top level "Component" functions

1. raffle.clj

(ns raffle
  (:require [raffle.system :refer [find-by-id]]
            [clojure.tools.logging :refer [info]]))

(defn raffle-it []
  (let [id (rand-int 42)]
    (info "and the winner is... " 
          (find-by-id id))))

2. database.clj

(ns raffle.database
  (:require [com.stuartsierra.component :as component]
            [clojure.tools.logging :refer [info]]))

(defrecord Database [conf]
  component/Lifecycle

  (start [this]
    ;; create db, connection, etc..
    (assoc this :find-by-id (fn [id] 
                              (info "database: looking by id.. [" id "]")
                              "Jimi Hendrix")))

  (stop [this]
    (dissoc this :db)))

(defn new-database [conf]
   (map->Database {:conf conf}))

3. system.clj

(ns raffle.system
  (:require  [com.stuartsierra.component :as component]
             [raffle.database :refer [new-database]]))

(defn raffle-system []
  (let [conf {:db {:foo 42 :bar 34}}]
    (component/system-map
      :database (new-database conf))))

(def system nil)

;; defined in other, more related namespace
(defn find-by-id [id]
  ((-> system :database :find-by-id) id))

(defn rock-n-roll! []
  (alter-var-root #'system (constantly (raffle-system)))
  (alter-var-root #'system component/start))

4. repl.clj

user=> (use 'raffle 'raffle.system)
nil
user=> (rock-n-roll!)
#<SystemMap>
user=> (raffle-it)
21:21:38.290 [nREPL-worker-4] INFO  raffle.database - database: looking by id.. [ 1 ]
21:21:38.290 [nREPL-worker-4] INFO  raffle - and the winner is...  Jimi Hendrix