dyn-edn

Often, especially in production, you don’t know all of the configuration until your application is actually started. For example, in a cloud provider, important IP addresses and port numbers are often assigned dynamically. This information is provided to the processes via environment variables.

This approach has been codified as part of the 12-Factor App manifesto.

However, in most Clojure projects, configuration is done primarily in terms of EDN format configuration files.

This library provides a number of reader macros that allow an EDN configuration file to reference data provided in environment variables, JVM system properties, or elsewhere.

API Documentation

Overview

dyn-edn introduces a little bit of indirection into the otherwise fixed content of an EDN file, in the form of reader macros.

The dynamic bits are properties:

• Shell environment variables

• JVM System properties

• Explicitly provided properties

The following reader macros are available:

#dyn/prop

Accesses dynamic properties. The value is either a key, or a vector of a key and a default value.

#dyn/join

Joins a number of values together to form a single string; this is used when building a single string from a mix of properties and static text.

#dyn/long

Converts a string to a long value. Typically used with #dyn/prop.

#dyn/boolean

Converts a string to a boolean value. Typically used with #dyn/prop.

#dyn/keyword

Converts a string to a keyword value. Typically used with #dyn/prop.

Here’s an example showing all the variants:

{:connection-pool
  {:user-name #dyn/prop [DB_USER "accountsuser"]
   :user-pw #dyn/prop DB_PW
   :url  #dyn/join ["jdbc:postgresql://"
                       #dyn/prop [DB_HOST "localhost"]
                       ":"
                       #dyn/prop [DB_PORT "5432"]
                       "/accounts"]}
 :web-server
  {:port #dyn/long #dyn/prop "WEB_PORT"}}

In this example, the DB_USER, DB_PW, DB_HOST, and DB_PORT, and WEB_PORT environment variables all play a role. DB_USER and DB_PORT are optional, since default values have been provided.

Let’s assume that the DB_HOST environment variable is db.example.org, DB_PW is change-me, and WEB_PORT was 8192, and the other referenced environment variables are unset.

After parsing and reader macro expansion, the resulting data will be:

{:connection-pool
  {:user-name "accountsuser"
   :user-pw   "change-me"
   :url        "jdbc:postgresql://db.example.org:5432/accounts"]}
 :web-server
  {:port 8192}}

Notice that combining #dyn/long and #dyn/prop has ensured that the web server port number is present as a number, not as a string.

Property Lookup

The #dyn/prop macro’s value is either single key, or a vector of a key and a default value. The key may be a symbol, keyword, or string.

An exception is thrown if the property is not found and there is no default.

For environment variables or JVM system properties, the dynamic value that replaces the macro will always be a string. For explicit properties, the dynamic value is typically a string, but can be any Clojure data: string, number, keyword, even a map or vector.

Underneath the covers, the key is used for a simple lookup in a map. The map is a merge of all environment variables, all JVM system properties, and any application-provided properties.

As a convenience, each environment variable is added to the map twice: one using a string key, and once with the string key converted to a symbol.

JVM system properties are added using string keys and string values.

Application properties are added exactly as provided; typically this means keyword keys and any kind of data values.

Usage

The env-readers function returns a map of readers; it may optionally be passed additional properties beyond those obtained from docker secrets, environment variables and JVM system properties.

(require '[clojure.edn :as edn]
         '[clojure.java.io :as io]
         '[com.walmartlabs.dyn-edn :refer [env-readers])

(->> "config.edn"
     io/resource
     slurp
     (edn/read-string {:readers (env-readers)})

Usage with Docker secrets

In a docker swarm secrets can be passed to a container when the container is added as a service to the swarm. In the default case, these secrets are mounted from tmpfs at /run/secrets although this location can be specified when the service is created.

e.g.

$ printf <secret> | docker secret create my_secret -

$ docker service create --replicas 1 --name <servicename> \
    --secret src=my-secret,target="/mysecrets/mysecret" \
    --publish published=8081,target=8081 <image:tag>

The above will create a secret called my-secret and make it available as a file at /mysecrets/mysecret.

The following can be used to recover the secret within the Docker conainer:

(require '[clojure.edn :as edn]
         '[clojure.java.io :as io]
         '[com.walmartlabs.dyn-edn :refer [env-readers])

(->> "config.edn"
     io/resource
     slurp
     (edn/read-string
       {:readers (env-readers
                   {:docker-secrets-dir "/mysecrets"})})

If the :docker-secrets-dir is not available as a property, it’s assumed that all secrets have been mounted at /run/secrets (which is Docker’s default location).

Note	In order for secrets to be available under this library, it is assumed that all secrets are mounted in the same directory.

Using the example above

$ printf 5432 | docker secret create DB_PORT -

$ docker service create --replicas 1 --name <servicename> \
    --secret DB_PORT \
    --publish published=8081,target=8081 <image:tag>

and if DB_PORT is not in the Environment of the container, the outcome will be the same. The value of the parameter will be read from the file at /run/secrets/DB_HOST and made available.

Note	the value of an env variable with a particular name will overwrite a secret with that name i.e. an environment variable has precedence.

License

Copyright (c) [2018]-present, Wal-Mart Store, Inc.

Distributed under the Apache Software License 2.0.