[Zope-dev] Racks and Persistence (was Alternative Storages)

[Jason Spisak]

Phillip J. Eby:

This clears things up *a lot*.
Thank you very much.  Sometimes reading the sources doesn't answer all the
questions.

> At 01:03 AM 5/4/00 +0000, Jason Spisak wrote:
> >
> >What about storing the class data itself in a Rack?  Is there any
> >limitation 
> >to what you can put into a Rack.  I am only asking this to clarify what we
> >are heading towards.  Somewhere to put just the instance data.
> 
> Racks hold a collection of objects derived from the RackMountable base
> class.  The objects may be ZClass instances, or instances of a Python
> ZClass base, as long as in either case they derive from RackMountable.
> RackMountable (as of ZPatterns 0.3.0) contains a C mixin class that gives
> Persistent objects the ability to coexist with (renamed)
> __get/set/delattr__ methods.  It also contains hooks that delegate various
> operations to the Rack, such as propertysheet management.
> 
> When a RackMountable is retrieved from a Rack, the Rack calls _setRack() on
> the RackMountable to tell it where it came from.  From then on, the
> RackMountable can delegate its data management operations to the Rack.
> 
> Conceptually, a Rack is like a database table.  It holds only one kind of
> object (although there are ways around that if it's really needed), and has
> a key by which items are stored/retrieved.  The basic Rack interface
> consists of Rack.getItem(key) and Rack.newItem(key).  Both return an
> instance of the class the Rack stores.  You then talk to that object
> directly for any operations you need, such as setting its initial
> properties, or telling it to delete itself from the Rack.  Racks have a
> "Methods" tab, however, where you can add your own DTML, Python, or
> whatever else to add extended operations such as searching, automatic
> generation of sequential ID's, etc.  By default, Racks use the "id"
> attribute as a key, but this can be overridden in a subclass if you desire.
> 
> 
>  
> >So a "standard" ZODB connection from which a class can get it's data which
> >can be fulfilled by anything inherited from BaseStorage?
> 
> Um, no, it would need to be something which could provide a ZODB
> connection.  

<Lights going on>

>The connection would be back-ended to something inheriting
> from BaseStorage, yes.  In practice, all that Racks need is someplace to
> retrieve a BTree object with all its attached children (PersistentMapping
> objects containing instance data and/or propertysheets).
> 
> 
> >> The thing about these "persistence providers", though, is that they need to
> >> provide some kind of root for each thing that wants to use them.  
> >
> >Is that for acquisition sake or class container sake or what exactly?
> 
> It's a ZODB internal issue.  A ZODB has to have a root object, which is
> normally a PersistentMapping.  Items placed in that root are never garbage
> collected.  That root's namespace would have to be shared among the users
> of that ZODB in some fashion, and garbage collection would be important so
> that deleting a Rack from the primary ZODB would ensure deletion of its
> data from the secondary ZODB.  Or, alternatively, the secondary ZODB must
> expose access to the root object to manually remove sub-roots.
> 

Again thanks,

Jason Spisak
CIO
HireTechs.com
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