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From "Daniel Spicar (JIRA)" <>
Subject [jira] [Commented] (CLEREZZA-460) Use whiteboard pattern for Renderlets
Date Thu, 31 Mar 2011 09:35:05 GMT


Daniel Spicar commented on CLEREZZA-460:

I investigated the problem of duplicated renderlet registration today. The problem does not
lie in the fact of duplicates primarily. The major problem is that the number of registered
renderlets can grow very fast when bundles/services are started stopped (a refresh in the
OSGi framework may call the activate methods of many other bundles). 

I am not sure which rendering specification is chosen when duplicates exist. But this may
also be problematic.

When implementing TypeRenderlets there the OSGi framework takes care of service registration
and unregistration properly. The problem appears when either the RenderletManagerImpl or the
ScalaServerPageService register Renderlets (upon request from another bundle). Then the Renderlets
are registered using the bundleContext of the RenderletManagerImpl/ScalaServerPageService,
and thus they are not unregistered when the bundle that called the registerScalaServerPage/registerRenderlet
method disappears The corresponding Renderlets would only be unregistered when the typerendering.core/typerendering.scalaserpages
bundle disappears.

One way of solving this is to check all registered renderlets to prevent duplication (semantics,
add if no equal renderlet exists (what exactly is equal?), update with new specification when
an equal renderlet exists.). But I think this defeats the purpose of changing to the whiteboard
pattern, at least for Renderlets registered through the named services.

Another approach is to register the renderlets using the bundlecontext of the bundle that
called registerrRenderlet/registerScalaServerPage. I did not test it but I expect that then
the registered renderlets are unregistered when their bundle disappears. I think this requires
a change of the Interface to get that bundlecontext. This interface change is not very sensible
for the deprecated, old style RenderletManager. Also this will not really prevent duplicates,
but will make sure that each Renderlet registered in a bundle will be unregistered when that
bundle disappears. However another bundle may register the exact same renderlet again, then
duplicates would exist. But this seems to be less of a problem than uncontrolled growth of
registered renderlets that may occur currently.

Anyway. For now I suggest a quick fix in RenderletManagerImpl that checks all already registered
renderlets before registering new renderlets.

> Use whiteboard pattern for Renderlets
> -------------------------------------
>                 Key: CLEREZZA-460
>                 URL:
>             Project: Clerezza
>          Issue Type: Improvement
>            Reporter: Reto Bachmann-Gmür
>            Assignee: Reto Bachmann-Gmür
>         Attachments: patch-typerendering-core.diff
> The current mechanism for renderlets is overly complex. Instead of describing renderlets
in a graph rendelets should be services that have properties indicating the rdf type and the
mode-pattern they handle.
> The priority for selecting renderlets should be:
> - type, according to type-priority list
> - length of regex math (the matching regex with the most alphanumeric characters wins)
> - startlevel of the bundle providing the service

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