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From Alexandros Karypidis <akary...@yahoo.gr>
Subject Re: Fetch database changes from external sources
Date Thu, 15 May 2008 10:52:11 GMT
Hi,

First of all, thanks for the input.

1) I am not sure what the second-level cache is. Going through the user 
manual and searching the web did not help. All I could find that is 
cache-related are the following properties, which I set as follows 
(hoping that they indeed control the "second-level cache"):

	<property name="openjpa.DataCache" value="false"/>
	<property name="openjpa.QueryCache" value="false"/>


2) As far as calling evict is concerned, I added these calls between the 
two find() calls:

	OpenJPAEntityManagerFactory jemFactory =
		(OpenJPAEntityManagerFactory) emFactory;
	jemFactory.getQueryResultCache().evictAll();
	jemFactory.getStoreCache().evictAll();
	
	OpenJPAEntityManager jem = (OpenJPAEntityManager) em;
	jem.evictAll(Record.class);

Neither modification helped and as far as the second approach is 
concerned, I would prefer to restrict my code to standard JPA only.

In any case OpenJPA still uses some cache to return the object the 
second time around.

Can you point me towards something more concrete regarding this "second 
level cache" and how to disable it? Is it perhaps referred to by some 
other name? I really couldn't find information when web-searching for 
stuff like "openjpa second level cache".

Thanks again for your time.

Craig L Russell wrote:

> Hi,
> 
> It sounds like you're using the second-level cache which is dutifully 
> returning the current contents that are cached.
> 
> Have you tried using the evict methods of the second level cache, or 
> disabling it for the application?
> 
> Craig
> 
> On May 14, 2008, at 2:37 AM, Alexandros Karypidis wrote:
> 
>> Hi,
>>
>> First of all, I've done research through the archives for the 
>> situation I am facing and found this:
>>
>> http://mail-archives.apache.org/mod_mbox/openjpa-users/200706.mbox/%3C7262f25e0706152022u4942a14dkec98e30b520350b7@mail.gmail.com%3E

>>
>>
>> I am facing the exact problem mentioned in that thread: I need to 
>> fetch an object several times from the database, whose columns are 
>> updated from another program and pick up the changes made by that 
>> program. However, I can't get OpenJPA to re-execute SQL and retrieve 
>> the object's new data from the database.
>>
>> In the thread of the link I provide, I was lead to believe that 
>> em.refresh(obj) should do the trick. However, OpenJPA does not execute 
>> any SQL (I have SQL=TRACE in the log options and I only see one select 
>> statement) and keeps the old data in the object.
>>
>> I use OpenJPA release 1.0.2 and the code snippet with which I test is 
>> the following:
>>
>> =========================================================
>> EntityManagerFactory emFactory =
>>     Persistence.createEntityManagerFactory(PERSISTENCE_UNIT);
>> EntityManager em = emFactory.createEntityManager();
>>
>> Record r = em.find(Record.class, 1);
>> System.out.println("VALUE: " + r.getStrField());
>>
>> System.out.print("PRESS ENTER TO REDISPLAY...");
>> BufferedReader stdin = new BufferedReader(new InputStreamReader(
>>         System.in));
>> stdin.readLine();
>>
>> // ------> PROBLEM AREA <------
>> em.refresh(r); // does not work
>> // ((OpenJPAEntityManager)em).evict(r); // also does not work
>> // em.clear(); // works but detaches everything
>> // em.close(); // ... re-create. works, costs too much
>> // ------> PROBLEM AREA <------
>>
>> r = em.find(Record.class, 1);
>> System.out.println("VALUE: " + r.getStrField());
>> =========================================================
>>
>> I change the strField of the "Record" object using a database editor 
>> before I press enter to re-fetch the value.
>>
>> So, my question is, why does OpenJPA ignore the call to refresh() if 
>> the object is still attached? I don't see it as reasonable to detach 
>> all objects just to pick up a change in the database. I figured it may 
>> be a transaction isolation issue, but then did the following test 
>> which also did not work:
>>
>> =========================================================
>> EntityManagerFactory emFactory =
>>     Persistence.createEntityManagerFactory(PERSISTENCE_UNIT);
>> EntityManager em = emFactory.createEntityManager();
>>
>> em.getTransaction().begin();
>> Record r = em.find(Record.class, 1);
>> System.out.println("VALUE: " + r.getStrField());
>> em.getTransaction().commit();
>>
>> System.out.print("PRESS ENTER TO REDISPLAY...");
>> BufferedReader stdin = new BufferedReader(new InputStreamReader(
>>         System.in));
>> stdin.readLine();
>>
>> em.getTransaction().begin();
>> r = em.find(Record.class, 1);
>> em.refresh(r);
>> em.getTransaction().commit();
>> System.out.println("VALUE: " + r.getStrField());
>> =========================================================
>>
>> So even when reading the object through 2 different transactions, I 
>> still can't get its data to be re-fetched...
>>
>> Funilly enough, if i call rollback() on the transaction, it does work! 
>> However, it seems too much of a kludge to base my solution on that...
>>
>> Thanks in advace to anyone who takes the time to help out.
>>
> 
> Craig Russell
> Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
> 408 276-5638 mailto:Craig.Russell@sun.com
> P.S. A good JDO? O, Gasp!
> 


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