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From Hans Pikkemaat <>
Subject Re: Object Caching
Date Thu, 12 Nov 2009 11:46:30 GMT

What I can see when I use paging in combination with SQLTemplate is this:

Cayenne first runs the main SQLTemplate query which is stored in memory
When I get the first page it determines the key values of the main query 
which it then
uses in a new query which will return the main table plus the detail 
table data.
This will produce the main table object through which the detail table 
is accessible.

The problem here is that the key of the main table is used only. The 
SQLTemplate query was manually
constructed and does a query on the main table and a left join to the 
detail table so this will produce
a duplicate key value where a main table record has 2 related detail 
table records.

This doesnt have to be a problem, actually the query does return the 
number of records used as page
size. But internally in cayenne something weird happens. Somehow the 
duplicate records are remove
and the IncrementalFaultList.checkPageResultConsistency method throws an 
exception for this.

Because the main query returns the main object but also the detail 
object I find it strange
that the query generated for the page only uses the main table key. I 
would expect that
it also would use the key of the detail table.

An example. Say I have a main table key 1 and related detail records 
with key 1, 2 and 3.
Say I run the SQLTemplate which returns key 1 but only key 1 and 2 for 
the detail table.

The page query will now run for all detail records and return all 
records which I did not

 From this I'm concluding that if an SQLTemplate is used it is not 
usefull (read: faulty) to
include the detail table in this query. When paging is used all the 
detail tables are automatically

If I write the main SQLTemplate query such it only returns the main 
object then the
Exception does not occur.

My conclusion is then that if you want to use paging with SQLTemplate 
the main
query should only return the main table. Prefetching will then return 
ALL related
table records.



Andrus Adamchik wrote:
> Yeah, still need to check that one.
> On Nov 12, 2009, at 10:43 AM, Hans Pikkemaat wrote:
>> Hi,
>> Yes, the paginated query would indeed be the only way for me to go  
>> forward.
>> The problem however is that I get the exception I posted earlier.
>> tx
>> Hans
>> Andrus Adamchik wrote:
>>> 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  forward.
>>> 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.
>>> Andrus
>>> [1]
>>> 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
>>>> HPI
>>>> 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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