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Date: Tue, 03 Dec 2013 18:20:52 +0000
From: Andy Seaborne <andy@apache.org>
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To: dev@clerezza.apache.org
Subject: Re: Problem with SingleTdbDatasetProvider
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On 03/12/13 15:13, Reto Bachmann-Gm�r wrote:
> So here's some code to reproduced the ConcurrentModificationException
> directly with jena (using transactions):
>
>          String directory = "target/Dataset1";
>          Dataset dataset = TDBFactory.createDataset(directory);
>          {
>              dataset.begin(ReadWrite.WRITE);
>              Model foo1 = ModelFactory.createDefaultModel();
>              foo1.add(RDFS.Class, RDF.type, RDFS.Class);
>              foo1.add(RDFS.Class, RDF.type, RDFS.Resource);
>              dataset.addNamedModel(URNXFOO1, foo1);
>              dataset.commit();
>              dataset.end();
>          }
>          {
>              dataset.begin(ReadWrite.WRITE);
>              Model foo2 = ModelFactory.createDefaultModel();
>              dataset.addNamedModel(URNXFOO2, foo2);
>              dataset.commit();
>              dataset.end();
>          }
>          {
>              dataset.begin(ReadWrite.WRITE);
>              Model foo1 = dataset.getNamedModel(URNXFOO1);
>              Model foo2 = dataset.getNamedModel(URNXFOO2);
>              StmtIterator iter = foo1.listStatements();
>              while (iter.hasNext()) {
>                  foo2.add(iter.nextStatement());
>              }

You are modifying the dataset while iterating over it.

Model foo2 is just a view of the dataset, not an isolated copy.  'fraid 
you can't modify the dataset and get a consistent view for the iterator 
at the same time.  The code just happens to check otherwise there would 
be non-deterministic behaviour.  (The check isn't perfect BTW.)

This does it as well - and no transactions.

     public static void main1() throws Exception {
         String directory = "target/Dataset1";
         String URNXFOO1 = "urn:x-foo:1" ;
         String URNXFOO2 = "urn:x-foo:2" ;

         Dataset dataset = TDBFactory.createDataset();
         dataset.getNamedModel(URNXFOO1).add(RDFS.Class, RDF.type, 
RDFS.Class);
         dataset.getNamedModel(URNXFOO1).add(RDFS.Class, RDF.type, 
RDFS.Resource);

         Model foo1 = dataset.getNamedModel(URNXFOO1);
         Model foo2 = dataset.getNamedModel(URNXFOO2);
         StmtIterator iter = foo1.listStatements();
         while (iter.hasNext()) {
             foo2.add(iter.nextStatement());
         }
     }


	Andy

>              dataset.commit();
>              dataset.end();
>          }
>
>
> On Wed, Oct 30, 2013 at 2:42 PM, Reto Bachmann-Gm�r <reto@wymiwyg.com>wrote:
>
>> Hi Minto
>>
>> Interesting problem you have there.
>>>
>>
>> Thanks ;)
>>
>>>
>>> Best is to fix the dreaded ConcurrentModificationException. Occasionally
>>> I run into it it als well. But most probably it is not that trivial to
>>> solve.
>>>
>>
>> On hand we had some concurrency issues that caused the exception. I fixed
>> (some of) them recently, the tests are now passing.
>>
>> On the other hand the exception is coming from Jena (as well as as from
>> java collections) if the dataset (respectively the collection) is modified
>> while iterating over it. Inthe case of addAll the modification of the
>> underlying dataset is necessary, so this is not about some
>> timing/concurrency.
>>
>> Cheers,
>> Reto
>>
>>
>>
>>
>>>
>>> The only work around that I see is copy the MGraph to a different
>>> provider and do the normal addAll(). This other provider does not
>>> necessarily need to be in memory. Basically it is 2x addAll(). One to a
>>> different provider and one back.
>>>
>>> My knowledge is too limited to comment on forwarding it to Jena.
>>>
>>> Hope this is of some help to you.
>>>
>>> Regards,
>>>
>>> Minto
>>>
>>> Reto Bachmann-Gm�r schreef op 30-10-2013 12:46:
>>>> Hi
>>>>
>>>> I'm having a problem using addAll two add one Mgraph from
>>>> SingeTdbDatasetProvider to another such MGraph.
>>>>
>>>> The problem is that the iterator over that added graph will return a
>>>> ConcurrentModificationException as soon as a triple has been added to
>>> the
>>>> target graph.
>>>>
>>>> I don't know how to solve this. Copying the graph to be added to memory
>>>> doesn't seem to be a compealing solution. Maybe the add-all could be
>>>> forwarded to Jena but this would solve the problem only in some cases,
>>> not
>>>> if there is any wrapper on the added graph or if union-graphs are used.
>>>>
>>>> Cheers,
>>>> Reto
>>>>
>>>
>>>
>>> --
>>> ir. ing. Minto van der Sluis
>>> Software innovator / renovator
>>> Xup BV
>>>
>>> Mobiel: +31 (0) 626 014541
>>>
>>>
>>
>