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From Hans Pikkemaat <>
Subject Re: Object Caching
Date Thu, 12 Nov 2009 08:07:35 GMT

So this means that if I use a generic query that the query results are 
always stored
completely in the object store (or the query cache if I configure it).

Objects are returned in a list so as long I have a reference to this 
list (because I'm
traversing it) these objects are not garbage collected.

If I use the query cache the full query results are cached. This means 
that I can only
tell it to remove the whole query.

Effectively this means I'm unable to run a big query and process the 
results as a stream.
So I cannot process the first results and then somehow make them 
available for
garbage collection.

The only option I have would be the iterated query but this is only 
usefull for queries
one 1 table without any relations because it is not possible to use 
prefetching nor is
it possible to manually construct relations between obects.

My conclusion here is that cayenne is simply not suitable for doing 
large batch wise
query processing because of the memory implications.



Andrus Adamchik wrote:
> As mentioned in the docs, individual objects and query lists are  
> cached independently. Of course query lists contain a subset of cached  
> object store objects inside the lists. An object won't get gc'd if it  
> is also stored in the query list.
> Now list cache expiration is controlled via query cache factory. By  
> default this is an LRU map, so as long as the map has enough space to  
> hold lists (its capacity == # of lists, not # of objects), the objects  
> won't get gc'd.
> You can explicitly remove entries from the cache via QueryCache remove  
> and removeGroup methods. Or you can use a different QueryCacheFactory  
> that implements some custom expiration/cleanup mechanism.
> Andrus
> On Nov 11, 2009, at 3:43 PM, Hans Pikkemaat wrote:
>> Hi,
>> I use the latest version of cayenne, 3.0b and am experimenting with  
>> the object caching features.
>> The documentation states that committed objects are purged from the  
>> cache because it uses weak references.
>> (
>> If I however run a query using SQLTemplate which caches the objects  
>> into the dataContext local cache (objectstore),
>> the objects don't seem to be purged at all. If I simply run the  
>> query dump the contents using an iterator on the resulting
>> List then the nr of registered objects in the objectstore stays the  
>> same (dataContext.getObjectStore().registeredObjectsCount()).
>> Even if I manually run System.gc() I don't see any changes (I know  
>> this can be normal as gc() doesn't guarantee anything)
>> What am I doing wrong? Under which circumstances will cayenne purge  
>> the cache?
>> tx
>> Hans

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Hans Pikkemaat
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