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Re: backend overlays


I'm not sure I compltely understand your point; however, if you're talking
about the need to explicitly enable overlays rather than have them
automatically instantiated by the ocurrence of their configure directives,
well, this is required for dynamically loaded or custom overlays, unless
we provide a means to register their specific configuration directivs in
the frontend config hamdler.  I initially suggested, for this purpose, to
prefix all overlay directives with the name of the overlay (e.g.
proxycache-* for the pcache overlay), so the overlay might simply register
its bi_db_config handler with the prefix and have it invoked for all the
related directives.

One issue that comes to mind is that, as happened to me with the "rwm"
overlay in back-ldap, automatic and explicit overlay instantiation may
conflict.  For the purpose, I recently added code to backover.c
(over_db_config(), ITS#3395) to detect if an overlay has been instantiated
inside a call to bi_db_config, and to check for multiple instantiations of
the same overlay.  In fact, typically multiple instantiations of an
overlay are unnecessary, but in principle they should be allowed, and
overlays should specify if they have to be unique or not (no clear
examples come to my mind, but I'm vguely thinking of a general-purpose
overlay that can be configured to do a single specific tsk, so if one
needs more tasks of the same category this can be obtained by
instantiating multiple generic overlays, each one doing one of the
specific tasks).

If this is completely off-topic, please disregard.


> Every now and then I look at things like the proxycache overlay with its
> internal database, and wonder if there would ever be a need to put an
> overlay on that database. I suppose one way to handle that would be to
> add explicit proxycache directives for configuring such a feature.
> Another thought that came to mind was being able to declare overlays
> inside a "backend" section, such that these overlays would automatically
> be instantiated whenever a particular type of backend is used. Comments?
> --
>   -- Howard Chu
>   Chief Architect, Symas Corp.       Director, Highland Sun
>   http://www.symas.com               http://highlandsun.com/hyc
>   Symas: Premier OpenSource Development and Support

Pierangelo Masarati

    SysNet - via Dossi,8 27100 Pavia Tel: +390382573859 Fax: +390382476497