[jboss-as7-dev] Default values in the model

[David Bosschaert]

On 29/07/2011 01:31, Brian Stansberry wrote:
> Emanuel,
>
> Or chat yesterday about the downsides of storing default values in the
> in-memory management model got me thinking a bit.
>
> Downsides:
>
> 1) The in-memory model no longer represents the user configuration
> values; it also stores defaults. This is an issue when marshalling back
> to xml; you either have to make the xml verbose by writing back all the
> defaults, or you don't write back defaults and lose the fact the user
> included the value in the original.
>
> 2) Possible versioning issues in a domain where different hosts run
> different versions; i.e. if the version on the domain controller host
> stores and pushes to servers a default value that a server running an
> earlier version doesn't understand.  This would really be a bug, since a
> change in a default value should spur an increment in the subsystem's
> schema and appropriate handling by the parser. But still, storing
> defaults increases the chances of this kind of bug cropping up.
>
> Possible solution:
>
> What's been requested previously is that management clients be able to
> see the default values, without having to go read the resource
> description metadata to find them. That request can be satisfied via the
> read-resource and read-attribute handlers. There is no need to store the
> default in the model; those handlers can read the resource description
> metadata to find the default if the model doesn't have a value stored.
>
> That will slow those handlers down a little bit since the description
> has to be generated and read, but those handlers aren't called as part
> of our internal operations (e.g. boot) so doing it that way won't slow
> down boot, deployment etc.
>
> An issue that doing it this way raises is a client calling read-resource
> or read-attribute doesn't know if the returned value is the actual user
> config or is a default. A solution to that is to have those operations
> accept a new parameter "defaults" which if false does not include the
> defaults.
>
> I'm not sure what the best default value for that "default" param would
> be. My instinct is "true".
>
> WDYT?
>

I personally wouldn't do this. I would leave the default out of the 
read-resource etc result. That would keep things clear. When you obtain 
the model you'll see that the value isn't specified. A management client 
can easily also obtain the default from the description and nicely 
present this to the user. The management client may actually want to 
mark the default value as such for the users in a slightly different way 
so having it come in somewhat separate can actually be helpful I think.

My 2c,

David