If you've never read the Twelve Factor App methodology I strongly recommend you do, it's a great guide born from real life experience of building many, many web applications.

One of the rules in the Twelve Factor App methodology is that you "Store config in the environment" or, more specifically, store config that is likely to change across environments in the environment. It makes sense as it ties the target environment and its necessary configuration together instead of having to juggle if ENV=test|dev style flags around your code base or swap out different config files per build.

One other positive side effect of this approach is that secure configuration doesn't accidentally get checked into source. Every one needs database credentials or client secrets but no-one want to share them with the world.

Supporting this approach in Clojure is fairly simple and there are a few ways to do it.

System/getenv

Clojure sits on the JVM and the JVM has the System namespace which provides utilities and classes for accessing the running systems features. System.getenv and System.getenv(String) give us access to environment variables from Clojure.

We can also provide a fallback for environment variables that don't exist

( or ( System/getenv "NOEXIST_DATABASE_URL" ) "http://localhost/mydb" ) ;=> http://localhost/mydb

In this case NOEXIST_DATABASE_URL isn't set as an environment variable so we fallback to our hardcoded version.

This isn't uncommon and often its done to support development environments. For example Heroku (where the original 12 factor app methodology arose) add environment variables for any addons you may activate such as DATABASE_URL for the Postgres DB addon. So rather than write different handlers per environment many people would either.

Use the fallback URL to represent their development environment Add the environment variable to their shell

Both of the approaches have problems. Using a fallback URL can lead to problems where an environment isn't correctly configured and starts using the wrong database. If it can write to it there will be trouble, if it can't you'll be scratching your head wondering whats going wrong for longer then necessary. Setting a project specific environment variable in your shell can lead to pain when things get overwritten across projects and pollution if you're setting it in an automated fashion ( .bashrc ) or you'll simply forget to set it per use when doing it manually.

weavejester/environ

A neater solution is available in the form of environ - a simple library for managing environment variables. The clever thing environ does is merge environment variables from multiple sources into a single map.

So by adding it to our project.clj

[ environ "0.5.0" ]

We can import and use it where we need,

( require [ environ.core :refer [ env ]]) ( env :database-url ) ;=> http://localhost/mydb

So how are the environment variables sourced? Well environ looks in a number of areas in order,

A .lein-env file in the project directory Environment variables Java system properties

It also keywordises the variable names as you can see from the example above.

Great, so why is this more useful? The big benefit I've discovered is the .lein-env file in the root of my project. This file holds a map of environment values that can be useful during development. There are 2 ways to create this file.

The RIGHT way as set out by the creator of the project, and, The WRONG way as in how I use it

environ also has a plugin available lein-environ which sucks profile specific settings from ~/.lein/profiles.clj and/or a project specific profiles.clj (which should not be checked into source) and creates the .lein-env file when Leiningen does its thing (effectively making .lein-env a transient file). A .lein-env file looks a little something like this.

{ :env { :database-url "http://localhost/mydb" :client-token "QWERTY12345" } }

Caveat: Now you can create this manually (this is what I have done for a small project I'm working on) but it's not officially recommended for a few reasons. Firstly the minute you add the lein-environ plugin to the project your important setting will get wiped out and secondly you lose out on the ability to vary setting across profiles as well that you'd get from profiles.clj .

Big thanks to @weavejester the creator of environ for the tip off about my dubious use of .lein-env

So back to .lein-env - You end up with the .lein-env file on your machine with development (or profile) specific values and you forget about fallbacks and setting environment variables and the potential conflicts with other projects. Then on your production and test systems you can source values from actual environment variables without having to change your strategy or use (if (= "test" (:env config))) style checks.