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A library for declaring configuration vars and setting their values in a centralized fashion. Included tooling allows one to gather and emit all config vars and their docstrings, default values, etc.

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Stable (0.13.0) API Docs

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Applications and libraries wishing to declare config vars should add the following to the project.clj file:

Clojars Project

Applications may also include the following to ease generating a config.edn file:

:aliases {"config" ["run" "-m" "outpace.config.generate"]}


Most configuration systems rely on consumers pulling named values from something akin to a global map (cf. environ). This yields a number of negative consequences:

  • It is difficult to identify what configuration values are required by a system.
  • There is no single place to put documentation for a configurable entry.
  • Multiple consumers may independently pull directly from the configuration source, leaving only the configuration key to reveal their shared dependency.
  • Default values for missing configuration can be inconsistent across consumers.
  • Sources of configuration values are widely varied, e.g., properties files, system environment variables.

This library attempts to address the above issues by:

  • Using declaration of configurable vars, which can be documented, defaulted, and used as a canonical source of data for other code just as any other library-specific var would be.
  • Providing introspection into the configurable surface of an application and its dependencies.
  • Relying on pushing values to all configuration vars, and doing so from a single source.


Configuration is provided in an EDN map of namespaced symbols (naming a config var) to the value to be bound to the corresponding var:

com.example/greeting       "Hello World!"
com.example/tree           {:id 1, :children #{{:id 2} {:id 3}}}
com.example/aws-secret-key #config/env "AWS_SECRET_KEY"
com.example/path           #config/property "some.path"
com.example/db-password    #config/file "db-password.txt"
com.example/secret-edn     #config/edn #config/file "secret_key.edn"

As shown above, custom data-reader tags can be used to pull values from external sources.

The configuration EDN map is provided to an application in one of the following ways:

  1. A config.edn file in the current working directory.
  2. A config.edn java system property (e.g., a command line arg -Dconfig.edn=...). The value can be any string consumable by
  3. A resource.config.edn java system property pointing to a resource file.
  4. Setting outpace.config.bootstrap/explicit-config-source to any non-nil value consumable by

The :profiles entry of your project.clj file can be used to set the system property for an environment-specific configuration EDN file:

:profiles {:test {:jvm-opts ["-Dconfig.edn=test-config.edn"]}
           :prod {:jvm-opts ["-Dconfig.edn=prod-config.edn"]}}

Alternatively, different profiles can share the same config file name, but use different resource paths:

:profiles {:test {:resource-paths ["test_resources"] :jvm-opts ["-Dresource.config.edn=app-config.edn"]}
           :prod {:resource-paths ["prod_resources"] :jvm-opts ["-Dresource.config.edn=app-config.edn"]}}

Config Usage

Declaring config vars is straightforward:

(require '[outpace.config :refer [defconfig]])

(defconfig my-var)

(defconfig var-with-default 42)

(defconfig ^:dynamic *rebindable-var*)

(defconfig ^:required required-var)

(defconfig ^{:validate [number? "Must be a number."
                        even?   "Must be even."]}

As shown above, the defconfig form supports anything a regular def form does, as well as the following metadata:

  • :required When true, an exception will be thrown if no default nor configured value is provided. See also defconfig!
  • :validate A vector of alternating single-arity predicates and error messages. After a value is set on the var, an exception will be thrown when a predicate, passed the set value, yields false.

The outpace.config namespace includes the current state of the configuration, and while it can be used by code to explicitly pull config values, this is strongly discouraged; just use defconfig.

External Config Values

Recognizing that it is not always appropriate to provide configuration values directly in the config file, custom data-readers can be used to instead convert a tagged literal in the config to an externally-provided value. This allows one to still grasp the full configuration of an app, and at least know where a value will come from, if not the value itself.

The provided data-readers' tags are:

  • #config/env Tags a string, interpreted as the name of an environment variable, and yields the string value of the environment variable. If the environment does not have that entry, then the var will use its default value or remain unbound.
  • #config/property Tags a string, interpreted as the name of java system property, and yields the string value of the property. If there is no property defined by this name, then the var will use its default value or remain unbound.
  • #config/file Tags a string, interpreted as a path to a file, and yields the string contents of the file. If the file does not exist, then the var will use its default value or remain unbound.
  • #config/edn Tags a string, interpreted as a single EDN-formatted object, and yields the read object. When composed with #config/env, #config/file, or #config/property, if the external value is not provided, then the var will use its default value or remain unbound.

