XMLInput/OutputFactory.
Right now it seems to constrain development. That way Abdera can just create a new instance
and reuse that instead of sharing it with ALL other thirdparty libraries who are also using
Axiom.

I would be glad to learn your thoughts and insights into these.

ps. I agree with your thoughts on Abdera trunk to work off of Axiom trunk. But I also believe
Axiom could warn before removing backwards compatibility; I understand in some cases it's
just not possible.

> 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
>         Attachments: ABDERA-267.patch
>
>
> *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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