Re: concurrent access to LRS

Tue, 12 Dec 2006 23:20:46 -0800 

Roger-
JDBC drivers are supposed to be thread-safe, but I wouldn't be surprised of some of them had problems. 


What happens if you synchronized on the result collection when  
iterating through them?




On Dec 12, 2006, at 7:44 PM, roger.keays wrote:



Hi Marc,


Marc Prud wrote:


Was a transaction committed or rolled back between the two requests?
Many databases and/or JDBC drivers do not allow a result set cursor
to span multiple transactions, which might be the cause of this error

(which, admittedly, we are not handling as gracefully as we could  
be).





No, there is no SQL activity between the requests except for that  
produced
by the LRS. I suspected it could be a JDBC driver issue which is  
why I tried
MySQL also. I see pretty well the exact same behaviour on MySQL  
though.



It seems that the result list is closing because of an exception  
caused by
two threads both trying to use the database connection at the same  
time. The

first exception is usually a nonsensical one from the JDBC driver -

NullPointerException, stuff about BigDecimals (of which I use  
none), SQL

errors - "portal c_8 does not exist' (?).


The only constant I can find is that it works fine if I load the  
frames one

by one, so my best guess is it is a concurrency issue.

Thanks,

Roger



On Dec 12, 2006, at 6:26 PM, roger.keays wrote:



Now that I have an em which lives for longer than one request (as
per my
previous thread) I have noticed some concurrency issues accessing
the large
result set. Since my app uses frames, there are occasions when two
frames
will be refreshed at the same time. When this occurs, and the both
try to

iterate the LRS I will frequently get either a  
NullPointerException or

NoSuchElementException (result list is closed). Loading the frames
one by
one works fine.

I've tried to the following with no avail:
    * Adding openjpa.Multithreaded=true
    * Synchronizing the LRS' getter method
    * Removing connection pooling (i.e. use openjpa's datasource)
    * Using mysql instead of postgresql
    * Disabling the query cache
    * Disabling the data cache

As you can see, I'm a bit stuck. Any suggestions welcome.

The NullPointerException stacktrace is:

java.lang.NullPointerException

org.apache.openjpa.jdbc.kernel.SelectResultObjectProvider.next
(SelectResultObjectProvider.java:96)

org.apache.openjpa.lib.rop.WindowResultList.getInternal
(WindowResultList.java:126)

org.apache.openjpa.lib.rop.AbstractNonSequentialResultList.get
(AbstractNonSequentialResultList.java:69)

org.apache.openjpa.kernel.DelegatingResultList.get
(DelegatingResultList.java:241)
 javax.faces.model.ListDataModel.getRowData(ListDataModel.java:141)
 javax.faces.model.ListDataModel.setRowIndex(ListDataModel.java:175)
 javax.faces.model.ListDataModel.setWrappedData(ListDataModel.java:
209)
 javax.faces.model.ListDataModel.<init>(ListDataModel.java:68)


org.apache.myfaces.component.html.ext.HtmlDataTableHack.createDataMo 
de

l(HtmlDataTableHack.java:448)

org.apache.myfaces.component.html.ext.HtmlDataTable.createDataModel
(HtmlDataTable.java:847)

org.apache.myfaces.component.html.ext.HtmlDataTableHack.getDataModel
(HtmlDataTableHack.java:414)

org.apache.myfaces.component.html.ext.HtmlDataTable.getDataModel
(HtmlDataTable.java:839)

org.apache.myfaces.component.html.ext.HtmlDataTableHack.getRowCount
(HtmlDataTableHack.java:87)


org.apache.myfaces.shared_tomahawk.renderkit.html.HtmlTableRendererB 
as

e.encodeInnerHtml(HtmlTableRendererBase.java:226)


org.apache.myfaces.shared_tomahawk.renderkit.html.HtmlTableRendererB 
as

e.encodeChildren(HtmlTableRendererBase.java:134)


org.apache.myfaces.renderkit.html.ext.HtmlTableRenderer.encodeChildr 
en

(HtmlTableRenderer.java:187)

javax.faces.component.UIComponentBase.encodeChildren
(UIComponentBase.java:828)
 javax.faces.component.UIComponent.encodeAll(UIComponent.java:883)
 javax.faces.component.UIComponent.encodeAll(UIComponent.java:889)
 com.sun.facelets.FaceletViewHandler.renderView
(FaceletViewHandler.java:578)

and the NoSuchElementException:

java.util.NoSuchElementException: The result list has been closed.

org.apache.openjpa.lib.rop.AbstractResultList.assertOpen
(AbstractResultList.java:91)
 org.apache.openjpa.lib.rop.WindowResultList.size
(WindowResultList.java:81)

org.apache.openjpa.kernel.DelegatingResultList.size
(DelegatingResultList.java:133)

 javax.faces.model.ListDataModel.isRowAvailable 
(ListDataModel.java:99)

 javax.faces.model.ListDataModel.setRowIndex(ListDataModel.java:174)

org.apache.myfaces.component.html.ext.HtmlDataTableHack.setRowIndex
(HtmlDataTableHack.java:243)

org.apache.myfaces.component.html.ext.HtmlDataTable.setRowIndex
(HtmlDataTable.java:263)


org.apache.myfaces.shared_tomahawk.renderkit.html.HtmlTableRendererB 
as

e.encodeInnerHtml(HtmlTableRendererBase.java:257)


org.apache.myfaces.shared_tomahawk.renderkit.html.HtmlTableRendererB 
as

e.encodeChildren(HtmlTableRendererBase.java:134)


org.apache.myfaces.renderkit.html.ext.HtmlTableRenderer.encodeChildr 
en

(HtmlTableRenderer.java:187)

javax.faces.component.UIComponentBase.encodeChildren
(UIComponentBase.java:828)
 javax.faces.component.UIComponent.encodeAll(UIComponent.java:883)
 javax.faces.component.UIComponent.encodeAll(UIComponent.java:889)
 com.sun.facelets.FaceletViewHandler.renderView
(FaceletViewHandler.java:578)

Cheers,

Roger
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