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Writing a layer by example

In this document, we will be writing a charm layer for Vanilla. Vanilla is an open source, themeable, pluggable, and multi-lingual forum software, which enables communities to host a forum for discussion topics at scale. Powered by PHP and MySQL, Vanilla is a fine example of a three-tiered application:

  • Database (MySQL)
  • Middleware (PHP App)
  • Load Balancing via HTTP interface

Prepare your workspace

Building off of $JUJU_REPOSITORY, we want to add two more environment variables to your session. We recommend adding these into your shell configuration file so that they are always available.


First off, you require a local charm repository in which to work. This involves creating three directories -- layers, interfaces, and charms -- and setting some environment variables.

The layers directory contains the source code of the layered charm covered in our examples. The interfaces directory is where you'd place any interface-layers you may wish to write, and the charms directory holds the assembled, ready to deploy charm.


mkdir -p $LAYER_PATH $LAYER_PATH/vanilla

cd $LAYER_PATH/vanilla

Note: Exporting the environment variables in this way only sets the variables for the current terminal. If you wish to make these changes persist, add the same export statements to a resource file that are evaluated when you create a new console such as ~/.bashrc depending on your shell.

Determine a base layer and additional relations

To best leverage existing work, it is important to choose the right base layer for your charm. The available layers and interfaces can be found at

For our example, Vanilla is a PHP5 application, so the layer we will use is the apache-php5 layer. This is a "runtime" layer that provides a common basis for specific applications to be built on top of. Out of the box, this will give us Apache2 running on a configurable port, mod-php5, and support for the http interface for running behind a proxy for high availability and scaling. On top of that, we'll use the mysql interface to facilitate our database interface. We will put these in a layer.yaml file, which tells the builder what layers and interfaces to combine with yours to create the finished charm:

includes: ['layer:apache-php', 'interface:mysql']

Fill in your charm layer

With that decided, we're ready to start creating our charm layer. The README, icon, and copyright file will be the same. The metadata.yaml will be the same except that the website relation will be managed for us, so we can leave out the provides section:

name: vanilla
    summary: Vanilla is an open-source, pluggable, themeable, multi-lingual forum.
    maintainer: Your Name <your@email.tld>
    description: |
    Vanilla is designed to deploy and grow small communities to scale.
    This charm deploys Vanilla Forums as outlined by the Vanilla Forums installation guide.
      - social
        interface: mysql

The apache-php layer requires some additional configuration to tell it how to install and configure your application. This goes in an apache.yaml file:

      - 'php5-mysql'
      - 'php5-gd'
        options: 'Indexes FollowSymLinks MultiViews'

This tells the apache-php layer to install some additional packages for PHP, where to fetch your application from (as well as its cryptographic hash to verify the payload), and a few options for the vhost entry. The application will be installed under /var/www/{site_name} where {site_name} is the name of the block under sites (in this case vanilla).

Implementing your layer

The apache-php layer and mysql interface use charms.reactive to coordinate with your layer. This makes it easy to coordinate the state of multiple layers, relations, configuration options, etc. Code for reactive handlers should live under a reactive/ directory in your charm layer. In our case, we'll use reactive/ The apache-php layer sets an apache.available state when your application is done being installed, and the mysql interface sets a similar {relation_name}.available state (where {relation_name} depends on what you called the relation in your metadata.yaml) to indicate that MySQL has provided you with a complete set of database credentials. Thus, by watching for both of those states, you can easily tell when the right time to set up your application is:

    @when('apache.available', 'database.available')
        def setup_vanilla(mysql):

You'll notice that the MySQL relation is passed in so that you can easily access the database connection information. Since the Apache installation and MySQL relation negotiation are all handled for us, the only thing left to do is create the conf/config.php file for Vanilla. To make things easy, we will render a template to populate the values. We'll also handle a few more state changes to provide better status reporting and react to our database going away in an intelligent manner:

    import pwd
    import os
    from charmhelpers.core.hookenv import status_set
    from charmhelpers.core.templating import render
    from charms.reactive import when, when_not
    from charms.reactive import set_state, remove_state

    @when('apache.available', 'database.available')
    def setup_vanilla(mysql):
                   'db': mysql,
        uid = pwd.getpwnam('www-data').pw_uid
        os.chown('/var/www/vanilla/cache', uid, -1)
        os.chown('/var/www/vanilla/uploads', uid, -1)
        status_set('maintenance', 'Starting Apache')

    def missing_mysql():
        status_set('blocked', 'Please add relation to MySQL')

    def waiting_mysql(mysql):
        status_set('waiting', 'Waiting for MySQL')

    def started():
        status_set('active', 'Ready')

By setting the apache.start state, we are letting the apache-php layer know that we have finished configuring the application and it is ready to start.

The templates/vanilla_config.php file is straightforward:

<?php if (!defined('APPLICATION')) exit();
$Configuration['Database']['Host'] = '{{ }}';
$Configuration['Database']['Name'] = '{{ db.database() }}';
$Configuration['Database']['User'] = '{{ db.user() }}';
$Configuration['Database']['Password'] = '{{ db.password() }}';