The document discusses different approaches for managing application lifecycles and dependencies in Clojure applications. It presents atoms, maps, and components as options, evaluating each approach. Components are determined to be the best option, as they provide lifecycle management capabilities, allow dependencies to be defined, and keep application logic together and reload-friendly, while still maintaining pure functions. Some caveats are mentioned, such as components being invasive and records sometimes being weird to work with.

1. Components!
Taming Clojure applications
David Dossot

2. Every application, ever*
● Subsystem lifecycle
● Long running processes
● Connections pools
● Caches
* almost

3. The Clojure Curse*
* http://www.winestockwebdesign.com/Essays/Lisp_Curse.html
● Clojure can do anything
● No official app framework
● NIH is a risk
● We went there...

4. Take #1 ~ Atoms ?
(def ^:dynamic server (atom nil))
(defn start [config]
(reset! server
(make-server config)))
(defn stop []
(.stop @server))

5. Take #1 ~ Atoms ✘
● Easy :)
● Shared state :(
● All over the place :(
● Reload unfriendly :(
● No dependencies :(

6. Take #2 ~ Maps ?
(defn start [execution-context config]
(assoc execution-context
:server (make-server config)))
(defn stop [execution-context]
(.stop (:server execution-context))
(dissoc execution-context :server))

7. Take #2 ~ Maps ✘
● Pure functions :)
● Reload friendly :)
● All in one place :)
● Ad-hoc lifecycle :(
● No dependencies :(

8. Take #3 ~ Components ?
(defrecord Server [server config]
component/Lifecycle
(start [this]
(if server ; already started
this
(assoc this :server (make-server config))))
(stop [this]
(if-not server ; already stopped
this
(do
(.stop server)
(assoc this :server nil)))))

9. Take #3 ~ Components ✔
● Pure functions :)
● Reload friendly :)
● All in one place :)
● Lifecycle API :)
● Dependencies :)

10. Caveat emptor
● Invasive
● All fns that reify the module's behaviour
●
Wish clj had Erlang's parameterized modules*
● Records can be weird
● lein clean is your friend
* experimental feature, now removed :(

11. Resources
https://github.com/stuartsierra/component
https://youtu.be/13cmHf_kt-Q