Component Lifecycleλ︎
Clojure has several library to manage the lifecycle of components that make up the application, especially those components with state. Components can be started and stopped in a specific order.
mount integrant component system
Reloade Workflowλ︎
Set up a project to work with component lifecycle service, for example by adding components to the dev.clj file under a dev directory.
To load the service code automatically, add require and in-ns expressions to dev/user.clj and use the -M:env/dev alias when starting the REPL from Spacemacs.
Example project with component lifecycleλ︎
Using mount, its common to define a dev.clj file with go, stop and restart functions that manage the lifecycle of mount components. A start function contains the list of components with optional state.
(defn start []
(with-logging-status)
(mount/start #'app.conf/environment
#'app.db/connection
#'app.www/business-app
#'app.service/nrepl))
The go function calls start and marks all components as ready.
The stop function stops all components, removing all non-persistent state.
The reset function that calls stop, refreshes the namespaces so that stale definitions are removed and starts all components (loading in any new code).
Define a system which contains one or more component definitions.
A component definition may include references to other components and signal handlers that specify behavior.
Example: a :printer and :stack as service components. When the system receives the :donut.system/start signal, the :printer starts to process the data on the :stack
(ns donut.examples.printer
(:require [donut.system :as system]))
(def printer-system
{::system/defs
{:services
{:stack #::system{:start (fn [{{:keys [items]} ::system/config}]
(atom (vec (range items))))
:stop (fn [{::system/keys[instance]}]
(reset! instance []))
:config {:items 10}}}
:app
{:printer #::system{:start (fn [{{:keys [stack]} ::system/config}]
(doto (Thread.
(fn [] (prn "peek:" (peek @stack))
(swap! stack pop)
(Thread/sleep 1000)
(recur)))
(.start)))
:stop (fn [{::system/keys [instance]}]
(.interrupt instance))
:config {:stack (system/ref [:services :stack])}}}}})
;; start the system, let it run for 5 seconsystem, then stop it
(comment
(let [system-in-flight (system/signal printer-system ::system/start)]
(Thread/sleep 5000)
(system/signal system-in-flight ::system/stop)))
Lifecycle with cider-refreshλ︎
Create a .dir-locals.el file that calls the relevant mount functions when using cider-refresh.
Space p e to open a .dirs.locals.el in the current project, creating the file if it does not exist.
During development call the reset function to stop components, clean the namespace and start all components again.
Space f s to save the file. Refresh a buffer from the project or open a new file to trigger the reading of the .dir-locals.el configuration by Emacs.
Alternatively, if a namespace refresh is not required, configure the .dirs-local.el file to call stop then start.
((clojure-mode . ((cider-refresh-before-fn . "practicalli.dev/stop")
(cider-refresh-after-fn . "practicalli.dev/start"))))
This code calls the stop function from the component lifecycle library at the start of the cider-refresh function. At the end of cider-refresh, the start function is called to restart all the components in the defined order in the project.
Prevent cider-ns-refresh calling component lifecycle functions
Ctrl+- , e n or Space u - 1 , en calls cider-ns-refresh but prevents the refresh functions defined in cider-ns-refresh-before-fn and cider-ns-refresh-after-fn from being invoked.