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From Jukka Zitting <jukka.zitt...@gmail.com>
Subject On virtual content
Date Tue, 18 Sep 2012 15:56:53 GMT
Hi,

We have a good reasons for adding support for "virtual content", i.e.
content that's accessible through the Oak API, but that's not actually
stored in the underlying MicroKernel. The main reasons that come to my
mind are:

1) As we implement workspace support, we need some way to make the
shared /jcr:system subtree visible to all workspaces.
2) The JCR spec mandates some auto-created content to be available
already before transient changes are persisted. That should be fairly
straightforward to implement with virtual content.
3) We could "mount" parts of external repositories to a single content tree.
4) We could implement "views" like in Hippo's facet feature.

The same feature could probably also be easily used to hide existing
content for access control or other reasons.

So how could we best implement something like this?

The implementation should probably go to somewhere between the Oak API
and the MicroKernel, ideally either as an extension point in the Tree
or NodeState implementations. Additionally, to make sure that the
implementation won't interfere with the performance of normal content
access, this mechanism should probably only be invoked _after_ an
attempt to access normal content has been made. Apart from these
general ideas I don't yet have a good picture of what the detailed
implementation could or should look like.

Like with custom indexes, the configuration settings for such virtual
content should in my mind go into the repository as normal content.
For example, the following hypothetical repository tree would define a
workspace subtree with a view to a shared /jcr:system subtree and a
custom view for the latest 100 nodes in /latest:

    /
        /jcr:system {the global jcr:system tree}
            /... {namespace & node type registries, version store, etc.}
        /my-workspace [jcr:mixinTypes = oak:virtual]
            /oak:virtual
                /system [jcr:mixinType = oak:mount,
                         globalPath = /jcr:system,
                         mountName = jcr:system]
            /latest [jcr:mixinTypes = oak:virtual]
                /oak:virtual
                    /view [jcr:mixinType = oak:view,
                           query = "SELECT * FROM [nt:base] ...
                                    ORDER BY [jcr:created] DESC LIMIT 100"]

As before, please share your critiques, improvements and/or alternatives.

BR,

Jukka Zitting

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