Custom data-readers whose tag namespace is config will be automatically loaded during config initialization. See outpace.config/read-env for an example of how to properly implement a custom data-reader.

Backup configuration sources

It is possible to use #config/or to be able to use multiple sources of external configuration values. It tags a vector where the first external value will be used.

In the following example, the value from the environment variable APP_HOME will be used. However, if it is not provided, it will fall back to the app.home system property. If neither external value is availabe, the var will use its default value or remain unbound.

#config/or [#config/env "APP_HOME"
            #config/property "app.home"]

Generator Usage

The outpace.config.generate namespace exists to generate a config.edn file containing everything one may need to know about the state of the config vars in the application and its dependent namespaces. If a config.edn file is already present, its contents will be loaded, and thus preserved by the replacing file.

To generate a config.edn file, invoke the following in the same directory as your project.clj file:

lein run -m outpace.config.generate

Alternately, one can just invoke lein config by adding the following to project.clj:

:aliases {"config" ["run" "-m" "outpace.config.generate"]}

The generator can take an optional :check flag (e.g., lein config :check) that causes it to print unbound and unused config vars to stdout. It will not generate a config file.

The generator can also take a :strict flag (e.g., lein config :strict) that will result in an exception after file generation if there are any config vars with neither a default value nor configured value. This can be used to provide feedback to automated build systems.

The following is an example of a generated config.edn file:



; This is the docstring for the 'foo' var. This
; var does not have a default value.


com.example/bar 123


; The docstring for aaa.
com.example/aaa :configured-aaa

; The docstring for bbb. This var has a default value.
com.example/bbb :configured-bbb #_:default-bbb

; The docstring for ccc. This var has a default value.
#_com.example/ccc #_:default-ccc


The first section lists commented-out config vars that do not have a default value nor configured value, thus will be unbound at runtime. If a config value is provided, these entries will be re-categorized after regeneration.

The second section lists config entries that have no corresponding config var. This may happen after code change, or when a dependent library has been removed. If the config var reappears, these entries will be re-categorized after regeneration.

The third section lists all config vars used by the system, and their respective values. For reference purposes, commented-out default values will be included after the configured value. Likewise, commented-out entries will be included when their default values are used.

Change Log


  • Changes where the attempt to load outpace.config-aws occurs.


  • Tries to load outpace.config-aws as part of the outpace.config namespace


  • Add :check flag for generate.


  • Do not read configuration when compiling.


  • Add support for loading configuration from resources via the resources.config.edn system property. @rafalprzywarski


  • Add support for #config/or


  • Fixed outpace.config.repl/reload to re-read config.


  • Add #config/property data-reader which sets a config var's value to the contents of a Java property.


  • A required config (e.g. defconfig!) will not cause an error when running the generator.
  • When running the generator if an error occurs, its details will be printed.


  • extract and provides? now recursively visit data structures. This allows something like the following in your config.edn file (which did not work before):

    { {:access-key #config/env "AWS_ACCESS_KEY_ID"
                        :secret-key #config/env "AWS_SECRET_ACCESS_KEY"}}


  • Add #config/edn data-reader which can be composed with other readers to interpret values from content.


  • Add #config/file data-reader which sets a config var's value to the contents of a file.


  • Add :validate metadata support to defconfig.
  • Generator now pretty-prints wide values.


  • Add ability to set the config-source explicitly in-code. Allows custom code to set the config-source when neither providing a local config.edn file nor setting a system-property can be used (e.g., webapps).


  • Add support for using custom data-readers when loading the config EDN. Tags with a config namespace (e.g., #config/foo) will have the corresponding data-reader function's namespace automatically loaded.


  • Add in-repl ability to reload with a different config.


Copyright © Outpace Systems, Inc.

Released under the Apache License, Version 2.0

Can you improve this documentation? These fine people already did:
Alexander Taggart, Paul Stadig, Daniel Solano Gómez, Devin Walters, Timothy Pratley, Jake McCrary, Rafal Przywarski & Ron Toland
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