JCR + N-Tier architecture design issues: need some help

["maarten.volders@pandora.be" <ma...@telenet.be>]

Hi,

I'm new to the JCR philosophy and also a little bit confused on how we should this new technology in an N-Tier architecture. I'm used on working with Hibernate, pojo's and ORM mappings in this kind of environment and have the following questions about JCR implementations:

Let's take for example a simple blogging website (that's seems to be the "hello world" example for JCR :) ... )
- Is JCR only present behind the DAO layer or do the nodes and so traverse through all layers
- If the JCR in not shielded behind pojo's and the DAO, I find it difficult to understand how we should expose JCR retrieved data? Should we use the JCR api also in the presentation layer ... i hope not!!!
- If the JCR is indeed behind the DAO ... how do we go on from the DAO to the presentation layer? Do we still use pojo, for example with a pojo called BlogMessage which have references to its parent and child BlogMessages? If so what is the best practise to create and update data that is changed in these pojo's. I'm a little bit lost with the use of pojo's in combination with JCR!!!
- What about transactions ... is that possible with JCR
- If the JCR impl is backed by i.e. Hibernate ... how will the JCR data will be saved. Is there still a ORM mapping behind the scenes or how is this done?

So, it would be great that someone explains a real life example on how to use JCR in an N-Tier architecture...

Thanks

M.

Re: JCR + N-Tier architecture design issues: need some help

[Christophe Lombart <ch...@gmail.com>]

> i think you may find some more info in graffito. (right, christophe?)
> http://incubator.apache.org/graffito/

Correct, We are working on such "ocm" framework. I like
"object-content-mapping" - Thanks David !

It is still under dev and all helps are welcome. The Graffito project
is splitted into severals subprojects, you have to check out only the
"jcr-mapping" subproject.  Other subprojects are used for portal
integration and building the Graffito CMS services.
You are welcome to joing the Graffito mailing list to speak about this
jcr mapping framework.


regards,
Christophe

Re: JCR + N-Tier architecture design issues: need some help

[David Nuescheler <da...@gmail.com>]

hi maarten,

thanks for your question.

> I'm new to the JCR philosophy and also a little bit confused on how
> we should this new technology in an N-Tier architecture. I'm used
> on working with Hibernate, pojo's and ORM mappings in this kind
> of environment and have the following questions about JCR
> implementations:
> Let's take for example a simple blogging website (that's seems to be
> the "hello world" example for JCR :) ... )
hehe ;)

> - Is JCR only present behind the DAO layer or do the nodes and
> so traverse through all layers
i think that this is subject to the "implementation style" and the
"application". personally, i like a mix of pojo and direct access to
the nodes and properties.

sometimes it is very convenient to be able to just access the
nodes and properties directly from your presentation-layer for very
simple things (mostly generic display).

if there is only a hint of "business logic" involved, i usually appreciate a
pojo instead. once you have the pojo there is not much reason to go
back to the nodes and properties directly.

> - If the JCR in not shielded behind pojo's and the DAO, I find it difficult
> to understand how we should expose JCR retrieved data? Should we
> use the JCR api also in the presentation layer ... i hope not!!!
well, it is up to the application how much you want to shield.

let's say an application like http://jsr170tools.day.com/crx/browser/index.jsp
does not experience any benefit of wrapping the nodes and properties.

also, in my experience a lot of the just "template presentation logic" does not
really benefit from that either.

> - If the JCR is indeed behind the DAO ... how do we go on from the DAO
> to the presentation layer? Do we still use pojo, for example with a pojo
> called BlogMessage which have references to its parent and
> child BlogMessages? If so what is the best practise to create and
> update data that is changed in these pojo's. I'm a little bit lost with the
> use of pojo's in combination with JCR!!!

yes, a pojo is the right way of doing that.

i think at this point there is still a bit of hand coding involved
in the creating the the java classes from the nodetypes or
vice-versa.
updates can be subscribed to through observation.

personally, i would love to have a robust, annotations-based
"object-content-mapping" (ocm) framework.
on the bright side of things a "content repository" (opposed
to an rdbms) can actually deal with inheritance (and even aspects)
in a native fashion and therefore architecturally lends itself much better
for pojo persistence.

> - What about transactions ... is that possible with JCR
yes.

> - If the JCR impl is backed by i.e. Hibernate ... how
> will the JCR data will be saved. Is there still a ORM mapping
> behind the scenes or how is this done?
i never saw much value in backing a content repository by an orm-layer.

i am a bit more of a fan of a relatively direct jdbc backing, since i
think that there is no real value at a low level like the
persistence manager to introduce additional abstraction.
i am sure other people feel differently. ;)

> So, it would be great that someone explains a real life example on
> how to use JCR in an N-Tier architecture...

i think you may find some more info in graffito. (right, christophe?)
http://incubator.apache.org/graffito/

as i said i am convinced that a content repository is an ideal
store to persist pojo's and i think that it is a matter of time
until robust generic mapping frameworks are available.

regards,
david