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From Andrus Adamchik <>
Subject Re: Object Caching
Date Thu, 12 Nov 2009 08:19:17 GMT
For paginated queries we contemplated a strategy of a list with  
constant size of fully resolved objects. I.e. when a page is swapped  
in, some other (LRU?) page is swapped out. We decided against it, as  
in a general case it is hard to consistently predict which page should  
be swapped out.

However it should be rather easy to write such a list for a specific  
case with a known access order (e.g. a standard iteration order). In  
fact I would vote to even include such implementation in Cayenne going  

More specifically, you can extend IncrementalFaultList [1], overriding  
'resolveInterval' to swap out previously read pages, turning them back  
into ids. And the good part is that you can use your extension  
directly without any need to modify the rest of Cayenne.



On Nov 12, 2009, at 10:07 AM, Hans Pikkemaat wrote:
> Hi,
> 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.
> tx
> 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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