Clojurians Log v2

When you say "low level dependency changes might mean updating the system map of 20+ microservices" can you give an example of what you mean? Integrant has a few different tools for dealing with problems like these: inheritance in keywords, refsets and modules. But I think I need an example or some more information on the specifics before I can be sure.

Right now, for all 20+ of our services, we do something like this, except there are many more components/dependencies involved:

(-> (component/system-map :component-1 ( ... ) :component-2 ( ... ) ... )
    (component/system-using 
      {:component-1 [:component-2 :another-component]
       :component-2 [ ... ]}))

Ah, I see now. Yes, that's exactly the problem that modules are designed to solve. A module effectively defines a subsystem that is merged into the system map. For example, you might have code that looks like:

;; Setup inheritance so that we can use the more generic
;; :app.database/sql key
(derive :app.database.sql/postgres :app.database/sql)

;; Create an expansion that is merged into the system
(defmethod ig/expand-key :app.module/database [_key _config]
  {:app.database.sql/postgres
   {:queue (ig/ref :app.queue/simple)
    :cache (ig/ref :app.cache/redis)}
   :app.queue/simple {...}
   :app.queue/cache {...}})

;; In your configuration:
(def config
  {:app.handler {:db (ig/ref :app.database/sql)}
   :app.module/database {}})

(defn -main []
  (-> config
      (ig/expand) ;; expand out modules
      (ig/init)   ;; start system

Can you have multiple levels of subsystems? e.g. 3 subs, merge 1 onto 2, then the result onto 3?

And, if you can, I assume you don't hit key contention because unlike component, in integrant you can use fully qualified keys everywhere?

Expansions are not applied recursively by default, but you can create a module that is a superset of other modules. For example:

(defmethod ig/expand-key :app.module/abc [_ opts]
  (merge (ig/expand-key :app.module/a opts)
         (ig/expand-key :app.module/b opts)
         (ig/expand-key :app.module/c opts)))

You could also write a function that recursively called ig/expand until there are no more expansions (i.e. when the configuration no longer changes), but I didn't want that to be the default behavior of ig/expand, as I felt that could result in hard-to-debug issues.

Sounds great. I'm going to explore this. Thanks so much for the guidance.