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.
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.
#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/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 (though DB_USER
is optional, since it has a default value).
After parsing and reader macro expansion, the resulting data might be:
{:connection-pool
{:user-name "accountsuser"
:user-pw "change-me"
:url "jdbc:postgresql://db.example.org:5432/accounts"]}
:web-server
{:port 8192}}
Property Lookup
The #dyn/prop
macro’s first (or only) argument is a key.
It may be a symbol, keyword, or string.
The optional second value is the default used when property lookup fails.
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 can be any Clojure data: string, number, keyword, even a map or vector.
Usage
The env-readers
function returns a map of readers; it may optionally be passed additional
properties beyond those obtained from 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)})
License
Copyright © 2018 Walmart Labs
Distributed under the Apache Software License 2.0.