docs/dev/design_documents/resource_before_notifications.md
Let users trigger another resource prior to an action happening.
As a Chef user,
I want to make one resource action a precondition for another,
So that when two resources depend on each other, I can make an update succeed.
As a Chef user,
I want this action only to happen if the other resource *does* update,
So that I don't unnecessarily run my preconditions on every single Chef run.
As a Chef user,
I want Chef to run an action if a resource updates,
So that I don't have to implement the resource test logic in my recipe.
We add a new :before timing which causes a notification to happen
before the resource actually updates. If the resource will not actually update,
this event does not fire.
The events you can specify would become:
:before - before the resource updates, but only if an update will occur.:immediately - after the resource updates, but only if an update occurred.:delayed - after the resource updates, at the end of the run.package "foo" do
action :upgrade
# The package upgrade will fail if we try to upgrade while it runs!!!
notifies :stop, "service[blah]", :before
end
service "blah" do
end
This will work for both subscribes and notifies.
This will only affect resources, which have :before on them, and will
not modify any existing resources or recipes.
There is a tricky implementation detail here, because both the test part -- "is my package version lower than the latest?" -- and the set part -- "call rpm and update the package"-- of a resource are both part of the action. Chef only runs one action to run at a time, and it seems ill-advised to change that without a lot of extra thinking.
To get around this without breaking the model, we propose that resources with an
:before action run a why-run test of the action and trigger off of
that, before running the action for real. The flow of this resource:
package "foo" do
action :upgrade
notifies :stop, "service[blah]", :before
end
The execution of the package upgrade looks like this:
:before events are on the resource:
a. raise an error if the resource does not support why-run.
b. Turn on why-run temporarily.
c. Run the action.
d. Send the notification if updated_by_last_action? is true.
e. Turn off why-run.:immediate or :delayed notifications if updated_by_last_action? is true.