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https://issues.apache.org/jira/browse/MUSE-270?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12557947#action_12557947
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Vinh Nguyen commented on MUSE-270:
----------------------------------
Looks like this issue is becoming very serious in high scale environments.
Because Muse uses EMPTY_DOC in so many places and objects, the general result
is that when multiple threads attempt to access any object that's backed by
EMPTY_DOC, various errors occur . Mostly NullPointerExceptions happen because
retrieved data comes out null.
So the problem isn't really apparent just with notifications or EPRs. I
encountered the same issue when multiple clients were calling Subscribe
concurrently, and an NullPointerException occurred in SubscribeResponse line 89
as indicated in the code below:
String currentString = XmlUtils.getElementText(xml,
WsnConstants.CURRENT_TIME_QNAME);
String terminationString = XmlUtils.getElementText(xml,
WsnConstants.TERMINATION_TIME_QNAME);
try
{
if (currentString != null)
_currentTime = XsdUtils.getLocalTime(currentString);
if (terminationString != null)
_terminationTime = XsdUtils.getLocalTime(terminationString);
<--- NullPointerException occurred here because terminationString was
null !!!
}
The workaround to this issue is to not allow mutilple Subscribes (or any other
operation) to execute concurrently, but this is not feasible.
This issue doesn't seem to happen in low-scale environments, or in
single-processor machines where threads don't truly run in parallel. But it
happens often in high-scale or multi-processor machines.
I think the real solution is to remove EMPTY_DOC completely. There might be a
performance hit because now a new Document needs to be created for every
object, but that's not as serious as the multi-thread issue. Also, trying to
update code everywhere to synchronize access to an object backed by EMPTY_DOC
(which really is because of Xerces limitation of not being thread-safe) is too
much of a burden. Muse is supposed to hide the fact that Xerces is used for
DOM creation.
> EMPTY_DOC thread stability issues
> ---------------------------------
>
> Key: MUSE-270
> URL: https://issues.apache.org/jira/browse/MUSE-270
> Project: Muse
> Issue Type: Bug
> Components: Utilities - General, QName, and XML, WS-Addressing and
> SOAP
> Affects Versions: 2.0.0 M1, 2.0.0 M2, 2.0.0, 2.1.0, 2.2.0, 2.3.0
> Environment: Impacts all platforms and configurations
> Reporter: Rich Lucente
> Assignee: Dan Jemiolo
> Fix For: 2.3.0
>
> Attachments: epr-tests.zip, MuseThreadSafety.zip, WsnErrorsTest.zip
>
>
> This bug is being opened to track the EMPTY_DOC thread stability issue
> currently being discussed on the muse-dev mailing list. XmlUtils provides
> EMPTY_DOC as a scratchpad instance of a DOM Document for creation of
> elements. This has been shown to cause exceptions in a multi-threaded
> environment. The issue is compounded by the widespread use of EMPTY_DOC in
> the code due to its convenience and the reduction in object creation when
> constructing XML fragments.
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