[katello-devel] Custom callbacks for optional backend services

[Justin Sherrill]

While we are working on the pulpv2 stuff, we are breaking apart our 
models a bit more and trying separate duties out more.  As part of this 
we should expect models to be able to operate on their own when backend 
services are disabled (even if they dont' do a whole lot).

One such example where this breaks is in Systems and System Groups:


https://github.com/jlsherrill/katello/blob/pulpv2/src/app/models/system_group.rb#L24

You can see here that we've broken up System even more, splitting out 
the elastic search code and authorization (in addition to the glue 
modules).  That specific line though will not function properly without 
Pulp and Elastic search being present.  You will get an error trying to 
add a system to a system group because "add_pulp_consumer_group" will 
have not been defined.  Since SAM is talking about adding system this 
will become more of a problem.

An interesting solution would be to use custom callbacks:

http://www.intridea.com/blog/2012/3/22/define-custom-callbacks-for-activerecord-and-more#

So we could define our own events that get triggered (for example 
'before_system_group_added')  and pulp glue layer could simply define a 
trigger on that callback.  In the SAM case that trigger'd method would 
never be defined, so it could go on its way happily.

Thoughts?

-Justin