system.clj

(defn routes
  "Returns a web handler function"
  [db kafka]
  ,,,)

;; example HttpServer resource which implements the essential lifecycle methods
(defrecord HttpServer [port handler server]
  ResourceLifecycle
  (start [this]
    (assoc this :server (http-server-start! port handler))
  (stop [this]
    (http-server-stop! server)))
  

;; api module takes a db, kafka and config values and starts an http server
;; the http server will restart whenever any of the arguments (port or handler)
;; change
(defmodule api
  [{:keys [db kafka config]}]
  (http-server {:port (:http/port config)
                       :handler (routes db kafka)})
  nil)

;; Modules are boundaries of encapsulation for resources.
;; If an exception is thrown while starting a resource, it will stop all resources
;; used within the module and its children.
;; A parent can choose to catch the exception at the component boundary and handle it
;; it in some way.

;; service is the root module. reads in config, starts the db and kafka connections.
;; if config changes, will restart db and kafka connections and 
(defmodule service
  [config]
  (let [db (database {:host (:db/host config) :port (:db/port config)})
        kafka (kafka (:kafka/opts config))]
    ;; call api as a child module. we do not handle errors, so service will stop
    ;; if an error occurs while starting any resources in the api module
    (api {:db db :kafka kafka})))

;; this a var to hold the current state of our system
(def sys (system))

;; call this every time you make changes that require restarting any part of the global system
(load! sys (service (read-config!)))

;; tear down the system completely
(unload! sys)

;; multiple systems at once
(def sys1 (system))
(def sys2 (system))

(load! sys1 (service ,,,))
(load! sys2 (service ,,,))