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From "Daniel Lee" <tsunfang...@gmail.com>
Subject Re: missing getAll(List keys) method?
Date Tue, 29 May 2007 21:33:57 GMT
Hi Craig,

The discussed API (getAll) is for fetching objects that's already cached in
the DataCache.  From what I understand, OpenJPA executes the following code
when loading (find()) a customer which exists in the DataCache.  It loads
not only the customer but also the objects in any eager (direct and
indirect) relationships.  In the earlier example (a customer with 100 orders
and each order has different products) the direct relationships
are all orders placed by the customer and the indirect relationships are all
products in these orders).

   1. BrokerImpl.find() calls DataCacheStoreManager.initialize() to
   initialize a new state manager for an object (a customer with 100 orders
   for example).
   2. initialize() then issues get() to DataCache to see whether the data
   (customer) is already cached.  After successfully getting the customer (data
   != null) from the datacache, DataCachePCData.load(sm, fetch, edata) is
   invoked to load all the eager relationships (orders in the example) of the
   object (customer).
   3. PCDataImpl.load() loops through the relationship field to call
   loadField() for each relationship which is not yet loaded.  In this example,
   it is the relationship the customer to its orders (eager,
   one-to-many) relationship
   4. loadField() calls toField() which is defined in AbstractPCData.
   5. toField() LOOPS through all elements (orders) to invoke
   toNestedField() for each element.  This is 100 toNestedFields calls for the
   100 orders in the example.
   6. toNestedField() calls toRelationField(sm, vmd, data, fetch,
   context) which actually calls find() and recursively get back to step 1
   above for loading "a" order.  This will end up calling get() 100 times to
   the DataCache for the 100 orders and can possibly get into another loop for
   loading all products in each order, etc.

Because of the loop in step 5 above, a single "find(customerA)" statement
actually triggers 100 DataCahce.get() for its orders and could be hundreds
or thousands more of the get() calls for the products ordered by the
customer.  This is a performance hit as I understand.

If we have getAll(List keys) method which returns a list of objects from the
datacache, we can change the logic to call the following new methods to get
all elements (orders/products) in one relationship in single call to
getAll(); instead of calling get() a hundred times for 100 orders.

   - toNestedFields() - called by toFields without the loop
   - toRelationFields() - called by toNestedFields; calls findAll()
   - findAll need to be able to initialize a List of sm and call
   initializeAll()
   - initializeAll() - call getAll() instead of get(), then iterate the
   return to call load

This is more like doing batch fetch from DataCache.  There should be some
significant performance improvement, especially in the distributed
environment in which the communication/serialization area is known be the
bottleneck of the whole process.  This implementation can also potentially
provide a lot better performance for the 3-rd party DataCache plug-ins which
provide and optimize getAll() process.

Hope this make the issue more clear this time.  Could you please let me know
if you have further questions or other concerns.  Many thanks.

Daniel

On 5/24/07, Craig L Russell <Craig.Russell@sun.com> wrote:

> Hi Daniel,
>
> On May 24, 2007, at 11:59 AM, Daniel Lee wrote:
>
> > Hi Craig,
> >
> > I think findAll() is different.  It is a client level API and the
> > getAll()
> > here is for internal fetch from data cache.
> >
> > In the example, when an application issue findAll() for a list of
> > customers.  It internally, for each customer with order(s), loads the
> > "eager" relationship (orders) from data cache if they are already
> > cached by
> > calling map.get(orderId) for each order placed by the customer.  It
> > again
> > load the items that are related to each order by calling map.get
> > (itemId) for
> > each item if the relationship to Order is declared as eager.  This is
> > potentially a performance bottleneck and findAll() does not avoid
> > this.
>
> Seems that this algorithm can be improved to use the broker's findAll
> mechanism when the instance is not found in the cache. The not-found
> instances can be found more efficiently than the code currently does.
>
> Craig
> >
> > Thanks.
> > Daniel
> >
> >
> > On 5/23/07, Craig L Russell <Craig.Russell@sun.com> wrote:
> >>
> >> Hi Daniel,
> >>
> >> Take a look at the findAll(Collection oids) method of
> >> OpenJPAEntityManager. This should do a better job than N get(Object
> >> key) methods.
> >>
> >> Craig
> >>
> >> On May 23, 2007, at 3:55 PM, Daniel Lee wrote:
> >>
> >> > Do we miss the getAll(List keys) method for data cache?
> >> >
> >> > When fetching objects with eager "to-many" relationships, the
> >> code is
> >> > calling get(Object key) multiple time (one for each object in the
> >> > relationship).  For example, it is doing 1 get() call for each
> >> > order placed
> >> > by a customer which we are fetching, that means 100 calls for a
> >> > customer
> >> > with 100 orders.  The performance can be greatly improved if we
> >> have
> >> > getAll(List keys) methods which returns all orders in one call.
> >> > This is
> >> > especially important in a distributed environment.
> >> >
> >> > Is there a way (new plug-in) to avoid the multiple-trip for single
> >> > relationship, or can we implement the code to improve the
> >> > performance in
> >> > this area?
> >> >
> >> > Many thanks.
> >> > Daniel
> >>
> >> Craig Russell
> >> Architect, Sun Java Enterprise System http://java.sun.com/products/
> >> jdo
> >> 408 276-5638 mailto:Craig.Russell@sun.com
> >> P.S. A good JDO? O, Gasp!
> >>
> >>
> >>
>
> Craig Russell
> Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
> 408 276-5638 mailto:Craig.Russell@sun.com
> P.S. A good JDO? O, Gasp!
>
>
>

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