Re: backend overlays

[Pierangelo Masarati <ando@sys-net.it>]

Howard Chu wrote:

> Pierangelo Masarati wrote:
>
> > If this is completely off-topic, please disregard.
> >  
> Let me try again... The proxycache overlay sits on top of (e.g.) 
> back-ldap. It also creates an instance of back-bdb for its local 
> storage. This instance of back-bdb is totally private to the 
> proxycache overlay, it is completely invisible to the rest of slapd so 
> you can't configure anything special on top of it. The first question 
> in my mind was "would you ever need to put another overlay on top of 
> this private back-bdb?"
>
> Obviously one way to do so would be to add config mechanisms to 
> proxycache so that its private database can be explicitly configured.
>
> The other thought that occurred to me was to declare overlays in the 
> backend-specific config directives. Then whenever a backend of that 
> type is used, it has these other overlays already included. The 
> problem of course is that if the overlay needs any suffix-dependent 
> configuration, there's no way to automate that.

I see another problem: in this case, all instances of that backend would 
get that certain stack of overlays, which might be undesirable.  I'd 
consider providing a mechanism (e.g. a sort of generalization of 
backover.c) that allows those overlays that use internal databases and 
are willing to allow overlay stacking to inject a slap_overinfo 
structure in place of the original BackendInfo; then, something like

proxycache-overlay syncprov
proxycache-syncprov-sessionlog 100

could be used to access the configuration of that overlay.  By using a 
hierarchical configuration directive pattern we should be able to 
propagate this to an arbitrary number of layers.  The directives would 
look quite odd, but at least we don't need to change the style of the 
configuration; of course, nested parentheses would be a big plus:

database ldap
overlay proxycache {
   proxycache hdb ...
   overlay syncprov {
       sessionlog 100
   }
}

p.




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