[[chef-dev]] Resources, Providers and Runners, oh my - A tale of concerns

[Mathias Meyer < >]

Hi all,

I'm trying to wrap my head around something, and I think I'm a bit
stuck. While working on CHEF-606 [1] (which was a direct result of
trying to fix CHEF-605 [2]) I ran across something I can only describe
as mixed concerns.

Chef::Runner has a similar functionality as Chef::Resource, they both
have a run_action method, but other than I expected, Runner#run_action
doesn't invoke Resource#run_action, it pretty much duplicates its
functionality. The difference seems to be that when the Runner builds
a provider instance it hands in some of its own instance variables
(@collection, @definitions, @cookbook_loader) which the Resource knows
nothing about, but Providers do. The Resource on the other hand only
builds Providers without references to these, and its run_action
method seems only to be used when running Resources directly from
Recipes.

That's where the concerns start to fall apart for me. I'd argue that
the Runner shouldn't have to know anything about Providers, especially
when all the logic dealing with them is already implemented in the
Resource. But there's cases where the mentioned instance variables are
expected to be set. That code seems to be mostly dealing with
lightweight resources, as they are the only tests that failed. The
code in RecipeDefinitionDSLCore references them, as far as I could
tell it's used to create ad-hoc Recipes or something like that,
correct me if I'm wrong.

If I consider this a chain, where the Runner only knows about
Resources, and the Resources are the only spot where the code deals
with invoking Providers, there's a blind spot in the middle. Providers
need access to objects the Resource doesn't know anything about. The
question is if the Resource should know them, or if it should just
hand them through. We started to move some code around, and play with
some ideas, as can be seen in [3], but we're not happy with it. It
basically hands the required objects into the Resource where they're
just handed over to the Provider. The Resource though comes with its
own instance variable @collection which doesn't seem to have any
relation to the one the Provider refers to.

I'm not exactly sure how to proceed on this, so I'm asking for some
input, because maybe we missed something along the way, or maybe
someone has other ideas how that can be fixed.

For me, putting all the code that deals with running an action and
notifying subscribed actions belongs into the Resource, it just seems
to be the right place for it. But the fact that there's a mix of
concerns here makes me think there's a bigger issue here.

Let me know, what you think.

Thanks!

Cheers, Mathias
[1] https://tickets.opscode.com/browse/CHEF-606
[2] https://tickets.opscode.com/browse/CHEF-605
[3] http://github.com/peritor/chef/tree/CHEF-606-WIP
-- 
http://paperplanes.de ;| http://holgarific.net
http://twitter.com/roidrage