[infinispan-dev] Providing a context for object de-serialization

[Mircea Markus]

On 10 Jul 2012, at 12:48, Dan Berindei wrote:

> On Tue, Jul 10, 2012 at 2:15 PM, Galder Zamarreño <galder at redhat.com> wrote:
> 
> On Jul 9, 2012, at 9:52 AM, Mircea Markus wrote:
> 
> > On 06/07/2012 22:48, Sanne Grinovero wrote:
> >> On 6 July 2012 15:06, Galder Zamarreño <galder at redhat.com> wrote:
> >>> On Jun 26, 2012, at 6:13 PM, Sanne Grinovero wrote:
> >>>
> >>>> Imagine I have a value object which needs to be stored in Infinispan:
> >>>>
> >>>> class Person {
> >>>>   final String nationality = ...
> >>>>   final String fullName = ...
> >>>> [constructor]
> >>>> }
> >>>>
> >>>> And now let's assume that - as you could expect - most Person
> >>>> instances have the same value for the nationality String, but a
> >>>> different name.
> >>>>
> >>>> I want to define a custom Externalizer for my type, but the current
> >>>> Externalizer API doesn't allow to refer to some common application
> >>>> context, which might be extremely useful to deserialize this Person
> >>>> instance:
> >>>>
> >>>> we could avoid filling the memory of my Grid by having multiple copies
> >>>> of the nationality String repeated all over, when a String [1] could
> >>>> be reused.
> >>>>
> >>>> Would it be a good idea to have the Externalizer instances have an
> >>>> initialization phase receiving a ComponentRegistry, so I could look up
> >>>> some custom service to de-duplicate or otherwise optimize my in-memory
> >>>> data representation?
> >>>> Personally I'd prefer to receive it injected via the constructor so
> >>>> that I could use a final field when my custom Externalizer is
> >>>> constructed.
> >>>>
> >>>> This is OGM related.
> >>> ^ Makes sense, but only solves one part of the problem.
> >>>
> >>> String is probably a bad example here [as you already said, due to 1], but a better example is if you have a Nationality class with country name, timezone…etc in it.
> >>>
> >>> My point is, your suggestion works for nodes to which data is replicated to, but in the original node where you've created 100 Person instances for Spanish nationaility, you'd still potentially have 100 instances.
> >>>
> >>> Did you have anything in mind for this?
> >> That's where the ComponentRegistry's role kicks in: the user
> >> application created these object instances before storing them in the
> >> original node, and if it is a bit cleverly designed it will have
> >> something like a Map of immutable Nationality instances, so that every
> >> time it needs Spanish it looks up the same instance.
> >>
> >> Consequentially the custom externalizer implementation needs access to
> >> the same service instance as used by the application, so that it can
> >> make use of the same pool rather than having to create his own pool
> >> instance: the essence of my proposal is really to have the user
> >> application and the Externalizer framework to share the same Factory.
> >>
> >>> Btw, not sure about the need of ComponentRegistry here. IMO, this kind of feature should work for Hot Rod clients too, where Externalizers might be used in the future, and where there's no ComponentRegistry (unless it's a RemoteCacheStore...)
> >> It doesn't need to be literally a ComponentRegistry interface
> >> implementation, just anything which allows the Externalizer to be
> >> initialized using some externally provided service as in the above
> >> example.
> >>
> >> This optimisation should have no functional impact but just an
> >> optionally implementable trick which saves some memory.. so if we can
> >> think of a way to do the same for Hot Rod that's very cool but doesn't
> >> necessarily have to use the same components and (internal) interfaces.
> >>
> >> I'm thinking of this as a similar "optionality" as we have when
> >> choosing between Serializable vs. custom Externalizers : people can
> >> plug one in if they know what they're doing (like these instances
> >> should definitely be immutable) but everything just works fine if you
> >> don't.
> >> I'm not really sure if there is a wide range of applications, nor have
> >> any idea of the amount of memory it could save in practice... just and
> >> idea I wanted to sketch.
> > I think there might be quite useful; the flyweight pattern[1] was
> > created to solve exactly this kind of *existing* problems.
> > Just as a note, there is a simple, not necessarily nice, workaround for
> > this: make the object pool statically accessible (or even better Enums).
> 
> It's wise to avoid static object pools, cos they can lead to classloader leak issues. Enums might be better...
> 
> 
> Sanne already mentioned in another email that OGM doesn't know the actual data type at compile time, so switching to an enum is definitely not an option.
The way I understand this, the pool would be required in the custom Externalizer implementation. More specific in the o.i.marshall.Externalizer.readObject.
Even if the type of objects are not know at compile time, the associated Externalizer implementation where the caching logic resides is  aware of its fields's types - which can be enums.   

> 
> Although it might work well enough when you know the fields ahead of time, a single static cache does seem a bit simplistic for the general case. I think in general you'd want a cache per field, e.g. so that you can give up on caching once there are too many different values for that field.
> 

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