Axwalk Juju Introspection

Channel Version Revision Published Runs on
latest/stable 0 0 18 Mar 2021
Ubuntu 16.04
juju deploy axwalk-juju-introspection
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This is the base layer for all charms built using layers. It provides all of the standard Juju hooks and runs the charms.reactive.main loop for them. It also bootstraps the charm-helpers and charms.reactive libraries and all of their dependencies for use by the charm.


To create a charm layer using this base layer, you need only include it in a layer.yaml file:

includes: ['layer:basic']

This will fetch this layer from and incorporate it into your charm layer. You can then add handlers under the reactive/ directory. Note that any file under reactive/ will be expected to contain handlers, whether as Python decorated functions or executables using the external handler protocol.

Charm Dependencies

Each layer can include a wheelhouse.txt file with Python requirement lines. For example, this layer's wheelhouse.txt includes:


All of these dependencies from each layer will be fetched (and updated) at build time and will be automatically installed by this base layer before any reactive handlers are run.

Note that the wheelhouse.txt file is intended for charm dependencies only. That is, for libraries that the charm code itself needs to do its job of deploying and configuring the payload. If the payload itself has Python dependencies, those should be handled separately, by the charm.

See PyPI for packages under the charms. namespace which might be useful for your charm.

Layer Namespace

Each layer has a reserved section in the charms.layer. Python package namespace, which it can populate by including a lib/charms/layer/<layer-name>.py file or by placing files under lib/charms/layer/<layer-name>/. (If the layer name includes hyphens, replace them with underscores.) These can be helpers that the layer uses internally, or it can expose classes or functions to be used by other layers to interact with that layer.

For example, a layer named foo could include a lib/charms/layer/ file with some helper functions that other layers could access using:

from import my_helper
Layer Options

Any layer can define options in its layer.yaml. Those options can then be set by other layers to change the behavior of your layer. The options are defined using jsonschema, which is the same way that action paramters are defined.

For example, the foo layer could include the following option definitons:

includes: ['layer:basic']
defines:  # define some options for this layer (the layer "foo")
  enable-bar:  # define an "enable-bar" option for this layer
    description: If true, enable support for "bar".
    type: boolean
    default: false

A layer using foo could then set it:

includes: ['layer:foo']
  foo:  # setting options for the "foo" layer
    enable-bar: true  # set the "enable-bar" option to true

The foo layer can then use the charms.layer.options helper to load the values for the options that it defined. For example:

from charms import layer

def do_thing():
  layer_opts = layer.options('foo')  # load all of the options for the "foo" layer
  if layer_opts['enable-bar']:  # check the value of the "enable-bar" option
      hookenv.log("Bar is enabled")

You can also access layer options in other handlers, such as Bash, using the command-line interface:


@when 'state'
function do_thing() {
    if layer_option foo enable-bar; then
        juju-log "Bar is enabled"
        juju-log "bar-value is: $(layer_option foo bar-value)"


Note that options of type boolean will set the exit code, while other types will be printed out.


This layer provides hooks that other layers can react to using the decorators of the charms.reactive library:

  • config-changed
  • install
  • leader-elected
  • leader-settings-changed
  • start
  • stop
  • upgrade-charm
  • update-status

Other hooks are not implemented at this time. A new layer can implement storage or relation hooks in their own layer by putting them in the hooks directory.

Note: Because update-status is invoked every 5 minutes, you should take care to ensure that your reactive handlers only invoke expensive operations when absolutely necessary. It is recommended that you use helpers like @only_once, @when_file_changed, and data_changed to ensure that handlers run only when necessary.

Layer Configuration

This layer supports the following options, which can be set in layer.yaml:

  • packages A list of system packages to be installed before the reactive handlers are invoked.

  • use_venv If set to true, the charm dependencies from the various layers' wheelhouse.txt files will be installed in a Python virtualenv located at $CHARM_DIR/../.venv. This keeps charm dependencies from conflicting with payload dependencies, but you must take care to preserve the environment and interpreter if using execl or subprocess.

  • include_system_packages If set to true and using a venv, include the --system-site-packages options to make system Python libraries visible within the venv.

An example layer.yaml using these options might be:

includes: ['layer:basic']
    packages: ['git']
    use_venv: true
    include_system_packages: true

Reactive States

This layer will set the following states:

  • config.changed Any config option has changed from its previous value. This state is cleared automatically at the end of each hook invocation.

  • config.changed.<option> A specific config option has changed. <option> will be replaced by the config option name from config.yaml. This state is cleared automatically at the end of each hook invocation.

  • config.set.<option> A specific config option has a True or non-empty value set. <option> will be replaced by the config option name from config.yaml. This state is cleared automatically at the end of each hook invocation.

  • config.default.<option> A specific config option is set to its default value. <option> will be replaced by the config option name from config.yaml. This state is cleared automatically at the end of each hook invocation.

An example using the config states would be:

def my_opt_changed():


This layer currently does not define any actions.