system owes to component both in spirit and in name. The component library helps implement the reloaded pattern as championed by Stuart Sierra. system is built on top of it, offering a set of readymade components. The idea is to expand the set over time with contributions from the community. It currently includes:

• Jetty (HTTP server)
• HTTP kit (Async HTTP server)
• aleph (Async HTTP server)
• Datomic (Immutable database)
• H2 (H2 relational database)
• Monger (MongoDB client)
• Sente (Websockets/Ajax communications library)
• nREPL (Clojure network REPL )
• Langohr (RabbitMQ client)
• Etsy (Etsy client)
• hara.io.watch (File watcher)

A good REPL experience is a prerogative of Lisp languages. Reloaded components enable this goodness in Clojureland. Since they require an amount of setup, the first steps when starting a new project are generally devoted to infrastructure. My first attempt to tackle the boilerplate was a Leiningen template. The problem is that Leiningen templates are hard to maintain and difficult to retrofit on existing projects. I was finding myself repeatedly updating the template for future use. Then it dawned on me that a library would better fit the bill. And so system came to light. It’s now the first dependency I add to any project, allowing me to focus from the get-go on the substance of my application.

Yes.

Add the following to the Leiningen dependencies in project.clj.

An example project is available under the example folder. It is a self-contained project that will help you get started.

First, assemble your system(s) in a namespace of your choosing. Here we define two systems, development and production.

(ns my-app.systems
  (:require 
   [com.stuartsierra.component :as component]
   (system.components 
    [jetty :refer [new-web-server]]
    [repl-server :refer [new-repl-server]]
    [datomic :refer [new-datomic-db]]
    [mongo :refer [new-mongo-db]])
   [my-app.webapp :refer [handler]]
   [environ.core :refer [env]]))

(defn dev-system []
  (component/system-map
   :datomic-db (new-datomic-db (env :db-url))
   :mongo-db (new-mongo-db)
   :web (new-web-server (env :http-port) handler)))

(defn prod-system []
  "Assembles and returns components for an application in production"
  []
    (component/system-map
     :datomic-db (new-datomic-db (env :db-url))
     :mongo-db (new-mongo-db (env :mongo-url))
     :web (new-web-server (env :http-port) handler)
     :repl-server (new-repl-server (env :repl-port))))

Then, in user.clj:

(ns user
  (:require [reloaded.repl :refer [system init start stop go reset]]
            [my-app.systems :refer [dev-system]]))

(reloaded.repl/set-init! dev-system)

You can then manipulate the system in the REPL: (go), (reset) or (stop). The system map is accessible at any time, it resides in a var named, as you can guess, #'system.

In production, in core.clj:

(ns my-app.core
  (:gen-class)
  (:require [my-app.systems :refer [prod-system]]))

(defn -main 
  []
  "Start the application"
  (alter-var-root #'prod-system component/start)

Or, if you want to keep a handler on your system in production:

(ns my-app.core
  (:gen-class)
  (:require [reloaded.repl :refer [system init start stop go reset]]
            [my-app.systems :refer [prod-system]]))

(defn -main 
  []
  "Start the application"
  (reloaded.repl/set-init! prod-system)
  (go))

A convenience macro, defsystem, allows you to declare systems succinctly:

(ns my-app.systems
  (:require 
   [system.core :refer [defsystem]]
   (system.components 
    [jetty :refer [new-web-server]]
    [repl-server :refer [new-repl-server]]
    [datomic :refer [new-datomic-db]]
    [mongo :refer [new-mongo-db]])
   [my-app.webapp :refer [handler]]
   [environ.core :refer [env]]))

(defsystem dev-system 
  [:datomic-db (new-datomic-db (env :db-url))
   :mongo-db (new-mongo-db)
   :web (new-web-server (env :http-port) handler)])

(defsystem prod-system 
  [:datomic-db (new-datomic-db (env :db-url))
   :mongo-db (new-mongo-db (env :mongo-url))
   :web (new-web-server (env :http-port) handler)
   :repl-server (new-repl-server (env :repl-port))])

Note: Component allows you to define dependency relationships within systems. Please don’t use said macro for those cases. Be sure to consult component’s API to see the range of options available to you.

At runtime, the system var can be used anywhere after requiring it from the reloaded.repl namespace:

(ns front-end.webapp.handler
 (:require [reloaded.repl :refer [system]]))

For a discussion around a specifice example (i.e. Sente and http-kit), please look here.

Here are a couple of links that are sure to shed more light on the motivations of the reloaded workflow.

The canonical reference: My Clojure Workflow, Reloaded

And more references touching on the topic.

• Clojure in the Large
• Retrofitting the Reloaded pattern into Clojure projects
• 5 faces of dependency injection in Clojure
• REPL functions to support the reloaded workflow

Please fork and issue a pull request to add more components. Please don’t forget to include tests. You can refer to the existing ones to get started.

I wish to thank Stuart Sierra for the pioneering and guidance. Special thanks to James Reeves for the reloaded.repl library and general inspiration. Thanks to Peter Taoussanis, the friendly OSS contributor, who helped to ‘componentize’ sente, an amazing library on its own right.