Make it possible to start a subsystem? about component HOT 4 CLOSED

Hi! Thanks for your interest in Component.

I have experimented with sub-systems or partially-started systems in the past (that was the original idea behind the component-keys argument to start-system) and found that it doesn't work well in practice. You get confusing behavior if different components are "started" at different times. This is a limitation of the design of the Component library, and I have never found a satisfactory solution.

Instead, since Systems are basically just maps, I recommend creating partial systems: You can assoc, dissoc and merge systems of components to create a system that contains just the components you want, then start that.

Best wishes for your componentizing!

from component.

Hi, thank you! I understand.

Thereal issue is getting the correct subset of the system map, including all transitive dependencies. It would be helpful if Component had a helper function for that...

from component.

getting the correct subset of the system map, including all transitive dependencies.

You can! All the pieces are there. It would look something like this:

(require '[com.stuartsierra.component :as component]
         '[com.stuartsierra.dependency :as dep])

(defn subsystem [system subsystem-keys]
  (let [graph (component/dependency-graph system (keys system))
        selected (into (set subsystem-keys)
                       (mapcat #(dep/transitive-dependencies graph %))
                       subsystem-keys)]
    (merge (component/system-map)
           (select-keys system selected))))

Note: this is completely untested! 😀

from component.

Nice, thank you! I did not know about dep/transitive-dependencies

from component.