Best Practice for Charm Authors

This document is to capture charm best practices from the community. We expect devops to be strongly opinionated, therefore some strong opinions don't make sense as policy, but do make sense to share with others to disseminate expertise.

If you'd like to share some best practice and have it added to this page we recommend you post to the Juju Discourse forum with some ideas on what you'd like to see added. Ideally we'd like to document all sorts of great ideas on how people are using Juju.

Best Practice Tips for using Juju in general

Best Practice Tips for Charm Authors

  • Avoid symlinks if you can, people can use Windows for charms. If you prefer having your hook code in one file you can achieve the same affect by stubbing hooks which import your file and invoke the methods that are wrapped with the Hook decorator.
  • Juju can also run on other Linuxes, so if you want to make something more agnostic, you can leverage configuration management tools for things like the installation hooks to abstract those bits.
  • If you need to deploy things offline or on a network restricted network, consider using Juju resources.
  • Implement a pattern that can be easily unit testable, and submit unit tests with your charm.

Examples of Best Practice Charms

Here is a list of charms that leverage a particular technology that you might want to look at as examples:

  • Ansible integration - The ElasticSearch charm
  • Chef integration - The Rails charm
  • Puppet integration - Work in Progress with someone at Puppet Labs.
  • Charm written in Python - Any of the Zabbix charms
  • Tomcat integration - The OpenBook charm or the OpenMRS charm
  • Choosing a Java SDK based on target platform (useful for POWER8) - Check out the big data branch of charm tools. (Temporary location, this will be incorporated into charm tools soon.)
  • Example cinder charm - The cinder-vnx charm, use this if you're interested in adding support for your specific storage hardware to Ubuntu OpenStack.
  • Example neutron charm - The neutron-openvswitch charm, use this if you're interested in adding support for your specific networking hardware to Ubuntu OpenStack.

Juju Best Practices and Tips from Canonical's Infrastructure Team

Since Canonical IS uses Juju in production they have certain requirements from charms in order for them to run on a production OpenStack deployment. Though these are requirements for use inside Canonical, charms are not required to meet these criteria to be in the Charm Store, they are included here to share with the devops community.

Tips for production usage:

  • Provide an overview of the application in the README and metadata.
  • Use packaged software (i.e. Debian packages) where possible, and "backport" any packages needed outside of the archives from whatever PPA you have them to an internal accessible repository.
  • Do not duplicate any application components for which there are pre-existing charms.
  • Follow the coding guidelines for charms (see below).
  • Assemble code for your application outside of the Charm.
  • Create a /srv/${external-application-name}/${instance-type}/${application-name} directory for the code itself, and /srv/${external-application-name}/{${instance-type}-logs,scripts,etc,var} as needed.
  • Ensure the owner of the code isn't the same user than runs the code
  • Create monitoring checks for your application.
  • Which checks are prompt-critical (in other words, constitutes a user-affecting outage that would warrant waking an "oncall" sysadmin over a weekend)?
  • Confirm what data/logs from the application needs to be made visible to developers, and in what format?
  • Some organizations use proxies, do not assume that every port is available, consider using common ports like 80/443 to ensure your charm can run in as many providers as possible. For added flexibility we recommend exposing port configuration in the charm as a config option.
  • The configuration should not be immutable. This means that the charm should react to all configuration options as they are changed throughout the lifecycle of that application. Immutable configuration breaks the user experience, and should only be used to prevent data loss. If a user deploys a charm and later sets the configuration values the user would expect the charm to react to those changes accordingly.

Charm Coding Guidelines

If written in Bash:

  • Use set -e for all hooks written in Bash. This option tells bash to exit the script if a command returns a non true result. This option prevents the script from continuing when the script is known to be in an error state.
  • {variable}-value rather than $variable-value?
  • Use install rather than mkdir; chown

If written in Python:

  • Has pep8 been run against the relevant scripts?
  • Separation of code from content (i.e. all external files/templates are in the "files" or "templates" directory)?

As an example, this populates a template from any variables in the current environment: cheetah fill --env -p templates/celerymon_conf.tmpl > /etc/init/celerymon.conf

In this example, the template looks like this:

start on started celeryd
stop on stopping celeryd
exec sudo -u $USER_CODE_RUNNER sh -c "cd \$CODEDIR;
PYTHONPATH=\$PYTHONPATH ./ celerycam --pidfile
  • Do all config options have appropriate descriptions?
  • Are all hooks idempotent?
  • No hard coded values for things that may need changing - exposed via config.yaml options
  • No hard coding of full paths for system tools/binaries - we should ensure $PATH is set appropriately.
  • Has charm proof been run against the charm?
  • Has testing of adding units and removing units been done?
  • Has testing of changing all config options and verifying they get changed in the application (and applied, i.e. service reloaded if appropriate) been done?
  • Any cron entries should be in /etc/cron.d rather than stored in user crontabs.
  • This allows for easier visibility of active cronjobs across the whole system, as well as making editing things much easier.

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