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From Andrus Adamchik <and...@objectstyle.org>
Subject Re: Extract large data from database
Date Wed, 18 Mar 2009 14:52:29 GMT
Actually using a ResultIterator should be entirely safe in 3.0 in  
respect to memory management. If a user doesn't do something bad (like  
storing obtained objects in a list himself), and only one object is  
processed at a time, there won't be any memory issues.

>> The servlet i'm implementig will be called by a different webapp
>> (backoffice.. in a server 2 server way ) with a low traffic but i'm  
>> warried
>> about the warning reported in the article:
>>
>> "In web applications, programmers must ensure that no open  
>> ResultIterators
>> are kept between HTTP requests. Failure to do so may result in too  
>> many
>> database connections being locked, thus quickly exhausting  
>> connection pool.

This one is still valid. Keep ResultIterator request-scoped, and close  
it at the end.

>> In general, an application with Web GUI is NOT a good candidate for
>> implementation using ResultIterators."

Well, there are always exceptions... The general warning here is based  
on the assumption that large results are usually taking lots of time  
to fetch and process. So users may not see the response fast enough.  
If the response time is reasonable in your case, you can use  
ResultIterator.

Andrus

On Mar 18, 2009, at 4:17 PM, Robert Zeigler wrote:

> Problem is that the data context will cache the objects (up to the  
> max object limit); this is alleviated somewhat in 3.0 with the use  
> of weak references
> in the caching, but there's still potential for running out of memory.
>
> You can certainly use ResultIterator, with the caveat that you'll be  
> getting DataRows instead of DataObjects.
> Alternatively, periodically discard the context that you're using  
> and replace it with a fresh context, something like:
>
> Query query = createMyQuery();//SQLTemplate, or SelectQuery, or  
> EJBQLQuery, or whatever.
> while(notDone) {
>    ObjectContext context = ...;//create a new object context here.
>    //do things like set the query fetch limit and fetch offset.
>    List objs = context.performQuery(query);
>    for(Obj o : objs) {
>       //stream out the obj...
>    }
> }
>
> Robert
>
> On Mar 18, 2009, at 3/186:14 AM , mr.abanjo wrote:
>
>> Hi,i'm developing an application that must load a large ammount of  
>> data
>> (100.000 records) and stream them out  from a servlet.
>>
>> I created a "for" cycle that for every step load 200 record from the
>> database (using datacontext.performquery method), and then flush  
>> the data in
>> the serlvet output stream.
>> In this way i suppose that the max ammount of memory allocated by  
>> cayenne is
>> :
>>
>> object size * 200
>>
>> but seems that this is not true. When i try to get the data from the
>> servlet, after few seconds, i receive an "out of memory" error. (my  
>> 500Mb
>> allocated for the Heap are reached in a few time... :-(....)
>>
>> Which is the best practice in this case?
>> I can increase the heap, but i want to find a workaround that allow  
>> me to
>> have a small heap allocated during this operation.
>>
>> I found this article:
>> http://cayenne.apache.org/doc20/iterating-through-data-rows.html
>> Is this the right way?
>>
>> The servlet i'm implementig will be called by a different webapp
>> (backoffice.. in a server 2 server way ) with a low traffic but i'm  
>> warried
>> about the warning reported in the article:
>>
>> "In web applications, programmers must ensure that no open  
>> ResultIterators
>> are kept between HTTP requests. Failure to do so may result in too  
>> many
>> database connections being locked, thus quickly exhausting  
>> connection pool.
>> In general, an application with Web GUI is NOT a good candidate for
>> implementation using ResultIterators."
>>
>> Thanks
>> Davide
>
>


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