[jdev] Re: Idavoll 2

[Ralph Meijer]

On Wed, Jul 06, 2005 at 11:03:19PM -0400, Herman Li wrote:
> Hi,
> 
> I'm using the latest version of Idavoll2 and have a question about 
> meta-data. Is there a way to add or modify the meta-data? Currently when 
> I use disco#info on the node, it returns 3 pieces of data (leaf node, 
> persistent, deliver payload). Can I change these values and add 
> meta-data of my own?

Caution, longish explanation. Let me tell a bit about the architecture
of Idavoll.

The grand idea is that Idavoll is architected with three distinct types
of components: one that handles the protocol stuff, a protocol-agnostic
backend and a storage facility. An application can make its own
selection from the implementations of each of these types of components
to do its thing. 

If you look at the code, you see that there is one protocol
implementation (in pubsub.py) for doing pubsub over Jabber using
JEP-0060. However, the code is set up in such a way that you could hook
up more than one protocol to a backend. You can think about adding a web
interface for managing nodes and subscriptions, getting publish events
via XMLRPC or sending notifications via e-mail. Protocol implementations
are always an 'Adaptor' from the backend interface to the used protocol.

So far, there is one implementation of the backend interface (defined in
backend.py). You can find it in generic_backend.py. The backend embodies
the business logic of the system. The idea here is that you can either
write your own backend (that implements the interface) or derive from
the generic backend to modify a few small business rules.

Then finally, there is the storage facility. There are currently two
implementations of the storage interface. One that uses pgsql and a
memory based one. The former persists its information in the database,
the latter looses everything when you shutdown the application.

To get back to your question. It depends on where the information is
comming from. The meta-data of a node usually consists of configurable
information and other information. For example you could store the
creation date of a node, and return that in the meta-data for that node.
But this piece of information is typically not configurable. If the
information you want to convey is stored with the node information (e.g.
in the pgsql database), then you need to adjust the storage facility. If
you can derive this information from the unmodified storage facility,
you can just alter the backend.

I hope this makes some sense, and if you more questions, do let me know.
Eventually this information will probably be put in a tutorial kind of
document.

-- 
Groetjes,

ralphm