[elephant-devel] Re: blog example w/ hunchentoot

[Ian Eslick]

I'm not sure that deserializing a persistent object without actually  
creating it would make much sense.  What would you return from the  
deserializer?  What operations would it need to support?

Think of it this way.  If you get a value from a persistent index and  
that value is a persistent object, then you want object identity, the  
ability to inspect it's type, slot access, etc.  The only way to do  
that is to create something in memory which provides this.  The only  
way to that is to make sure your lisp has had the chance to construct  
everything it needs via make-instance.

The :from-oid argument to make-instance overrides some of the normal  
make-instance behavior by inhibiting all initform initialization as  
the object's slots are assumed to already be valid, so you have  
exactly the behavior you want.

There are two caveats to this nice model: transient slots and make- 
instance methods.

Transient Slots: they have the semantics of ordinary lisp slots - you  
cannot expect them to persist between images or after their  
placeholder has been garbage collected.  If you drop all references  
to an object with transient slots, those slots are reclaimed by the  
GC.  If you try to access them again later, they are reset to their  
default state (unbound, initforms or make-instance computations).

Make-instance specializers: If you override the typical make-instance  
via :before, :after, :around or specialized on your class, you have  
to make sure that you predicate on :from-oid (this will be documented  
in the new manual) to avoid resetting persistent values.  If :from- 
oid is given a value, then you can assume the object is being  
reconstituted and you don't need to do anything special for the  
persistent slots.

However, if you want to use persistent slots to compute the value of  
some transient slots, then a make-instance specializer is a good  
place to do it.  indexed-btree is an example of this, an in-memory  
hash is cached in the transient slot for reads and writes are  
mirrored to a serialized hash in a persistent slot.

Ian

On Apr 23, 2007, at 6:42 PM, Pierre THIERRY wrote:

> Scribit Ian Eslick dies 22/04/2007 hora 10:39:
>> However, if  the objects are not already in-memory, the deserializer
>> will call  make-instance on that instance using the OID and class
>> stored in the DB.
>>
>> This does not call initforms or use initargs but will trigger any
>> initialize-instance method specializers you have defined (such as
>> after methods).
>
> Would it be possible to deserialize the objects without triggering all
> this initialization? There's a semantic issue, here, because an image
> using persistent objects should behave the same with or without being
> shut down and restarted. This is precisely a goal of persistence,  
> in the
> way I understand it.
>
> If initialization is triggered, you have a visible behaviour change  
> when
> the Lisp image is shut down and restarted.
>
> Do you think it would be reasonable to get rid of that?
>
> Curiously,
> Pierre
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