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Gerard · Guest

Hi all,

I'm building a OPF, and the OPF has a object cache wich keeps created or
retrieved objects in memory.
A retrieve operation goes like this:

See if the Id we ask for is in the cache.
If it's there
return the object.
Otherwise
Ask the storage layer for the object
If the object was retrieved from storage
Put it in the cache
Return the Object
Otherwise
Return Nil

As it is designed now, an object can only exist once in the cache, but
it can be referenced in many different places:
- Individual objects created by a call to a constructor Retrieve(ID)
method. e.g. aInvoice := TInvoice.Retrieve('1001')
- As a list element, from a ObjectList type attribute e.g.
aCustomer.Invoices[0] or Invoice.Items[2]
- As a list element from an independent list used to retrieve objects
with some specific criteria. e.g. aInvoiceList :=
TMyObjectList.Create(TInvoice, 'Total > 10000')
Lists are loaded from storage and then the cache is updated if needed.

I have a problem trying to figure how to manage explicit object freeing
in a efficient and clear way.
When an object is freed by a call to MyObject.Free, I cannot really free
it because it may be referenced in many places. But is keeping objects
in the cache forever (or at least until the cache is emptied) a good
solution?
If not, I guess I should have to implement some kind of reference counting?

I'd apreciate any thougths/experiences on this.

Regards, and have a nice weekend.

Gerard.

Peter Morris [Droopy eyes · Guest

Hi Gerard

If you introduce the idea of having an ObjectReference class instead of
using direct references you could safely unload objects at any point you
like as long as you never free the ObjectReference. For example

PurchaseOrder.OrderLines[0].Object

OrderLine.PurchaseOrder.Object

The ObjectReference holds the information it needs to fetch the object
itself from the DB, when you ask for the Object it loads it, when the object
is unloaded its ObjectReference.Object is set to NIL.

--
--
Pete

Blessed are the geek, for they shall public class GeekEarth : Earth {}
====
Audio compression components, DIB graphics controls, ECO extensions,
FastStrings
http://www.droopyeyes.com
http://mrpmorris.blogspot.com
====

Gerard · Guest

Hi Peter,

Bob Dawson · Guest

"Gerard" wrote

Harley Pebley · Guest

Guest

Hello Gerard,

What I have done is to reintroduce a Free method in order to take into
account the reference count mechanism and if the instance is no longer
referenced then I call the "real" free method:

TNKObject = class(TPersistent)
private
FRefCount: Integer;
procedure InternalFree;
procedure Release;
procedure AddRef;
public
procedure Free; reintroduce;
end;

procedure TNKObject.InternalFree;
begin
inherited Free;
end;

procedure TNKObject.Free;
begin
Release;
end;

procedure TNKObject.AddRef;
begin
BOCriticalSection.Acquire;
Inc(FRefCount);
BOCriticalSection.Release;
end;

procedure TNKObject.Release;
begin
if Self = nil then Exit;
Assert((FRefCount <> 0), Format('%s: RefCount<>0', [ClassName]));

BOCriticalSection.Acquire; // framework lock
Dec(FRefCount);
BOCriticalSection.Release;
if FRefCount = 0 then
InternalFree;
end;

One last thing, do not free your objects using FreeAndNil, instead make
your own BoFreeAndNil in order to call the properly reintroduced Free
method:

procedure BoFreeAndNil(var Obj);
var
Instance: TNKObject;
begin
Instance := TNKObject(Obj);
TNKObject(Instance) := nil;
Instance.Free;
end;

I hope it helps you.

Luis Solar.

Gerard ha escrito:

Harley Pebley · Guest

Peter Morris [Droopy eyes · Guest

Joao Morais · Guest

Gerard wrote:

Joao Morais · Guest

Harley Pebley wrote:

Harley Pebley · Guest

Harley Pebley · Guest

Joao Morais · Guest

Harley Pebley wrote:

Gerard · Guest

Thanks all for your suggestions and feedback.
I have much clearer idea now of what to do and how to do it.

Regards,

Gerard.

Bob Dawson · Guest

<lsolart (AT) gmail (DOT) com> wrote