Application Configuration

When deploying an application, the charm you use will often support or even require specific configuration options to be set.

Juju provides tools to help manage these options and respond to changes over the lifetime of the application deployment. These options apply to the entire application, as opposed to only a specific unit or relation. The configuration can be modified by an administrator at deployment time or after the applications are operational.

Discovering application configuration options

Each charm will have its own set of options and possible values. You can discover these in several ways:

  • By running the juju config command.
  • By viewing the charm in the charm store.
  • By examining the config.yaml file in the charm itself.

Configuring an application at deployment time

Configuration values for an application can be set during deployment in several ways:

  • by using a yaml-formatted file
  • by passing options/values directly on the command line
  • a combination of the above

All these methods use the --config= switch.

For example, upon investigation we discover that the Mediawiki charm allows us to set values for the 'name' of the wiki and the 'skin' to use. We can put these inside a configuration file.

mediawiki:
  name: Juju Wiki
  skin: monobook

Assuming the file is called myconfig.yaml, the application can be deployed and configured in this way:

juju deploy --config myconfig.yaml mediawiki

Warning: If the configuration file cannot be read or contains syntax errors or invalid options, an error message will be printed to this effect. However, the application will still be deployed .

To pass the options directly:

juju deploy --config name='Juju Wiki' --config skin='monobook'

A combination can also be used. If a duplication arises, the last-mentioned value gets used. For instance, below, the wiki will be assigned the name of 'Juju Wiki':

juju deploy --config name='Juju Wookie' --config myconfig.yaml

Configuring an application which is already deployed

It is possible to set or change configuration of an application which is already deployed.

Before you set any of these options, you may want to check what current options are already set, using the juju config <application> command. For example:

juju config mediawiki

Should return something like this:

charm: mediawiki
application: mediawiki
settings:
  admins:
    default: true
    description: Admin users to create, user:pass
    type: string
    value: null
  debug:
    default: true
    description: turn on debugging features of mediawiki
    type: boolean
    value: false
  logo:
    default: true
    description: URL to fetch logo from
    type: string
    value: null
  name:
    default: true
    description: The name, or Title of the Wiki
    type: string
    value: Please set name of wiki
  skin:
    default: true
    description: skin for the Wiki
    type: string
    value: vector
  use_suffix:
    description: If we should put '/mediawiki' suffix on the url
    type: boolean
    value: false

You can set the options using juju config <application>, specifying multiple space-separated key=value pairs if necessary:

juju config mediawiki skin=monoblock name='Juju Wiki'

It is also possible to set the configuration options from a YAML file after the application has been deployed:

juju config mediawiki --file path/to/myconfig.yaml

Setting an option back to its default value is achieved by using the same command, with the --reset switch, followed by a comma-separated list of the values to return to the default setting:

juju config mediawiki --reset admins,name