The base layer: layer-basic

Apache 2.0 License

This is the base layer for all reactive Charms. It provides all of the standard Juju hooks and starts the reactive framework when these hooks get executed. It also bootstraps the charm-helpers and charms.reactive libraries, and all of their dependencies for use by the Charm. Check out the code for the basic layer on Github.

Usage

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 interfaces.juju.solutions 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.

Hooks

This layer provides hooks so that the reactive framework gets started when these hooks run, and so that other layers can react to these hooks 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 other hooks such as storage 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 @when_file_changed, and @data_changed to ensure that handlers run only when necessary.

Reactive flags for Charm config

This layer will set the following flags:

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

An example using the config flags would be:

@when('config.changed.my-opt')
def my_opt_changed():
    update_config()
    restart_service()

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.

    Note

    The packages layer option is intended for charm dependencies only. That is, for libraries and applications that the charm code itself needs to do its job of deploying and configuring the payload. If the payload (the application you’re deploying) itself has dependencies, those should be handled separately, by your Charm using for example the Apt layer

  • 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 $JUJU_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']
options:
  basic:
    packages: ['git']
    use_venv: true
    include_system_packages: true

Wheelhouse.txt for Charm Python dependencies

layer-basic provides two methods to install dependencies of your charm code: wheelhouse.txt for python dependencies and the packages layer option for apt dependencies.

Each layer can include a wheelhouse.txt file with Python requirement lines. The format of this file is the same as pip’s requirements.txt file. For example, this layer’s wheelhouse.txt includes:

pip>=7.0.0,<8.0.0
charmhelpers>=0.4.0,<1.0.0
charms.reactive>=0.1.0,<2.0.0

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.

See PyPI for packages under the charms. namespace which might be useful for your charm. See the packages layer option of this layer for installing apt dependencies of your Charm code.

Note

The wheelhouse.yaml are intended for charm dependencies only. That is, for libraries and applications that the charm code itself needs to do its job of deploying and configuring the payload. If the payload (the application you’re deploying) itself has dependencies, those should be handled separately.

Exec.d Support

It is often necessary to configure and reconfigure machines after provisioning, but before attempting to run the charm. Common examples are specialized network configuration, enabling of custom hardware, non-standard disk partitioning and filesystems, adding secrets and keys required for using a secured network.

The reactive framework’s base layer invokes this mechanism as early as possible, before any network access is made or dependencies unpacked or non-standard modules imported (including the charms.reactive framework itself).

Operators needing to use this functionality may branch a charm and create an exec.d directory in it. The exec.d directory in turn contains one or more subdirectories, each of which contains an executable called charm-pre-install and any other required resources. The charm-pre-install executables are run, and if successful, state saved so they will not be run again.

$JUJU_CHARM_DIR/exec.d/mynamespace/charm-pre-install

An alternative to branching a charm is to compose a new charm that contains the exec.d directory, using the original charm as a layer,

A charm author could also abuse this mechanism to modify the charm environment in unusual ways, but for most purposes it is saner to use charmhelpers.core.hookenv.atstart().

General layer info

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/foo.py file with some helper functions that other layers could access using:

from charms.layer.foo 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']
options:
  foo:  # setting options for the "foo" layer
    enable-bar: true  # set the "enable-bar" option to true

The foo layer can then use the API provided by the options layer (which is automatically included via the basic layer) to load the values for the options that it defined. For example:

from charms import layer

@when('flag')
def do_thing():
  # check the value of the "enable-bar" option for the "foo" layer
  if layer.options.get('foo', 'enable-bar'):
      hookenv.log("Bar is enabled")

  # or get all of the options for the "foo" layer as a dict
  foo_opts = layer.options.get('foo')

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

. charms.reactive.sh

@when 'flag'
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)"
    fi
}

reactive_handler_main

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