Wrench

Wrench is a library to manage your clojure app's configuration. It is designed with specific goals in mind:

• All values are available during initialization of your code
• That means you can use it in your defs (and def-like macros, like defroutes)
• All values come from environment variables, as 12 factors menifesto recommends
• Each configuration could be accompanied with a custom spec
• One can ensure that whole config matches provided specs during runtime
• Configuration values are coerced to their spec from string and edn (enables values like [8080 8888])
• Definition and usage of each key are easily traceable, since they are simple vars

In addition to environment variables, for local development, wrench reads from dev-config.edn.

Installation

Add [wrench "0.3.0"] to the dependency section in your project.clj file.

Wrench requires 1.9 version of Clojure.

Usage

Simplest way to use it is to define a config with a simple def. For instance, if you want to read environment variable USER you would do following:

(require '[wrench.core :as cfg])
(cfg/def user)

You can also customize name of the variable and provide specification:

(cfg/def port {:name "HTTP_PORT"
               :spec int?})

In this case loaded value would be coerced to an int.

There are plenty of other options:

• :doc will be symbol's documentation
• :spec spec-compatible (including any predicate) to validate the value, defaults to string?
• :name name of the environment variable, defaults to uppercased name of the var (ignoring namespace) with dashes replaced with underscores
• :require fails validation, if value is missing, default is false
• :default to provide a fallback value if it is missing, default is nil
• :secret to hide value from *out* during validation, default is false

(cfg/def oauth-secret {:doc    "OAuth secret to validate token"
                       :require true
                       :secret  true})

(cfg/def host {:doc "Remote host for a dependency service"
               :name "SERVICE_NAME_HOST"
               :require true})

Then use those vars as you would use any other constant, i.e.:

(cfg/def port {:name "NREPL_PORT"
               :spec int?
               :default 7888})

(mount/defstate nrepl-server
  :start (nrepl-server/start-server :port port)
  :stop (nrepl-server/stop-server nrepl-server))

If a value does not pass validation, ::cfg/invalid will be used.

To ensure you have everything configured properly, validate your config before app starts:

(defn -main [& args]
  (println "Starting service!")
  (if-not (cfg/validate-and-print)
    (System/exit 1)))

If everything is alright, then configuration will be printed to *out*, replacing values marked as :secret with <SECRET>:

Loaded config:
-  #'some.service/port 8080
-  #'some.service/token <SECRET>

If there were errors during validation or required keys are missing, then aggregated summary will be printed and false returned:

Failed to load config:
- configuration #'some.service/token is required and is missing
- configuration #'some.service/port present, but does not conform spec: something-wrong

Testing

Idiomatic with-redefs could be used to alter var's value:

;; service.clj
(ns some.service
  (:require [wrench.core :as cfg]))
  
(cfg/def user {:default "Rich"}) 

;; service-test.clj
(ns some.service-test
  (:require [clojure.test :refer :all]
            [some.service :as svc]))
 
(deftest a-test
  (testing "Let's talk about javascript"
    (with-redefs [svc/user "David"]
      (is (= svc/user "David")))))

You can also permanently alter config, by providing alternative values to vars:

(cfg/reset! :var {#'svc/user "David"})

or environment variables

(cfg/reset! :env {"PORTS" "[8080 8081 8082]"})
; or load from file
(cfg/reset! :env (cfg/from-file "dev-config.edn"))
;; or in combination
(cfg/reset! :env (from-file "dev-config.edn")
            :var {#'ports [8080 8081]})

Those changes will be applied to global scope, your experience may vary depending on your test runner.

REPL and reloaded workflow

If during REPL development you ever need whole configuration map, it is available using:

(cfg/config)

Note, that wrench relies on default reloading mechanics, maning that changes in environment variables or in external config file would not trigger reloading variables with (clojure.tools.namespace.repl/refresh).

To mitigate this you could either use refresh-all or reload config manually before your system starts:

(defn reset "reloads modified source files, and restarts the states" []
  (stop)
  (cfg/reset! :env (cfg/from-file "dev-config.edn"))
  (ns-tools/refresh :after 'user/go))

License

Copyright © 2018 Anton Chebotaev

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.