As of 5.2, the way it works is: The snapshots in EODatabase have a
reference count. Each editing context that fetches an EO increments
the reference count. The EO holds onto that EO via a WeakReference.
When the WeakReference is reclaimed, the snapshot reference count can
decrease (note CAN, not IMMEDIATELY WILL -- the editing context keeps
reference queue which is only processed periodically). When the
count gets to zero, the database forgets the snapshot. If you have
entity caching enabled, then EODatabase ignore reference count (or
keeps it at "1" as a minimum) and it will not go away in a read-only
scenario. If you modify any entity of that type and saveChanges in
your EditingContext, a cached entity's cache will be entirely
flushed. (NB: Keep this in mind, because if you are caching a large
amount of data that is writable, it will NOT be very smart about
updating that cache -- It's blown away with every update and then it
immediately reloads the entire set of objects for that entity at the
next access)
If you have retainsAllRegisteredObjects enabled on your editing
context, it will NOT use WeakReferences. Under this circumstance,
the EO reference count is only decreased when 1) you dispose the
editingcontext or 2) you forget or invalidate the object.
When you modify an object in an editing context, the editingcontext
keeps a strong reference to the objects until you saveChanges (or
revert, reset, etc), at which point the strong references are cleared
and the only remaining reference is the weakreference like before.
If you have an undo manager enabled, it will keep a strong reference
to the affected EO's as long as the undo is around.
I do wonder if EC's should be using SoftReferences instead of
WeakReferences ... Would seem to be more friendly to the users of
those EO's.
If you are using WO pre 5.2, then none of the WeakReference stuff
applies, and everything is purely done with snapshot reference
counting -- it should behave like retainsAllRegisteredObjects = true
in 5.2.
ms
On Jan 21, 2006, at 7:44 PM, [EMAIL PROTECTED] wrote:
Hello Art;
Thank you for this documentation reference -- very interesting.
Snapshots are reference-counted ... So as long as a reference to a
snapshot from any editing context exists, the snapshot will
remain. But when the last reference is broken (e.g., when an
editing context is freed), the snapshot will be freed.
I see what you mean from the EODatabase javadoc in that there is
definitely reference counting supported, but are you sure this is
still used by the EOF machinery in WO5? The thing that makes me
wonder is that EC-centric reference-counting doesn't entirely
correlate to the experience I've had with EC behaviours in WO5.2.
I can keep one EC instance for processes which must load up at
least many hundreds of thousands of EO-s over a long period and
there are generally no memory blow-outs which one might otherwise
expect.
I would have *guessed* that a cunning arrangement between weak and
strong EC-EO-snapshot references were being used to ensure
snapshots were allowed to be destroyed appropriately using GC. I
would really love to see some concrete WO5-centric tech note about
the workings of "fetch timestamp lag" and "EC-based snapshot
maintenace" so one could be more certain about some of these
aspects of EOF.
cheers.
___
Andrew Lindesay
www.lindesay.co.nz
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