my.namespace.clj

(ns my.namespace
  (:require
    [com.stuartsierra.component :as component]))
  
(defrecord ExampleComponent [options foo]
  component/Lifecycle

  (start [this]
    (println ";; Starting ExampleComponent")
    ;; In the 'start' method, a component may assume that its
    ;; dependencies are available and have already been started.
    (assoc this :foo "foo"))

  (stop [this]
    (println ";; Stopping ExampleComponent")
    ;; Likewise, in the 'stop' method, a component may assume that its
    ;; dependencies will not be stopped until AFTER it is stopped.
    (dissoc this :foo)))

(defn example-component [config-options]
  (map->ExampleComponent {:options config-options}))

(defn example-system [config-options]
  (component/system-map
   :app (component/using
         (example-component config-options)
         {})))

repl_ex.clj

> (require '[com.stuartsierra.component :as component] '[my.namespace :as n] :reload)
> (def system (web-comp/example-system {}))
> (:app (component/start system))
#de.web.component.ExampleComponent{:options {}, :foo nil} ; no println, and map is not altered?
>