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From Ian Boston <...@tfd.co.uk>
Subject Re: javax.jcr.InvalidItemStateException: Item cannot be saved because it has been modified externally: node /
Date Thu, 01 Apr 2010 00:13:11 GMT

On 1 Apr 2010, at 02:55, Norman Maurer wrote:

> Hi Ian,
> 
> I'm 100 % sure the session is not shared.
> 
> About your (2).. I thought It should be no problem to add child nodes
> in a multi-threading app if the name silbing of the childs are unique.
> I'm wrong ?

The problem here is deep under the covers child nodes are stored in an array that has to be
re-written when updated. So if you add a child node the entire contents of the array is re-written,
making it impossible to perform an optimistic merge of concurrent changes.

You can concurrently modify individual child nodes, or separate properties, but you cant concurrently
add multiple child nodes to a parent node. There is discussion on dev@jackrabbit about how
JR3 will address this issue.

Ian

BTW, anyone, if I got that wrong, please correct, I dont want to spread misinformation by
mistake.


> 
> I will have a look at the code you posted later today..
> 
> 
> Thx,
> Norman
> 
> 
> 2010/3/31 Ian Boston <ieb@tfd.co.uk>:
>> There are a number of causes if its under heavy load:
>> 
>> 1. Multiple threads (mistakenly) sharing the same session.
>> 2. Multiple threads adding a child node the same parent node at exactly the same
time, although this might also show up as a failure to merge a modification.
>> 
>> If operating in a cluster you may find that events from modifications on other nodes
result in the same problem as the state of the local shared cache changes in reaction to events
from other nodes.
>> 
>> One solution is to maintain an application level, in memory lock on the parent node
to prevent concurrent modifications on the same parent node in JVM (although this doesn't
solve the cluster problem). eg [1]
>> 
>> A better strategy is to use a retry mechanism even propagating to the client with
a suitable response code. (eg in http  409)
>> 
>> Ian
>> 
>> 1. http://github.com/ieb/open-experiments/blob/master/slingtests/osgikernel/bundles/locking/src/main/java/org/sakaiproject/nakamura/locking/LockManagerImpl.java
>> 
>> On 30 Mar 2010, at 10:07, Norman Maurer wrote:
>> 
>>> Hi all,
>>> 
>>> I have an application which use many threads. Sometimes I see this
>>> error on heavy load:
>>> 
>>> javax.jcr.InvalidItemStateException: Item cannot be saved because it
>>> has been modified externally: node /
>>> 
>>> I wonder how this can happen because I don't add a Node directly to
>>> the rootNode add this time..
>>> 
>>> Is it maybe related to :
>>> https://issues.apache.org/jira/browse/JCR-2428
>>> 
>>> I'm using Jackrabbit 2.0.0 without transactions..
>>> 
>>> Bye,
>>> Norman
>> 
>> 


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