Episode 011: The Convention of Configuration

Nate is worried about the hardcoded credentials in the code.

• It's episode 011 on 01/11. It's binary!
• "As a developer, you quickly learn what's under your control and what's not. Very little is under your control."
• Don't accidentally check in the credentials.
• "We need a configuration management system. Oh wait! That's a totally different problem."
• What about putting the configuration into an EDN file?
• Let's call it config.edn
• Why EDN? Isn't JSON the "one, true format for config information"?
• Pros of EDN:
  • native to Clojure
  • can have comments
  • is extensible
• Why not environment variables?
• Environment variables are great for production, but in development files are better.
• Have to restart the REPL to change env variables.
• Make a component that reads the config. That will reload config when you reset with component.repl.
• Two main considerations:
  1. Dynamically reloading configuration during development
  2. Plumbing the configuration values through the app
• Make a namespace for app.config
• "Files are way more fungible than the environment."
• We want both options: env for production and a file for dev.
• Make two functions in app.config: from-env and from-file.
• Use schema to make sure both functions return the same config map.
• "A thing I have done before...you decide if it's clever."
• Define a default configuration and then merge the config maps with that.
• Better yet, from-env handles defaults and we merge the map from dev.edn into that.
• Can use environ with Leiningen profiles. Still requires restarting the REPL.
• Defaults should make sense for development, so you can just check out and run.
• For bad config, we want helpful error messages, not stack traces.
• What goes in configuration?
  • Items under your control
  • Items that vary
• Twitter API URL does not vary.
• Even if Twitter provided sandbox URLs, those URLs don't vary. The config option should be "production" and "sandbox", not the URL. The wrapper code will select the right URL.
• Avoid the nonsense of trying to infer "sandbox" from reading the URL.
• "The hallmark of good design is: the less thinking you have to do, the better."
• "The more semantic the config, the less thinking you have to do."
• It's important to think about the data first, where it comes from, and where it goes.

Clojure in this episode:

• merge
• try, catch
• slurp
• clojure.edn/read-string
• environ.core/env
• component.repl/reset

Related projects:

• Component
• component.repl
• Environ
• Leiningen, profile support
• Schema

Code sample from this episode:

(ns app.config
  (:require
    [clojure.edn :as edn]
    [environ.core :refer [env]]))

(defn from-env []
  {:twitter-key (or (env :twitter-key) "")
   :twitter-secret (or (env :twitter-secret) "")
   :initial-tweets (or (env :initial-tweets) 20))

(defn config []
  (merge (from-env)
         (edn/read-string (try (slurp "dev.edn") (catch Throwable e "{}")))))