donut-system-merging.clj

;; The question: how to clearly provide system def overrides?
;; There are two main scenarios where we want to do this:
;;
;; 1. Deriving a new "profile", e.g. deriving a test profile from a base profile
;;    to be used for all tests
;; 2. Defining point overrides, e.g. you might want to mock out a specific component
;;    only for a specific test
;;
;; Ultimately developers can use what works for them, but I want to figure out
;; the recommended approach. Also, I need to figure out the behavior of helpers
;; like `start` where you can specify a system along with overrides:

(ds/start :system-name {:overrides {:go :here}})

;; the meta-merge approach
;; https://github.com/weavejester/meta-merge
;;
;; meta-merge's default behavior is to recursively merge maps, and recursively
;; concatenate vectors
(meta-merge/meta-merge {:name    {:middle "bobby"}
                        :hobbies ["photography"]}
                       {:name    {:first "robert", :last "foobar"}
                        :hobbies ["writing"]})
;; =>
{:name    {:first  "robert"
           :middle "bobby"
           :last   "foobar"}
 :hobbies ["photography" "writing"]}


;; Here's how that can work with combining system defs:
(def base-system
  {::ds/defs
   {:group-name
    {:component-name {:start (fn [_ _ _])
                      :conf  {:port (ds/ref [:env :http-port])}}}}})

(def test-system
  (meta-merge/meta-merge
   base-system
   {::ds/defs
    {:group-name
     {:component-name {:conf {:port 9090}}}}}))

;; I like that for the most part you can construct minimal structures to specify
;; overrides. However, I still continually find myself creating little bugs
;; because I actually need *replace* behavior and I forget to add the ^:replace
;; metadata.

;; Heres another option:
(def test-system
  (update base-ssytem ::ds/defs ds/assoc-many
          {[:group-name :component-name :conf :port] 9090}))

;; this approach could be repetitive but I think the behavior would be unambiguous