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

[Brian Stansberry]

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1) The in-memory model no longer represents the user configuration 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?

-- 
Brian Stansberry
Principal Software Engineer
JBoss by Red Hat